Mep

Front 1

Absorption Coefficient

Back 1

The percentage of incoming energy which is absorbed. In measur­ing radiant energy (light or heat) it is a unitless ratio which may vary depending on wavelength. In acoustics, it is called a sabin and may vary depending on frequency.

Front 2

Absorption Refrigeration

Back 2

A cooling process resulting from the absorption of vapor by a brine solution that is then heated to remove the mois­ture. The heat may be supplied by solar or other heating sources.

Front 3

AC

Back 3

The abbreviation for either air condition­ing or alternating current, depending on the context.

Front 4

Acoustic Power Level

Back 4

See Power Level.

Front 5

Air Gap

Back 5

An unobstructed vertical path, open to the atmosphere, separating the outlet of a faucet from the overflow rim of the fixture it serves. The purpose is to prevent a momentary vacuum in the supply pipe from siphoning water from the fixture back into the supply pipe.

Front 6

Alternating Current

Back 6

An electric current that reverses its direction at regular intervals, gener­ally 60 times per second in the United States. A plot of the voltage over time is a sine wave.

Front 7

Ambient

Back 7

Relating to a general or all surround­ing condition. In thermal processes, it refers to the air temperature, as distinct from that of surfaces or objects. In lighting, it refers to the background light level, and in acoustics, it refers to the background noise level.

Front 8

ASHRAE

Back 8

The abbreviation for the American Society of Heating, Refrigerating, and Air Conditioning Engineers, the source of most of the standardized information on the subject.

Front 9

Aspect Ratio

Back 9

The ratio of the longer to shorter dimension of an air conditioning duct, that affects duct friction, or of a room, that relates to light reflection.

Front 10

Ball Drip

Back 10

The automatic drain valve at the base of a dry standpipe.

Front 11

Blow Down

Back 11

The drain that removes dirt that builds up in the floor pool of an evaporative cooling tower.

Front 12

BTU

Back 12

The abbreviation for British Thermal Unit, a unit of heat energy, which is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

Front 13

BTUH

Back 13

The abbreviation for Btu's per hour, an energy flow rate.

Front 14

Building Automation

Back 14

The control by auto­matic equipment of many functions in a large building, usually including the HVAC system, the fire detection and alarm system, and building security.

Front 15

CFM

Back 15

The abbreviation for cubic feet per minute, which is the flow rate of air in a mechan­ical system or duct.

Front 16

Chiller

Back 16

A piece of equipment which cools water for use in an air conditioning system.

Front 17

Chill Factor

Back 17

A fictitious temperature assigned to a combination of actual temperature and wind velocity that has the same physiolog­ical effect as still air at the chill factor temperature.

Front 18

Chlorination

Back 18

The addition of small amounts of chlorine to a water source to kill bacteria.

Front 19

Circuit Vent

Back 19

In sanitary drainage, any vent that serves two or more traps.

Front 20

Clerestory

Back 20

A window above eye level that admits daylight, such as the high windows in a cathedral.

Front 21

Code

Back 21

An organized body of rules and regula­tions adopted and enforced by a governmental unit, such as a building code.

Front 22

Coefficient of Performance

Back 22

The ratio of the amount of heat energy delivered by a heat pump to the amount of energy supplied, or the ratio of the amount of heat energy removed by a refrigeration machine to the amount of energy expended in its removal. It is similar to effi­ciency, but often exceeds 100 percent, and is therefore expressed as a number, i.e., 2.7, which is typical for a heat pump.

Front 23

Coefficient of Utilization

Back 23

The ratio of useful light arriving at the work plane to the amount of light emitted by the source. The CU depends on the reflectivity of different surfaces and the aspect ratios of the ceiling, wall, and floor cavities.

Front 24

Comfort Zone

Back 24

The combination of thermal and environmental conditions within which a human is comfortable, often shown on a psy-chrometric chart.

Front 25

Conductance (C)

Back 25

The rate at which a specific thickness of a given material conducts heat.

Front 26

Convection

Back 26

The heat transfer process that occurs when a warm fluid rises, displacing cold fluid which then falls.

Front 27

Cycle

Back 27

One complete set of repeating events, typically used with alternating current or sound.

Front 28

Cycles per Second (CPS)

Back 28

A measure of frequency in electric current or acoustics, i.e., the number of cycles per second of a wave or oscillation. In acoustics, the term has been largely replaced by Hertz, where 1 cps = 1 Hz.

Front 29

Daylighting

Back 29

The use of natural light from out­side to replace or augment electrical light out­doors, which produces energy savings.

Front 30

dBA

Back 30

A decibel measured in the A scale, that is weighted to account for the special sensitivi­ties of the human ear.

Front 31

Decibel (dB)

Back 31

A logarithmic measure of sound intensity level.

Front 32

Degree Day (DD)

Back 32

The amount by which the average outdoor temperature at a particular loca­tion is below 65°F for one day. Degree days may also be summed and stated for a month or a year.

Front 33

Delta Connection

Back 33

A method of connecting windings on a three phase transformer, end to end, that results in a triangular shape.

Front 34

Dew Point Temperature

Back 34

The temperature of air at that the water contained in the air begins to condense and form dew. The dew point for a given air sample is always lower than or equal to its current temperature.

Front 35

Diffuser

Back 35

A device through which the air from a duct enters a room, or a device through which the light from a fixture enters a room.

Front 36

Dry Bulb (DB)

Back 36

The temperature of air as read on an ordinary glass thermometer.

Front 37

Dry Pipe Sprinkler

Back 37

A sprinkler system whose pipes are normally pressurized with only air, thus being invulnerable to freezing temper­atures. Upon actuation, the air is vented and supply pressure forces water through the sys­tem.

Front 38

Economizer Cycle

Back 38

An energy-saving strat­egy in which a part of the HVAC system is shut off while the rest is used, such as shutting off the refrigeration when the outside air tempera­ture is low, while the fan continues to operate.

Front 39

Effective Temperature

Back 39

A fictitious tempera­ture having the same physiological effect as air of a standardized temperature, humidity, and velocity.

Front 40

Efficacy

Back 40

The ratio of the lumens emitted by a lamp to the electrical power consumed by the lamp.

Front 41

Emissivity

Back 41

A factor that represents the rate at which a given surface material gives off or emits radiant energy. The emissivity varies from 0 to 1.0, where 1.0 is the theoretical emissivity of a perfect black box at the same temperature.

Front 42

Enthalpy

Back 42

The total of sensible plus latent heat stored in the air. It is also known as Total Heat.

Front 43

Exit

Back 43

A continuous and unobstructed means of egress to a public way. Its minimum width is generally 44 inches.

Front 44

Fire Assembly

Back 44

A complete fire-resistive assembly consisting of a fire door, fire damper, or fire window and its mounting frame and hard­ware. The entire assembly, not just its compo­nents, must be approved and labeled by a testing agency that inspects the materials and workman­ship during fabrication at the factory. Available ratings are 3/4, 1, 1-1/2 and 3 hours.

Front 45

Fixture Unit

Back 45

A unit of liquid flow used in sizing both supply and drainage pipes.

Front 46

Flame Spread Rating

Back 46

A numerical classifi­cation indicating the rate at which flame will spread in or on a given material, in which higher numbers flame up more rapidly.

Front 47

Flushometer Valve

Back 47

A valve that releases a definite amount of water into a plumbing fixture each time it is actuated.

Front 48

Footcandle

Back 48

The basic unit of illumination arriving at a work plane. One footcandle is equal to one lumen per square foot.

Front 49

Forced Air System

Back 49

A heating or cooling sys­tem that uses a fan to circulate heated or cooled air through ducts to the occupied spaces.

Front 50

Forced Convection

Back 50

The movement of a fluid by a fan or a pump, in order to force heat exchange.

Front 51

Four Pipe System

Back 51

A hot and chilled water system having separate return lines for each sup­ply line, and with no mixing of the two streams.

Front 52

Frequency

Back 52

The number of cycles that occur per second, either in alternating current or acoustics. In acoustics, the frequency deter­mines the pitch.

Front 53

Frost Line

Back 53

In a given location, the maximum depth in soil that is expected to freeze in cold weather. Water piping must ordinarily be buried below the frost line to protect against freezing.

Front 54

Fusible Link

Back 54

A piece of wax or paraffin that melts at a predetermined temperature, setting off a sprinkler head, an alarm system, or otherwise actuating a fire protection device.

Front 55

Globe Thermometer

Back 55

A thermometer that measures Mean Radiant Temperature (MRT).

Front 56

Ground

Back 56

An electrical conductor connected to the earth or to a pipe extending into the ground, used to dissipate hazardous currents into the earth.

Front 57

Halon

Back 57

A gaseous fire extinguishing medium that smothers fires, often used in automatic sys­tems in computer rooms.

Front 58

Heat Pump

Back 58

A refrigeration loop used to bring heat into a space instead of removing heat from it. The term is also used for an entire system of such units attached to a recirculating heat sink.

Front 59

HEPA Filter

Back 59

A high efficiency particulate air filter, which removes dust and other tiny parti­cles from a moving air stream.

Front 60

Hertz (HZ)

Back 60

The frequency of a sound, or of an alternating current, equal to the number of cycles per second.

Front 61

High Intensity Discharge (HID)

Back 61

A family of lamps consisting of a quartz envelope inside a glass envelope. The inner quartz tube can stand higher temperatures, and allows for the current to arc between the two electrodes exciting a plasma of mercury, metal halide, or high pres­sure sodium.

Front 62

HP or BHP

Back 62

Horsepower or brake horse­power, a unit of power, equal to roughly 746 watts.

Front 63

Illumination

Back 63

The intensity of light falling on a surface, usually expressed in footcandles.

Front 64

Impact Isolation Class (IIC)

Back 64

A rating of the degree of isolation of a floor against the trans­mission of impact noises.

Front 65

Infiltration

Back 65

The leakage of air through cracks around windows and other building elements.

Front 66

Intensity Level

Back 66

The intensity of sound at a given location, measured in watts per square meter, or more commonly in dB where the ref­erence level is 10~12 watts/meter2 or 10~16 watts/cm2.

Front 67

Inverse Square Law

Back 67

A physical principle that states that the intensity of a phenomenon is inversely proportional to the square of the dis­tance from the source to the measuring device. It holds true for point sources of light, and for sound in an open field.

Front 68

Invert

Back 68

The lowest point of the inside of a drain, pipe, channel, or other liquid-carrying conduit.

Front 69

Ion Exchange

Back 69

A process of water softening in which calcium and magnesium ions are replaced by sodium ions. This process is also known as the zeolite process.

Front 70

lonization Detector

Back 70

A fire detector which detects the products of combustion (POC) even before they are visible to the naked eye.

Front 71

K Factor

Back 71

The thermal conductivity of one square foot of a material per inch of thickness, with a surface temperature difference of one degree F.

Front 72

Kilowatt (KW)

Back 72

A unit of electric power, equal to 1,000 watts.

Front 73

KVA

Back 73

A rating for transformers equal to the product of volts and amperes divided by 1,000. The product of the KVA and the power factor gives the power in kilowatts.

Front 74

Latent Heat

Back 74

The heat added to or removed from a substance when it changes its state. See Sensible Heat.

Front 75

Light Shelf

Back 75

An overhang, either outside or inside or both, that is used with a clerestory to reflect light up onto the ceiling, and reduce direct light adjacent to the window below.

Front 76

Lumen

Back 76

A unit of light, defined as the amount of light passing through one square foot at a distance of one foot from a one candlepower source.

Front 77

Luminaire

Back 77

A complete light fixture, includ­ing lamps.

Front 78

Mass Law

Back 78

The theoretical law that states that for each doubling of mass in a wall, there is a 6 dB drop in the amount of sound transmitted.In actual practice, it is usually closer to 5 dB.

Front 79

Mean Radiant Temperature (MRT)

Back 79

The weighted average of all of the temperatures of all of the surfaces visible from a given position.

Front 80

NC Curves

Back 80

A single-number system for spec­ifying a maximum SPL level in a given location, using standardized reference contours. The curves weight the frequencies to which the human ear is sensitive.

Front 81

NEC

Back 81

The abbreviation for National Electrical Code.

Front 82

Neutral

Back 82

The wire or conductor in an electrical system that is equidistant in voltage from the phase conductors of the system. It is not the same as ground.

Front 83

Noise Reduction Coefficient (NRC)

Back 83

A one-number rating system giving the average sound absorption coefficient of a material at frequen­cies of 250, 500, 1,000 and 2,000 Hz.

Front 84

Occupancy

Back 84

The purpose for which a building is intended to be used.

Front 85

Occupancy Group

Back 85

A designation for a group of several occupancies that have comparable fire safety considerations, and which are therefore grouped together by the code.

Front 86

OSHA

Back 86

The abbreviation for the Occupational Safety and Health Act, which regulates working conditions.

Front 87

Passive Solar Design

Back 87

The practice of orient­ing and sizing a building, its windows, and its internal masses in such a way that it responds to the sun and to the climate, without the use of mechanical equipment.

Front 88

Perm

Back 88

The unit of permeability for a given material, expressing the resistance of the mate­rial to the penetration of water or water vapor through it. One perm is equal to the flow of one grain of water vapor through one square foot of surface area per hour with a pressure difference of one inch of mercury.

Front 89

Permeability

Back 89

The property of permitting pas­sage of water or water vapor through a material without causing rupture or displacement.

Front 90

Potable Water

Back 90

Water that is suitable for drinking.

Front 91

Power Factor (PF)

Back 91

In an electrical circuit, the ratio of real power in watts to the product of voltage and current.

Front 92

Power Level (PWL)

Back 92

The logarithmic expression for the acoustical power at the source of a sound. It is also known as Acoustic Power Level.

Front 93

PPM

Back 93

The abbreviation for parts per million.

Front 94

Psychrometric Chart

Back 94

A graph showing the relationships between temperature, humidity, relative humidity, and enthalpy.

Front 95

Reheat

Back 95

The adding of sensible heat to a sup­ply air stream which has been previously cooled.

Front 96

Relative Humidity (RH)

Back 96

The ratio of the moisture content of the air to the maximum pos­sible content at the same temperature.

Front 97

Reverberation

Back 97

The persistence of sound in an enclosed space after the source has stopped.

Front 98

Reverberation Time

Back 98

The time it takes a 60 dB sound to completely die away in a closed room after the source has stopped.

Front 99

Sabin

Back 99

The unit of sound absorption equivalent to the absorption of one square foot of open window.

Front 100

Sensible Heat

Back 100

Heat that changes the temper­ature of a substance, and does not represent the addition of any moisture to the substance

Front 101

Shading Coefficient (SC)

Back 101

The ratio of the solar heat gained through a window with adding devices to the solar heat gained by a single pane double strength clear glass window. Shading devices, such as Venetian blinds, lower the SC.

Front 102

Siamese Fitting

Back 102

A Y-shaped hose attachment at the base of a building, which allows the fire department to connect a fire hydrant through a pumper truck to provide or augment water flow to a standpipe.

Front 103

Smoke Developed Rating

Back 103

A numerical rat­ing derived from a standardized fire test proce­dure. Larger numbers indicate a greater density of smoke.

Front 104

Smokeproof Enclosure

Back 104

A continuous enclosed stairway separated from the building at each floor by an open vestibule that allows smoke to vent away without entering the stair.

Front 105

Soil

Back 105

A sanitary drainage term referring to the waste from urinals, water closets, and fixtures of similar function.

Front 106

Sone

Back 106

A subjective system of measuring loud-ness, based on the reference point of one sone equal to a sound pressure level of 40 dB.

Front 107

Sound Level Meter

Back 107

A meter that measures the sound pressure level and gives a reading in dB.

Front 108

Sound Pressure Level (SPL)

Back 108

The logarith­mic expression of the pressure exerted by sound waves on the receiver. The reference pressure is 2 x 1O~5 newtons per square meter.

Front 109

Sound Transmission Class (STC)

Back 109

A single-number rating for the evaluation of a particular construction cross-section in terms of its transmission of airborne sound. The higher the STC rating, the more effective the construction is at stopping airborne sound.

Front 110

Sprinkler System

Back 110

A system used to extin­guish fires automatically by releasing water or other substances. See Dry Pipe Sprinkler and Wet Pipe Sprinkler.

Front 111

Stack Vent

Back 111

The portion of a soil or waste stack that is above the highest branch drain con­nected to the stack. Its sole function is to vent to the outside air.

Front 112

Standpipe

Back 112

A vertical supply pipe for firefight-ing. Dry standpipes are empty, and must be con­nected to a fire hydrant via a Siamese connection and a pumper truck. Wet standpipes are pressur­ized and filled with water, to serve attached hoses within the building, on each floor. Wet standpipes also have Siamese connections to allow the water flow to be augmented from fire hydrants.

Front 113

Star Connection

Back 113

A method of connecting the windings on a three-phase transformer in which one end of all three windings is connected to a common neutral center point, forming a Y shape. Same as Wye Connection.

Front 114

Starter

Back 114

A device that starts the arc in a neon or fluorescent lamp, or a contactor and overload relay used in starting some electric motors.

Front 115

Steam Trap

Back 115

A valve that permits passage of air or water, but not steam, often used with steam radiators.

Front 116

Sweating

Back 116

The method of soldering copper plumbing, or the condensation of water on cold pipes or building materials.

Front 117

Thermosiphon

Back 117

The method of using a heated surface and the resulting convection to move a fluid out of a space. In solar hot water heaters, the collector is below the storage tank, and the water is circulated automatically by convection when it is heated. Thermosiphoning may also be used to ventilate a building by sending the warmed air out the top, and siphoning in cooler air at the bottom.

Front 118

Three Pipe System

Back 118

A hot and chilled water system having a common return pipe for both supply lines. Ton The amount of cooling required to create a ton of ice in a 24 hour period, equal to a steady rate of 12,000 Btuh.

Front 119

Transmission Loss (TL)

Back 119

The reduction of sound that occurs when a given wall transmits sound from one room to an adjacent room, expressed in decibels.

Front 120

Trombe Wall

Back 120

A form of mass wall that trans­fers heat by causing convection into the room behind it, as well as conduction.

Front 121

Two Pipe System

Back 121

A hot or chilled water sys­tem having only a supply and return line. It can supply only heated or chilled water to a zone, but not both simultaneously.

Front 122

U Factor or U Value

Back 122

The thermal conductiv­ity of a particular wall section, expressed in Btu's per hour per degree Fahrenheit per square foot.

Front 123

Vent Stack

Back 123

A vertical pipe that vents several sanitary drainage lines, in order to break the siphoning suction which would occur when water drops through the system.

Front 124

Watt

Back 124

The basic unit of electrical power, equal to the product of volts and amperes in direct current systems, equal to 3.41 Btuh.

Front 125

Wavelength

Back 125

The length of one complete cycle or waveform, for light or sound waves. In light, the dominant wavelength determines the per­ceived color.

Front 126

Wet Bulb Temperature (WB)

Back 126

The tempera­ture attained by a glass thermometer whose bulb is covered with a wet sock and placed in an air stream moving at 1,000 feet per minute.

Front 127

Wet Pipe Sprinkler

Back 127

A sprinkler system that is continually pressurized with water. If a fusible sprinkler opens, water is immediately forced through the sprinkler head.

Front 128

Vacuum Breaker

Back 128

An automatic valve that admits air into a supply pipe rather than allow­ing the pipe to suction or siphon polluted water back into the supply.

Front 129

Variable Air Volume (VAV)

Back 129

An air condi­tioning system that accommodates thermal load changes by varying the flow of supply air into

Front 130

Zeolite

Back 130

A substance used in water softening, in which a filtering tank is recharged by passing a salt solution through it. The tank may then be used as a filter for the free ions associated with hard water.

Front 131

Zone

Back 131

A portion of a building controlled by a single thermostat, because its spaces have sim­ilar heating or cooling needs.

Front 133

Absolute filter (H)

Back 133

A high-efficiency mechanical filter built originally for removing radioactive particles and having an efficiency of 99.9% or higher.

Front 134

Absolute humidity (H)

Back 134

Weight of water vapor per unit vol­ume of air-steam mixture. Usually expressed in grains per cubic foot.

Front 135

Absolute pressure (G, H)

Back 135

The pressure above an absolute vacuum; the sum of the pressure shown by a gage {ox gauge) and that indicated by a barometer.

Front 136

Absolute temperature (H)

Back 136

Temperature measured from absolute zero; on the Fahrenheit scale, the absolute tem­perature is approximately °F + 459.6° and on the Celsius scale °C + 273°.

Front 137

Absorption (AS)

Back 137

The ability of a material to absorb acoustical energy. Measured in sabins.

Front 138

Absorption (H)

Back 138

An assimilation during which the absorbent undergoes a physical or chemical change ior both:. Absorbers such as lithium chloride and calcium chloride are used in dehumidifying. The process is also used in absorption refrigeration.

Front 139

Absorption coefficient (AS)

Back 139

The fraction of sound energy impinging on a surface that is absorbed by that surface, usually denoted by a. Frequency sensitive.

Front 140

Absorption refrigeration (H)

Back 140

A process whereby a secondary fluid absorbs the refrigerant and, in doing so, gives up heat. Afterward, it releases the refrigerant, during which time it absorbs heat.

Front 141

Acoustics (AS)

Back 141

The science of sound, including its generation, transmission, and effects.

Front 142

Acrylonitn'le-butadiene-styrene, ABS (PS)

Back 142

A thermoplastic compound from which fittings, pipe, and tubing are made.

Front 143

Active sludge (PS)

Back 143

Sewage sediment, rich in destructive bacteria, that can be used to break down fresh sewage more quickly.

Front 144

Adaptive reuse (G)

Back 144

Converting existing buildings to new functions to avoid demolishing them; generally considered a sustainable design by avoiding the need to dispose of existing building materials and conserving the use of new materials along with the resources needed to produce and erect them.

Front 145

Adiabatic (H)

Back 145

A change at constant total heat; an action or process during which no heat is added or subtracted.

Front 146

Aeration (PS)

Back 146

An artificial method in which water and air are brought into direct contact with each other to furnish oxygen to the water and to reduce obnoxious odors.

Front 147

Aerobic (PS)

Back 147

Living or active only in the presence of free oxygen (said of certain bacteria).

Front 148

Airborne sound (AS)

Back 148

Sound that is transmitted through air by a series of oscillating pressure fluctuations.

Front 149

Air change (H)

Back 149

The quantity of infiltration or ventilation air, in cubic feet per hour or minute, divided by the volume of the room.

Front 150

Airfoil fan (AS, H)

Back 150

A fan with an airfoil-shaped blade that moves the air in the general direction of the axis about which it rotates.

Front 151

Air gap (PS)

Back 151

The unobstructed vertical separation between the lowest opening from a pipe or faucet conveying water or waste to a plumbing fixture receptor.

Front 152

Alternating current, AC (E)

Back 152

Flow of electricity that cycles or alternates direction. The number of cycles per second is referred to as the frequency.

Front 153

Ambient (AS, H, I)

Back 153

Encompassing on all sides. Thus ambient air is the surrounding air; ambient light is the lighting envi­ronment of a space; ambient noise is the surrounding sound in a space.

Front 154

Ambient lighting (I)

Back 154

Lighting throughout an area that pro­duces general illumination.

Front 155

Ambient noise (AS)

Back 155

The sound associated with a given envi­ronment, usually a composite of many sources.

Front 156

American Standard Code for Information Interchange, ASCil (DP)

Back 156

A system for referring to letters, numbers, and com­mon symbols by code numbers. The numerical value assigned for each binary digit of an eight-digit byte is, from left to right, 128, 64, 32, 16, 8, 4, 2, and 1.

Front 157

American Wire Gauge, AWG (E)

Back 157

The standard system for measuring the size of wires in the United States.

Front 158

Ampacity (E)

Back 158

The current, in amperes, that a conductor can carry continuously under conditions of use without exceed­ing its temperature rating.

Front 159

Analog (DP, TC)

Back 159

Varying continuously (e.g., sound waves). Analog signals have their frequency and bandwidth meas­ured in hertz. See Digital.

Front 160

Anechoic (AS)

Back 160

An acoustical environment free of any reflected sound; a free field.

Front 161

Angstrom (I, TC)

Back 161

A unit of length, 0.1 nm or 1CT10 m, often used to measure wavelength, but not part of the SI system of units. See Nanometer.

Front 162

Apparatus dew point (H)

Back 162

Dew point temperature of the air leaving the conditioning apparatus.

Front 163

Aspect ratio (H)

Back 163

In a duct, the depth of elbow along (paral­lel to) the axis of bend divided by the width in the plane of the bend.

Front 164

Atmospheric pressure (G, H)

Back 164

Pressure indicated by a barom­eter. The standard atmospheric pressure is 760 mm of mer­cury (29.921 in. of mercury) or 14.7 lb per square inch (in.2) at sea level.

Front 165

Attenuation (AS)

Back 165

Lessening or reduction, e.g., from 80 dB to 70 dB.

Front 166

Aural (AS)

Back 166

Pertaining to the ear or to the sense of hearing. A-weighting (AS) Prescribed frequency response defined by ANSI Standard SI.4-1971. Used to obtain a single number representing the sound pressure level of a noise in a manner approximating the response of the ear, by deemphasizing the effects of the low and high frequen­cies. See dBA.

Front 167

Axial flow fan (H)

Back 167

A fan with a disk- or airfoil-shaped blade that is mounted on a shaft and that moves the air in the general direction of the shaft axis.

Front 168

Backflow (H, PS)

Back 168

In plumbing or HVAC systems, water flow into water-distributing pipes from any source other than that for which the system is designed.

Front 169

Backflow connection (PS)

Back 169

In plumbing, any connection or arrangement which permits backflow.

Front 170

Backflow preventer (PS)

Back 170

A device to prevent backflow into a water supply line from the connection at its outlet.

Front 171

Background sound (AS)

Back 171

Noise from all sources in an envi­ronment, exclusive of a specific sound of interest.

Front 172

Backup (DP)

Back 172

A copy of a program or document that can be used if the original is destroyed.

Front 173

Ballast

Back 173

A device used with fluorescent and HID lamps to provide the necessary starting voltage and to limit the cur­rent during operation of the lamp.

Front 174

Bandwidth (DP, TC)

Back 174

The highest frequency that can be trans­mitted in an analog operation. Also (especially for digital systems), the information-carrying capacity of a system.

Front 175

Baud (DP, TC)

Back 175

Strictly speaking, the number of signal-level transitions per second in digital data. For some common coding schemes, baud equals the number of bits per second, but this is not true for more complex coding, where the term is often misused. Telecommunication specialists prefer bits per second, which is less ambiguous.

Front 176

Baud rate (DP)

Back 176

The number of bits per second. Baud rates are most commonly used as a measure of how fast data are transmitted by a modem.

Front 177

Beam angle (I)

Back 177

The angle between the two directions for which the intensity is 50 percent of the maximum inten­sity, as measured in a plane through the nominal beam centerline.

Front 178

Bell-and-spigot joint (PF)

Back 178

The joint commonly used in cast-iron pipe. Each piece is made with an enlarged diameter or bell at one end into which the plain or spigot end of another piece is inserted when the pipe is being laid. The joint is then made tight with cement, oakum, lead, or rubber caulked into the bell around the spigot.

Front 179

Bellows (H, PS)

Back 179

An expansible metal device containing a fluid that will volatilize at some desired temperature, expand the device, and open or close an opening or a switch, as in controls and steam traps.

Front 180

Bimetal (H)

Back 180

Two metals of different coefficients of expan­sion welded together so that the piece will bend in one direction when heated and in the other when cooled. Thus it can be used in opening and closing electrical circuits, as in thermostats.

Front 181

Binary numbers (DP)

Back 181

The base-2 numbering system of almost all computers, as opposed to the base-10 (decimal) numbers in common use.

Front 182

Binary system (DP)

Back 182

A numbering system using 2 as base, as opposed to 10 as base in the decimal system. For example, 1 in binary system is 1; 10 is 2; 11 is 3; 100 is 4; 101 is 5; 110 is 6; 111 is 7; 1000 is 8; 1001 is 9; and 1110 is 10. The binary system is used in almost all computers.

Front 183

Bit (DP)

Back 183

Abbreviated from "binary digit," the smallest possi­ble unit of information. A bit represents one of two things: yes or no, on or off, or, as expressed in the binary numbers used in computers, 0 or 1.

Front 184

Blackbody (H, I)

Back 184

A body that absorbs all the radiant energy falling upon it.

Front 185

Boiler (H)

Back 185

A closed vessel in which fuel is burned to generate steam. (A vessel that produces hot water should be called a hot-water heater^)

Front 186

Boiler horsepower (H)

Back 186

The power required to evaporate 34.5 lb of water at 212°F per hour, or the equivalent of 33,475 Btu per hour (Btuh).

Front 187

Booting (DP)

Back 187

The starting up of a computer by loading an operating system into it. (The name boot comes from the idea that the operating system pulls itself up by its own bootstraps.)

Front 188

Branch circuit (E)

Back 188

The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

Front 189

Branch interval (PS)

Back 189

In plumbing, a vertical length of waste pipe, usually one story high, to which the horizontal branches from that floor are connected.

Front 190

Branch vent (PS)

Back 190

A vent connecting one or more individual vents with a vent stack.

Front 191

British thermal unit, Btu (G, H)

Back 191

The amount of heat required to raise the temperature of 1 lb (0.45 kg) of water 1 degree Fahrenheit (0.565°C).

Front 192

Broadband (TC)

Back 192

In general, covering a wide range of fre­quencies. The broadband label is sometimes used for a network that carries many different services or for video transmission.

Front 193

Buffer (DP)

Back 193

An area of memory or a separate memory cache that holds information until it is needed. Buffers are used to speed up printing, redraw the screen, etc.

Front 194

Bug (DP)

Back 194

A mistake or unexpected occurrence in a piece of software (or, less commonly, in a piece of hardware).

Front 195

Building drain (PS)

Back 195

The section of the horizontal drainpipe connecting the drainage pipes inside the building with the building sewer outside the inner face of the building wall.

Front 196

Building sewer (PS)

Back 196

The pipe extending from the outer end of the building drain to the public sewer.

Front 197

Building trap (PS)

Back 197

A trap in the building drain to prevent air circulation between the building drainage system and the building sewer.

Front 198

Bulletin board (bulletin board system), BBS (DP)

Back 198

A com­puter dedicated to maintaining messages and software and making them available over phone lines at no charge. People upload (contribute) and download (gather) messages by calling the bulletin board from their own computers. BX (E) An electrical cable consisting of a flexible metallic cov­ering inside of which are two or more insulated wires for carrying electricity.

Front 199

Byte (DP)

Back 199

A group of eight bits that the computer reads as a single letter, number, or symbol. For example, 01000101 is a typical byte of the binary system.

Front 200

Calorie (H)

Back 200

In engineering, the large calorie is usually used; it is defined as one one-hundredth of the energy or heat required to raise the temperature of 1 kg of water from 0°C to 100°C. The small calorie is the heat required to raise the temperature of 1 g of water.

Front 201

Candela, cd (I)

Back 201

The unit of measurement of luminous inten­sity of a light source in a given direction.

Front 202

Candlepower, Cp (I)

Back 202

Luminous intensity of a light source in a specific direction.

Front 203

Candlepower distribution curve (I)

Back 203

A curve, generally polar, representing the variation in luminous intensity of a lamp or luminaire in a plane through the light center.

Front 204

Cathode ray tube, CRT (DP, TC)

Back 204

The display technology used on virtually all computer monitors and television sets.

Front 205

Cathodic protection (PS)

Back 205

The control of the electrolytic cor­rosion of an underground or underwater metallic structure by the application of an electric current.

Front 206

Central processing unit, CPU (DP)

Back 206

The central part of a computer. The CPU includes the circuitry built around the CPU chip and mounted on the motherboard that actually performs the computer's calculations.

Front 207

Check valve (FP, H, PS)

Back 207

A valve designed to allow a fluid to pass through in one direction only. A common type has a plate suspended so that the reverse flow aids gravity in forc­ing the plate against a seat, shutting off reverse flow.

Front 208

Circuit breaker (E)

Back 208

A device designed to open and close a circuit manually or automatically on a predetermined overcurrent.

Front 209

Circuit vent (PS)

Back 209

A branch vent from two or more traps extending from in front of the last fixture connection of a horizontal branch to the plumbing vent stack.

Front 210

Clean room (H)

Back 210

A room in which the air is highly purified, particularly with regard to dust and other particulate mat­ter. Clean-room techniques are used in the processing of delicate materials and products.

Front 211

Clock rate (or clock speed) (DP)

Back 211

The operations of a com­puter are synchronized with a quartz crystal that pulses millions of times each second. These pulses determine, for example, how often the screen redraws an image and how often the CPU accesses RAM or a hard disk. The frequency of the pulses—how often they occur—is measured in megahertz (millions of cycles per second) and is called the clock rate or clock speed.

Front 212

Close nipple (H, FP, PS)

Back 212

A nipple with twice the length of a standard pipe thread.

Front 213

Closed cycle (H)

Back 213

A system in which the fluid is used over and over without introduction of new fluid, as in a hot-water heating or mechanical refrigeration system.

Front 214

Coefficient of performance, COP (H)

Back 214

In a heat pump, the ratio of the effect produced to the electrical input in consis­tent units.

Front 215

Coefficient of utilization, CU (I)

Back 215

The ratio of the luminous flux (lumens) from a luminaire, calculated as received on the work plane, to the luminous flux emitted by the luminaire's lamps alone.

Front 216

Color-rendering index, CRI (I)

Back 216

Measure of the degree of color shift objects undergo when illuminated by a light source, as compared with a reference source, normally incandescent.

Front 217

Color temperature (I)

Back 217

The absolute temperature of a black-body radiator having a chromaticity equal to that of the light source.

Front 218

Community antenna television, CATV (TC)

Back 218

Cable television, a broadband transmission system generally using 75-ohm coaxial cable that simultaneously carries many frequency-divided TV channels.

Front 219

Compiler (DP)

Back 219

Software that implements a program by trans­lating it all at once.

Front 220

Condensate (G)

Back 220

Water that has liquefied from steam or pre­cipitated out from air.

Front 221

Condensate (H, PS)

Back 221

Liquid formed by condensation.

Front 222

Condenser (H)

Back 222

Apparatus used to liquefy a gas.

Front 223

Condensing unit (H)

Back 223

An assembly attached to one base and including a refrigerating compressor, motor, condenser, and receiver, and the necessary accessories.

Front 224

Conductance (H)

Back 224

The quantity of heat (usually BTU) transmitted per unit of time (usually 1 hour) from a unit surface of material under a unit temperature (usually 1°F) differential between the surfaces. Unit abbreviation is Btu/hr-ft2oE

Front 225

Conduction (H)

Back 225

The transmission of heat from one part of a body to another part of the same body, or from one body to another in contact with it, without appreciable displace­ment of the particles of the body.

Front 226

Conductivity (H)

Back 226

The quantity of heat (usually Btu) trans­mitted per unit of time (usually 1 hour) from a unit surface (usually 1 sq ft) to an opposite unit of surface of one mate­rial per unit of thickness (usually 1 in., but occasionally 1 ft) under a unit temperature differential (usually 1°F) between the surfaces.

Front 227

Continuous load (E)

Back 227

A load such that the maximum current is expected to continue for 3 hours or more.

Front 228

Contrast (I)

Back 228

Difference in brightness between an object and its background.

Front 229

Convection (H)

Back 229

The transfer of heat from one point to another within a fluid (such as air or water) by the mixing of one portion of the fluid with another. If the motion is due to differences in density, from temperature differences, the convection is natural; if the motion is imparted mechan­ically, it is forced convection.

Front 230

Convector (H)

Back 230

A unit containing heating elements that allow air to be heated through natural convection without using external power.

Front 231

Cooling tower (H)

Back 231

A device for cooling water by evaporation in the outside air. A natural-draft cooling tower is a cooling tower in which the airflow through it is due to the natural chimney effect; a mechanical-draft tower uses fans.

Front 232

Coprocessor (DP)

Back 232

A chip that specializes in mathematics, graphics, or some other specific kind of computation. When the CPU is handed the kind of job the coprocessor special­izes in, it hands the job off to the coprocessor.

Front 233

Counterflow (H)

Back 233

In a heat exchanger, a situation in which the fluid absorbing heat and the fluid losing heat are directed so that the lower and higher temperature of the one are adja­cent to the lower and higher temperature of the other, respectively. Ordinarily, one fluid is flowing in the opposite direction from the other, hence the term.

Front 234

Critical angle (I, TC)

Back 234

The angle at which light undergoes total internal reflection.

Front 235

Critical velocity (H)

Back 235

The point above which streamline flow becomes turbulent.

Front 236

dBA (AS) (TC)

Back 236

Overall A-weighted sound pressure level expressed in decibels referenced to 20 micropascals. An A-weighting is a single number approximating the response of human hearing.

Front 237

Dead room (AS)

Back 237

A room whose boundaries and furnishings absorb a great amount of sound.

Front 238

Debug (DP)

Back 238

To search out bugs or defects in a piece of soft­ware and eliminate them.

Front 239

Decay rate (AS)

Back 239

The rate at which the sound pressure level (in dB) decreases when the source of the sound is eliminated.

Front 240

Decibel, dB (AS, TC)

Back 240

Ten times the logarithm to the base 10 of a quantity divided by a reference quantity; dB = 10 X logl0(X/Xref).

Front 241

Degree day, DD (H)

Back 241

The number of Fahrenheit degrees that the average outdoor temperature over a 24-hour period is less than 65°F.

Front 242

Demand charge (E)

Back 242

That part of a utility service charged for on the basis of the possible demand as distinguished from the energy actually consumed.

Front 243

Demand factor, DF (E)

Back 243

The ratio of the maximum demand to the total connected load of a system.

Front 244

Demand load, DL (E, H, PF)

Back 244

The load, in appropriate units, such as kW, Btuh, or gpm, that an electrical or mechanical system encounters.

Front 245

Density (G)

Back 245

The weight of a unit of volume—usually, pounds per cubic foot.

Front 246

Developed length (H, PF)

Back 246

The length along the centerline of a pipe and its fittings.

Front 247

Dew point, DP (G, H)

Back 247

The temperature of a gas or liquid at which condensation or evaporation occurs.

Front 248

Dielectric fitting (H, PS)

Back 248

A fitting having insulating parts or material that prohibits the flow of electric current.

Front 249

Diffraction (AS)

Back 249

Alteration of the direction of propagation of a sound wave in the vicinity of a boundary discontinuity on the edge of a reflecting or absorbing surface. Frequency sensitive.

Front 250

Diffuse sound field (AS)

Back 250

A sound field in which the intensity of the sound is independent of its direction; an area over which the average rate of sound energy flow is equal in all directions.

Front 251

Diffusion (H)

Back 251

The movement of individual molecules through narrow spaces by the molecular velocity and bouncing asso­ciated with individual gaseous or liquid molecules.

Front 252

Digital (DP, TC)

Back 252

Expressed in binary code to represent infor­mation. See Analog.

Front 253

Diode (DP, TC)

Back 253

An electronic device that lets current flow in one direction. Semiconductor diodes used in fiber optics contain a junction between regions of different doping. (Doping means adding impurity elements to pure semiconductors to improve their conductivity.) Diodes include light emitters (LEDs and laser diodes) and detec­tors (photodiodes).

Front 254

Direct current, DC (E)

Back 254

Flow of electricity continuously in one direction from positive to negative.

Front 255

Direct expansion (H)

Back 255

An arrangement wherein a refrigerant expands in an evaporator in the airstream as intended.

Front 256

Direct-fired heater (H)

Back 256

A fuel-burning device in which the heat from the fuel is transferred through metal to air, which is then introduced to the space to be heated.

Front 257

Direct glare (I)

Back 257

Glare resulting from high luminances or insufficiently shielded light sources in the field of view.

Front 258

Direct sound field (AS)

Back 258

A sound field in which the energy arrives at the receiver in a direct path from the source, with­out any contribution from reflections. See Reverberant sound field.

Front 259

Directivity index, Dl (AS)

Back 259

A measure of the directionality of sound from a specific source, in decibels. The difference between the actual sound pressure level of the source and the sound pressure that would exist if the same source were a point source radiating spherically. The directivity index is sensitive to frequency.

Front 260

Discomfort glare (I)

Back 260

Glare producing discomfort. This type of glare docs not necessarily interfere with visual performance or visibility.

Front 261

Discomfort index (H)

Back 261

Original name of temperature-humidity index (THI).

Front 262

Disconnecting means (E)

Back 262

A device by which the conductors of a circuit can be disconnected from their source of supply.

Front 263

Diversity factor (E, H, PS)

Back 263

The ratio of the sum of the indi­vidual maximum loads during a period to the simultaneous maximum loads of all the same units during the same period. Always unity or more.

Front 264

Downspout (PS)

Back 264

The rain leader (pipe) from the roof to the means of disposal.

Front 265

Drainage fixture unit, DFU (PS)

Back 265

A measure of probable dis­charge into the drainage system by various types of plumbing fixtures. By convention, 1 DFU is equivalent to 1 CFM.

Front 266

Dry-bulb temperature, DB (H)

Back 266

The temperature of air as measured by a thermometer.

Front 267

Dynamic head (H)

Back 267

The pressure equivalent of the velocity of a fluid. Customary units of head are usually feet or inches ofH2O.

Front 268

Dynamic random access memory, DRAM (DP)

Back 268

A memory chip that loses its memory when the computer is shut off.

Front 269

Effluent (PS)

Back 269

Treated or partially treated sewage flowing out of sewage treatment equipment.

Front 270

Electronic cleaner (H)

Back 270

An air cleaner in which matter in the airstream is electrically charged, then attracted to surfaces oppositely charged.

Front 271

Electronic data interchange, EDI (TC)

Back 271

A series of standards that provide for the exchange of data between computers over phone lines.

Front 272

Emissivity (H, I)

Back 272

The ratio of radiant energy emitted by a body to that emitted by a perfect blackbody. A perfect blackbody has an emissivity of 1, a perfect reflector an emis­sivity of 0.

Front 273

Energy management system, EMS (H, E)

Back 273

An automated control system designed to achieve higher energy efficiency or lower energy consumption in a building. An energy man­agement system is not limited to the operation of HVAC, lighting, and other power systems.

Front 274

Enthalpy (H)

Back 274

For most engineering purposes, heat content or total heat, above some base temperature. Specific enthalpy is the ratio of the total heat to the weight of a substance.

Front 275

Entropy (H)

Back 275

The ratio of the heat added to a substance to the absolute temperature at which the heat is added. Specific entropy is the ratio of total heat to weight of the substance.

Front 276

Equipment-grounding conductor (E)

Back 276

The conductor used to connect the non-current-carrying metal parts of equip­ment, raceways, and other enclosures to the system ground. (The equipment-grounding conductor is color-coded green.)

Front 277

Equivalent length (H, FP, PS)

Back 277

The resistance to flow of a dtict or pipe elbow, valve, damper, orifice, bend, or fitting, or some other obstruction, expressed in the number of feet of straight duct or pipe of the same diameter that would have the same resistance.

Front 278

Equivalent sphere illumination, ESI (I)

Back 278

The level of spherical illumination that would produce a task visibility equivalent to that produced by a specific lighting environment.

Front 279

Ethernet (DP)

Back 279

A relatively fast local area network (LAN) cabling system developed by Xerox. Ethernet components are also sold by other vendors.

Front 280

Evaporative cooling (H)

Back 280

Cooling by the evaporation of water in air when the wet bulb is considerably lower than its dry bulb. This method is widely used in dry climates.

Front 281

Expansion joint (H, PS)

Back 281

A joint designed to absorb longitu­dinal thermal expansion in the pipeline due to heat.

Front 282

Expansion loop (H, PS)

Back 282

A bend of large radius in a pipeline designed to absorb longitudinal thermal expansion in the line due to heat.

Front 283

Face velocity (H)

Back 283

The speed, in feet per minute, by which air leaves a register or a coil.

Front 284

Fault (E)

Back 284

A short circuit—either line to line or line to ground.

Front 285

Feeder (E)

Back 285

Circuit conductors between the service equip­ment and the final branch-circuit overcurrent device.

Front 286

Feet of head (H, PS)

Back 286

Pressure loss in psi divided by the factor 0.433.

Front 287

Fiber optics, FO (DP, I, TC)

Back 287

A technology in which light is used to transmit information from one point to another. The transmitting medium is constructed of thin filaments (strands) of glass wires through which light beams are trans­mitted. For illumination purposes, the light may be any one visible spectral wavelength. For data transmission, light must be of a single wavelength to be totally reflected (refracted) within the fiber. Light sources are usually gener­ated by laser or LED.

Front 288

Field angle (I)

Back 288

The angle between the two directions for which the intensity of light is 10 percent of the maximum intensity, as measured in a plane through the nominal cen-terline of the light beam.

Front 289

Finned tube (H)

Back 289

Tube or pipe containing fins used for heat transfer between water and air, usually by natural convection.

Front 290

Fixture branch (PS)

Back 290

In plumbing, the branch from the water-distributing pipe in the building to the fixture supply pipe.

Front 291

Fixture carrier (PS)

Back 291

A metal unit designed to support an off-the-floor plumbing fixture.

Front 292

Fixture drain (PS)

Back 292

The drain from the trap of a fixture to the point where it connects with another drainpipe.

Front 293

Fixture unit, FU (PF)

Back 293

An index of the relative rate of flow of water to a fixture (water supply fixture unit, WFU) or of sewage leaving a fixture (drainage fixture unit, DFU).

Front 294

Flanking transmission (AS)

Back 294

Transmission of sound from the source to the receiver by paths around barriers or other means intended to block the transmission of sound.

Front 295

Flow rate (H, PF)

Back 295

Cubic feet per minute (CFM) of air circu­lated in an air system or pounds of water per minute circulated through a hot-water heating system.

Front 296

Flushometer valve (PS)

Back 296

A device that discharges a predeter­mined quantity of water to fixtures for flushing purposes and is actuated by direct water pressure.

Front 297

Font (DP, TC)

Back 297

A collection of letters, numbers, punctua­tion marks, and symbols with an identifiable and consis­tent look.

Front 298

Foot-candle, fc (I)

Back 298

The illuminance on a surface 1 sq ft in area on which there is a uniformly distributed flux of 1 lm. Foot-Lambert, f L (I) A unit of luminance of a perfectly dif­fusing surface emitting or reflecting light at the rate of 1 lm per square foot.

Front 299

Formatting (DP)

Back 299

All the characteristics of text other than the actual characters that make it up. Formatting includes things such as italics, boldface, type size, margins, line spacing, and justification. Also another term for initializ­ing a disk.

Front 300

Free field (AS)

Back 300

A field free from boundaries that would otherwise tend to reflect sound. Frequency (AS, E) Number of complete oscillation cycles per unit of time. A unit of frequency often used is the hertz.

Front 301

Furnace (H)

Back 301

Either the combustion space in a fuel-burning device or a direct-fired air heater. In the latter case, not to be confused with a boiler.

Front 302

Fuse (E)

Back 302

An overcurrent protective device with a circuit-opening fusible part that is heated and severed by the pas­sage of overcurrent through it.

Front 303

Galvanizing (PS)

Back 303

A process whereby a surface of iron or steel is covered with a layer of zinc.

Front 304

Gigabyte, (DP)

Back 304

A measure of computer memory, disk space, and the like, equal to 1024 megabytes (1,073,741,824 bytes), or about 179 million words. Sometimes a gigabyte is treated as an even billion bytes, but plainly, that is almost 74 million bytes short. Sometimes abbreviated gig (more often in speech than in writing).

Front 305

Glare (I)

Back 305

The sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which the eyes are adapted. Glare may cause annoyance, discomfort, or loss of visual performance and visibility.

Front 306

Ground (E)

Back 306

A conducting connection, whether intentional or accidental, between electrical circuit or equipment and the earth.

Front 307

Ground fault (circuit) interrupter, GFI or GFCI

Back 307

(E) A device that senses ground faults and reacts by opening the circuit.

Front 308

Grounded conductor (E)

Back 308

A system or circuit conductor that is intentionally grounded.

Front 309

Grounding conductor (E)

Back 309

A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Front 310

Halon (FP)

Back 310

A bromtrifluoromethane gas that is effective in extinguishing fires.

Front 311

Handshake (DP)

Back 311

What computers do when communicating, in order to establish a connection and agree on protocols for the transmission of data.

Front 312

Head end (TC)

Back 312

The central facility where signals are com­bined and distributed in an airborne sound (AS) system.

Front 313

Heat pump (H)

Back 313

An all-electric heating/cooling device that takes energy for heating from outdoor air (or groundwater).

Front 314

Hertz, Hz (AS, DP, E)

Back 314

A measure of frequency (cycles per second).

Front 315

High-density (or high-resolution) television, HDTV (DP)

Back 315

Television with about double the resolution of present systems.

Front 316

High-intensity discharge (HID) lamp (I)

Back 316

A lamp whose light source is mercury, metal halide, or high-pressure sodium.

Front 317

High-pressure sodium (HPS) lamp

Back 317

HID lamp in which light is produced by radiation from sodium vapor.

Front 318

Home run (E)

Back 318

The wiring run between the panel and the first outlet in the branch circuit. (Looking upstream, the wiring run between the last outlet and the panel.)

Front 319

Horsepower (E, H, PF)

Back 319

A unit of power that equals 746 W, or 1 hp = 3/4 kW.

Front 320

Humidity (H)

Back 320

Usually, water vapor mixed with dry air. Absolute humidity is the weight of water (or steam) per unit volume of the air-water mixture.

Front 321

Hydrogen ion concentration, pH (H)

Back 321

The logarithm of the reciprocal of the gram ionic hydrogen equivalents per litei. On the pH scale 7.0 is neutral; smaller values to 0 are acid, progressively so; and higher values to 14 are alkaline, pi°~ gressively so.

Front 322

Icon (DP)

Back 322

A graphic computer symbol usually representing a file, folder, disk, or tool.

Front 323

Illuminance

Back 323

The density of the luminous flux incident on a surface; the quotient of the luminous flux divided by the area of the surface when the latter is uniformly illuminated. illuminance (lux or foot-candle) meter (I) An instrument formeasuring illuminance on a plane. Instruments that respond accurately to more than one spectral distribution are color-corrected.

Front 324

Incandescent lamp (I)

Back 324

A lamp in which light is produced by a filament heated to incandescence by an electric current.

Front 325

Index of refraction (I, TC)

Back 325

The ratio of the speed of light in a vacuum to the speed of light in a material, usually abbreviated n.

Front 326

indirect lighting (I)

Back 326

Lighting by luminaires distributing 90 to 100 percent of the emitted light upward.

Front 327

Input/output, I/O (DP)

Back 327

Signal fed into and transmitted out of a circuit.

Front 328

Integrated circuit, IC (DP, TC)

Back 328

An electronic device that contains hundreds or thousands of separate components, such as transistors, resistors, and switches. Encapsulated in a plastic enclosure, it is also called a chip.

Front 329

Integrated services digital network, ISDN (TC)

Back 329

A digital standard calling for 144-kbit/sec transmission, corre­sponding to two 64-kbit/sec digital voice channels and one 16-kbit/sec data channel.

Front 330

Intensity, or I (AS)

Back 330

The average rate of sound energy flow per unit of area in a direction perpendicular to the area, usually in W/m2.

Front 331

Intensity, or I (I)

Back 331

The luminous flux per unit solid angle, expressed in lumens per steradian (lm/sr) or candela (cd).

Front 332

Intensity level, IL (AS)

Back 332

Ten times the logarithm of the ratio of the sound intensity to a reference sound intensity (Iref) of 10~12 W/m2 (9.29 X 10~14 W/ft2).

Front 333

International Organization for Standardization, ISO (G)

Back 333

An international organization that establishes standards on scientific and technology quantities.

Front 334

Inverse square law (I)

Back 334

The law stating that the illuminance at a point on a surface varies directly with the intensity of a point source and inversely as the square of the distance between the source and the point.

Front 335

Invert (PS)

Back 335

Lowest point on the interior of a horizontal pipe.

Front 336

Isolux chart (I)

Back 336

A series of lines, plotted on any appropriate set of coordinates, each of which connects all the points on a surface having the same illumination.

Front 337

Kilobyte, K (DP)

Back 337

A measure of computer memory, disk space, and the like, equal to 1024 characters, or about 170 words.

Front 338

Laminar flow (H)

Back 338

Flow occurring in fluid layers, in a conduit or space, with no eddy currents. Each particle of fluid flows directly from one end to the other or from top to bottom, or in some planned manner.

Front 339

Lamp efficacy (I)

Back 339

The ratio of lumens produced by a lamp to the watts consumed. Expressed as lumens per watt (lpw).

Front 340

Lamp lumen depreciation, LLD (I)

Back 340

Multiplicative factor in calculations of illumination for reduction in the light output of a lamp over a period of time.

Front 341

LASER (TC)

Back 341

An acronym for light amplification by stimu­lated emission of radiation. Laser light is directional, covers a narrow range of wavelengths, and is more coherent than ordinary light. Semiconductor diodes are the standard light sources in fiber-optic systems.

Front 342

Laser diode (DP, TC)

Back 342

A semiconductor diode in which the injection of current carriers produces laser light by amplify­ing photons generated when holes and electrons recombine at the junction between p- and »-doped regions.

Front 343

Latent heat (H)

Back 343

Inherent heat in the. form of fluid without a phase change.

Front 344

Life-cycle cost analysis (G)

Back 344

Accounting of cash flows for installation, debt retirement, maintenance, utilities, and salvage value of a system or component over the life of the element being analyzed; generally done to evaluate alterna­tive options.

Front 345

Light-emitting diode, LED (AX, DP, TC)

Back 345

Small lamp illumi­nated by the movement of electrons in a semiconductor.

Front 346

Light loss factor, LLF (I)

Back 346

A factor used in calculating illumi­nance after a given period and under given conditions. (Formerly called maintenance factor.)

Front 347

Liquid crystal display, LCD (DP, TC)

Back 347

A product that uses liq­uid crystals sealed in glass. The pixels contain individual transistors to generate alphanumeric and graphic images.

Front 348

Live room (AS)

Back 348

A room that has very little sound absorption from the boundaries and furnishings. Opposite of dead room.

Front 349

Local area network, LAN (DP)

Back 349

A network of computers and related devices that transmits data among many nodes in a relatively small area, such as one office or one building. See Wide area network.

Front 350

Loop vent (PS)

Back 350

A branch vent serving more than one fixture and looping back to connect with a stack vent instead of the vent stack.

Front 351

Loudness (AS)

Back 351

A subjective description of the level of sound. Typically, a 10-dB increase in sound pressure level is judged to be twice as loud as the level before the increase.

Front 352

Low-pressure sodium (LPS) lamp, (I)

Back 352

A discharge lamp in which a single wavelength of visible yellow light is produced by radiation of sodium vapor at a low pressure.

Front 353

LP gas (H)

Back 353

Liquefied petroleum gas, a gas of high heating content, stored under high pressure in liquid form. Delivered to consumers in containers where piped gas is not available, and used by gas utilities for peaking (when demand exceeds source of natural gas).

Front 354

Lumen, lm (I)

Back 354

The unit of luminous flux; the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela (1 cd).

Front 355

Luminaire (I)

Back 355

A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply.

Front 356

Luminaire dirt depreciation, LDD (I)

Back 356

Multiplicative factor used in calculations of illuminance for reduced illuminance due to dirt collecting on the luminaires.

Front 357

Luminaire efficiency (i)

Back 357

The ratio of the luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.

Front 358

Luminance (I)

Back 358

The amount of light reflected or transmitted by an object.

Front 359

Lux, Ik (I)

Back 359

The metric unit of illuminance. One lux (1 lx) is one lumen per square meter (1 lm/m2).

Front 360

Macro (DP)

Back 360

A command that incorporates two or more other commands or actions. (The name comes from the idea that macro commands incorporate "micro" commands.)

Front 361

Maintenance factor, MF (I)

Back 361

A factor used in calculating illu­minance after a given period and under given conditions.

Front 362

Makeup air (H)

Back 362

Air brought into a building from the outside to replace that exhausted by a ventilating system.

Front 363

Makeup water (H, PS)

Back 363

Water supplied to replenish that lost by leaks, evaporation, etc.

Front 364

Malleable iron (PS)

Back 364

Cast iron heat-treated to reduce its brit-tleness. The process enables the material to stretch to some extent and to withstand greater shock.

Front 365

Masking (AS)

Back 365

The rendering undetectable of one sound of interest by other sounds.

Front 366

Mbh (H)

Back 366

Thousands of British thermal units (Btu) per hour (h).

Front 367

MCM (E)

Back 367

Thousand circular mil; used to describe large wire sizes.

Front 368

Meg (DP)

Back 368

An abbreviation for megabyte.

Front 369

Megabyte, MB (DP)

Back 369

A measure of computer memory, disk space, and the like, equal to 1024K (1,048,576 bytes). Sometimes people try to equate a megabyte with an even million characters.

Front 370

Megahertz, MHz (DP, TC)

Back 370

A million cycles, occurrences, alterations, or pulses per second. Used to describe the speed of computers' clock rates.

Front 371

Memory (DP)

Back 371

The retention of information electronically, on chips. There are two main types of memory: RAM, which is used for the short-term retention of information (that is, until the power is turned off), and ROM, which is used to store programs that are seldom, if ever, changed.

Front 372

Menu (DP)

Back 372

A list of commands to operate a computer. There are many types of menus: pop-up menus, submenus, etc.

Front 373

Mercury lamp (I)

Back 373

A high-intensity discharge (HID) lamp in which the major portion of the light is produced by radia­tion from mercury.

Front 374

Metal halide (MH) lamp, (I)

Back 374

A high-intensity discharge (HID) lamp in which the major portion of the light is pro­duced by radiation of metal halides and their products of dissociation, possibly in combination with metallic vapors such as mercury.

Front 375

Modulator-demodulator, or modem (DP)

Back 375

A device that lets computers talk to each other over phone lines. Modems are used to send digital signals through telephone lines, to be converted back to analog signals at the receiving end.

Front 376

Mollier chart (H)

Back 376

A form of graph, named for its inventor, covering heat properties of a fluid and having entropy as one coordinate and enthalpy as the other.

Front 377

Multiplex (DP, TC)

Back 377

To transmit two or more signals over a single channel.

Front 378

Multiuser (DP)

Back 378

Said of software or hardware that supports use by more than one person at one time.

Front 379

Nadir (I)

Back 379

Vertically downward directly below the luminaire or lamp; designated as 0°.

Front 380

Nanometer (AX, G, I, TC)

Back 380

A unit of length, 10"~9 m. It is part of the SI system and has largely replaced the non-SI unit angstrom (0.1 nm) in technical literature.

Front 381

Nanosecond (DP, G, I, TC)

Back 381

A billionth of a second. Used to measure the speed of memory chips, among other things. Abbreviated ns.

Front 382

National Television Standard, NTS (TC)

Back 382

The analog video broadcast standard used in North America and set by the National Television Standards Committee.

Front 383

National Television Standards Committee, NTSC (TC)

Back 383

Committee that sets television transmission standards in North America at 30 frames per second and 525 horizontal lines per frame.

Front 384

Near field (AS)

Back 384

The field in the vicinity of a real sound source wherein the sound level contours are different from those of a point source in a free field.

Front 385

Needle valve (H)

Back 385

A valve provided with a long tapering point in place of the ordinary valve disk. The tapering point permits fine gradation of the opening.

Front 386

Net present value (G)

Back 386

The theoretical inception cost or savings due to operation of an element or system including initial investment and cash flow from operations, generally discount­ing future cash flows to the present, thereby accounting for interest, inflation, and/or foregone profit.

Front 387

Network (DP, E)

Back 387

Two or more computers (or other computer-related devices) connected to share informa­tion. Usually, the term refers to a local area network.

Front 388

Network operating system, NOS (DP)

Back 388

A software program for a computer network that runs in a file server and con­trols access to files and other resources from multiple users.

Front 389

Nipple (H, PS, FP)

Back 389

A tubular pipe fitting usually threaded on both ends and under 12 in. long. Pipe over 12 in. long is regarded as cut pipe.

Front 390

Noise isolation class, NIS (AS)

Back 390

A single-number rating derived in ,the same manner as STC but based on NR (noise reduc­tion) rather than TL (sound transmission loss). It includes the acoustical absorption in the receiving room.

Front 391

Noise reduction (AS)

Back 391

The difference in decibels between the sound in one space and the sound in a second space atten­uated by some intervening medium; e.g., from one room to another.

Front 392

Noise reduction coefficient, NRC (AS)

Back 392

The average of the sound absorption coefficients in the 250-, 500-, 1000-, and 2000-Hz octave bands.

Front 393

Octave (AS, AV)

Back 393

An interval between two frequencies having a ratio of two to one (2:1).

Front 394

Octave band (AS, AV)

Back 394

A frequency band whose upper limit is twice the lower limit.

Front 395

Ohm's law (E)

Back 395

The relationship between current and volt­age in a circuit. The law states that current is proportional to voltage and inversely proportional to resistance. Algebraically, in DC circuits, / = V /R; in AC circuits, /= V/Z.

Front 396

One-pipe hot-water system (H)

Back 396

System that carries heated and cooled water to and from the radiators in the same "main" and in the same direction, with the hot water on top. This system is rare in modern-day applications.

Front 397

One-pipe steam system (H)

Back 397

A steam heating system in which steam supplied to a radiator travels in the same pipe but in opposite directions as the cold water or condensate returns from the radiator. This system is rare in modern-day applications.

Front 398

Open protocol (DP, TC)

Back 398

A set of standard procedures that are agreed upon by all manufacturers so that data can be trans­mitted without obstructions.

Front 399

Optic fiber cable, or OFC (DP, I, TC)

Back 399

An information trans­mission medium consisting of a core of glass or plastic surrounded by a refractive cladding and a protective jacket. It transmits stranded and ribbon configurations. See Fiber optics.

Front 400

Orifice (H, PS)

Back 400

An opening. The term is commonly applied to disks placed in pipelines or radiator valves to reduce the flow of a fluid to a desired amount.

Front 401

Orsat (H)

Back 401

A device for measuring the percentage of carbon dioxide, oxygen, and carbon monoxide in flue gases.

Front 402

Overcurrent (E)

Back 402

Any current in excess of the rated current of equipment or the ampacity of a conductor.

Front 403

Overcurrent device (E)

Back 403

A device, such as a fuse or a circuit breaker, designed to protect a circuit against excessive cur­rent by opening the circuit.

Front 404

Panel or panelboard (E)

Back 404

A box containing a group of over-current devices intended to supply branch circuits.

Front 405

Parabolic reflector lamp, PAR (I)

Back 405

A lamp with internal reflector having the contour of a parabola to achieve beam control.

Front 406

Partial pressure (H)

Back 406

That part of the total pressure of a mixture of gases contributed by one of the constituents.

Front 407

Pascal, Pa (AS)

Back 407

Measure of pressure in the SI system ( = 1.45 X 10~4psi).

Front 408

Payback period (G)

Back 408

Elapsed time after installation of an ele­ment or system for savings in utilities and maintenance to equal the additional cost of its installation compared with another competing option.

Front 409

Percentage humidity (H)

Back 409

Ratio of weight of vapor in a mix­ture of air and water vapor to the weight of vapor when the vapor is saturated at the same temperature. Contrast with relative humidity.

Front 410

Performance interaction (G)

Back 410

The effects that one or mores ystems or subsystems have on other systems or subsystems; e.g., better insulation reduces HVAC loads and electrical system capacity to support the HVAC.

Front 411

Phase alternate line, PAL (TC)

Back 411

A television transmission standard vtsed mostly in Europe; 25 frames per second and 625 lines per frame.

Front 412

Picture element, or pixel (DP, TC)

Back 412

Any of the little dots of light that make up die picture on a computer (or TV) screen. The more pixels there are in a given area—that is, the smaller and closer together they are—the higher is die resolution of the screen. Sometimes pixels are simply called dots. The number of pixels on a screen is usually expressed by the number of hor­izontal and vertical rows, such as 640 X 480 or 1800 X 1280.

Front 413

Pink noise (AS)

Back 413

Sound that has equal energy in each octaveor suboctave band.

Front 414

Pitch (AS)

Back 414

A subjective description of sound quality referring to the principal frequency content. The sound pressure level of a tone.

Front 415

Point source (AS, I)

Back 415

A source of essentially zero dimensions that radiates sound or light uniformly in all directions.

Front 416

Polarization (I)

Back 416

The process by which the transverse vibra­tions of light waves are oriented in a specific plane.

Front 417

Polyvinyl chloride, PVC (P)

Back 417

An inert plastic material com­monly used for pipes. It has high resistance to corrosion.

Front 418

Pounds per square inch pressure, psi (H, PF)

Back 418

A unit measure of pressure, or force per unit area.

Front 419

Power factor, PF (I)

Back 419

The ratio of the working power to the apparent power of an AC circuit.

Front 420

Presbycusis (AS)

Back 420

A condition of deterioration of hearing acuity due primarily to aging.

Front 421

Private branch exchange, PBX (AX, TC)

Back 421

A private telephone system that usually interconnects with public telephone sys­tems to serve a business or an organization. It can also pro­vide access to a computer from a data terminal. PBX and PABX are used interchangeably.

Front 422

Programmable ROM, PROM (DP)

Back 422

A read-only memory chip that can be changed with a special device.

Front 423

Propeller fan (H)

Back 423

A fan with airfoil blades that move the air in the general direction of the axis of the fan.

Front 424

Protocol (DP, TC)

Back 424

A set of standard procedures that control how information is transmitted between computer systems.

Front 425

Psychrometer (H)

Back 425

A device employing a wet-bulb and a dry-bulb thermometer to measure the humidity in the air.

Front 426

Psychrometric chart (H)

Back 426

A graph used in air conditioning and showing the properties of airsteam mixtures. Pure tone (AS) A sound in which the instantaneous sound pressure wave is a pure sinusoid.

Front 427

Raceway (E)

Back 427

An enclosed channel of metal or nonmetal designed for holding wires or cables.

Front 428

Radiation (H)

Back 428

The transfer of energy in wave form from a hot body to a (relatively) cold body, independently of matter between the two bodies.

Front 429

Radio frequency, RF (TC)

Back 429

Electromagnetic waves operating between 5 kHz and the MHz range. Although off-air (over-the-air) and satellite TV signals operate far above these frequencies (54 MHz and up to many GHz), these TV signals are frequently incorrectly referred to as RF sig­nals in commercial applications.

Front 430

Rapid-start (RS) fluorescent lamp

Back 430

A fluorescent lamp designed for operation with a ballast that starts the lamp light output within a split second.

Front 431

Reflectance (I)

Back 431

The ratio of the reflected light to the incident light falling on a surface.

Front 432

Reflected glare (I)

Back 432

Glare resulting from specular reflections of high luminances in polished or glossy surfaces in the field of view.

Front 433

Reflector (R) lamp (I)

Back 433

A lamp with an internal reflector to redirect the light output to a controlled direction. Refraction (I, TC) The process by which the direction of a ray of light changes as the ray passes obliquely from one medium to another in which its speed is different.

Front 434

Refrigerant (H)

Back 434

A substance that absorbs heat while vaporiz­ing and whose boiling point and other properties make it useful as a medium for refrigeration.

Front 435

Regenerative heating or cooling (H)

Back 435

An arrangement whereby heat rejected in one portion of a cycle is utilized in another portion of the cycle.

Front 436

Relative humidity, RH (H)

Back 436

The ratio of the water vapor (by weight) in air to the water vapor saturated in air at the same pressure and temperature.

Front 437

Relief vent (PS)

Back 437

A vent designed to provide circulation of air between drainage and vent systems or to act as an auxiliary vent.

Front 438

Residual pressure (FP)

Back 438

Pressure remaining in a system while water is being discharged from outlets.

Front 439

Resonance (AS, 6)

Back 439

A state in which the forces of oscillation of a system occur at or near a natural frequency of the system.

Front 440

Reverberant sound field (AS)

Back 440

Sound that is reflected from the boundaries of and furnishings within an enclosed space. Excludes direct sound. (See Direct sound field.)

Front 441

Reverberation (AS)

Back 441

The persistence of sound in an enclosed space as a result of repeated reflection or scattering of the sound.

Front 442

Reverberation time (AS)

Back 442

The time required for the sound pressure level in a reverberant sound field to decay 60 dB after the source has been extinguished.

Front 443

Riser diagram (E)

Back 443

Electrical block-type diagram showing the connection of major items of equipment and components.

Front 444

Romex (E)

Back 444

One of several trade names for NEC-type non-metallic sheathed flexible cable.

Front 445

Romex (NM) cable (E)

Back 445

A cable composed of flexible plastic sheathing inside of which are two or more insulated wires for carrying electricity.

Front 446

Room cavity ratio, RCR (I)

Back 446

A number indicating the propor­tions of a room cavity, calculated from the length, width, and height of the room.

Front 447

R-value (H)

Back 447

Resistance rating of thermal insulation.

Front 448

Sabin (AS)

Back 448

Unit of measure of acoustical absorption. Named after Wallace Clement Sabin, U.S. physicist.

Front 449

Saturated air (H)

Back 449

Air containing saturated water vapor, with both the air and the vapor at the same dry-bulb temperature.

Front 450

Saturated pressure (H)

Back 450

That pressure, for a given tempera­ture, at which the vapor and the liquid phases of a substance can exist in stable equilibrium.

Front 451

Saturated steam (H)

Back 451

Steam at the boiling temperature cor­responding to the pressure at which it exists. Dry saturated steam does not contain water particles in suspension. Wet saturated steam does.

Front 452

Seasonal energy efficiency ratio, SEER (H)

Back 452

Measure of the efficiency of HVAC equipment on a seasonal, rather than a design-load, basis.

Front 453

Self-contained cooling unit (H)

Back 453

A combination of apparatus for room cooling complete in one package. Usually consists of compressor, evaporator, condenser, fan motor, and air filter. Requires connection to electric line.

Front 454

Semiconductor (DP, TC)

Back 454

A material that has an electrical resistance somewhere between that of a conductor (e.g., metal) and that of an insulator (e.g., plastic). Silicon and germanium are the two most commonly used semiconduc­tors. The flow of electric current in a semiconductor can be changed by light or by electric or magnetic fields.

Front 455

Sensible cooling effect (H)

Back 455

The difference between the total cooling effect and the dehumidifying effect.

Front 456

Sensible heat (H)

Back 456

Heat that raises the air temperature.

Front 457

Septic tank (PS)

Back 457

A tank designed to separate solid waste from liquid waste.

Front 458

Service conductors (6)

Back 458

The supply conductors that extend from the street main or from transformers to the service equipment of the premises supplied.

Front 459

Simplex (TC)

Back 459

Single element (e.g., a simplex connector is a single-fiber connector).

Front 460

Sol-air temperature (H)

Back 460

That temperature of the outdoor air that, in contact with a completely shaded building surface, would give the same rate of heat entry into the outdoor surface of the outdoor air, the actual intensity of solar and sky radiation incident upon that surface, and the actual wind velocity.

Front 461

Sound power (AS)

Back 461

The rate at which acoustic energy is radi­ated. Usually measured in watts.

Front 462

Sound power level, PWL (AS)

Back 462

Ten times the logarithm of the ratio of sound power, in watts, to the reference level

Front 463

Sound pressure (AS)

Back 463

The fluctuations of pressure about atmos­pheric pressure. Usually measured in micropascals (uPa).

Front 464

Sound pressure level, SPL (AS)

Back 464

Ten times the logarithm of the ratio of the mean square pressure to the square of a reference pressure (Prcf) of 20 uPa.

Front 465

Sound transmission class, STC (AS)

Back 465

A single-number rating system designed to provide a cursory estimate of the sound-insulating properties of a wall or partition.

Front 466

Sound transmission loss, STL (AS)

Back 466

The difference in deci­bels of the sound pressure level on the receiver side of a partition or barrier from that on the source side, with the receiver side being free-field conditions.

Front 467

Specific heat (H)

Back 467

The heat absorbed by a unit weight of a substance per unit temperature rise of the substance.

Front 468

Specific humidity (H)

Back 468

Weight of water vapor, in grains or pounds, per pound of dry air.

Front 469

Specific volume (H)

Back 469

The volume occupied by a unit of air. Usually given in cubic feet per pound.

Front 470

Specular reflection (I)

Back 470

Mirrorlike reflection.

Front 471

Split system (H)

Back 471

Historically, a combination of warm-air heating and radiator heating; the term is also used for other combinations such as hot water-steam and steam-warm air. It is also applicable to a DX cooling system when the evap­orator (cooling) section is separated from the condensing (heat rejection) section.

Front 472

Stack (PS)

Back 472

The vertical main of a system of soil, waste, or vent piping extending through one or more stories.

Front 473

Stack vent (PS)

Back 473

The extension of a soil waste stack above the highest horizontal drain connected to the stack.

Front 474

Standard air (H)

Back 474

Air at 70°F and 29.921 in. of mercury and weighing 0.07488 lb per cu ft.

Front 475

Standing wave (AS)

Back 475

A sound wave that has a fixed distribu­tion in space, the result of progressive waves of identical frequencies. It is characterized by the existence of nodes or partial nodes and antinodes at fixed points in space.

Front 476

Static pressure (H)

Back 476

The pressure exerted by a fluid in all directions; the pressure which would tend to burst the con­tainer; the pressure exerted by the fluid if stationary.

Front 477

Steam trap (H)

Back 477

An apparatus for allowing water or air to pass but preventing passage of steam.

Front 478

Storm sewer (PS)

Back 478

A sewer used for conveying rain or surface or subsurface water.

Front 479

Streamline flow (H)

Back 479

Also termed viscous or laminar flow. In a streamline flow, the fluid particles are moving in a straight line parallel to the axis of the pipe or duct. With streamline flow, the friction varies directly as the velocity.

Front 480

Structure-borne sound (AS)

Back 480

Sound transmitted through solid material by means of vibration waves in the material.

Front 481

Subcooling (H)

Back 481

Cooling of a liquid refrigerant below the condensing temperature at constant pressure.

Front 482

Subsoil drain (PS)

Back 482

A drain that receives only subsurface or seepage water and conveys it to an approved place of disposal.

Front 483

Sump pump (PS)

Back 483

A mechanical device for removing liquid waste from a sump.

Front 484

Superheated steam (H)

Back 484

Steam at a temperature higher than the boiling temperature corresponding to the pressure at which it exists.

Front 485

Supply fixture unit, SFU (PS)

Back 485

A measure of the probable hydraulic demand on the water supply by various types of plumbing fixtures.

Front 486

Surcharge (PS)

Back 486

Rising of water levels above sewer inlets owing to instantaneous loads in excess of flow capacity, gen­erally a temporary phenomenon.

Front 487

Sustainability (G)

Back 487

Quality of an activity, policy, design, or design element to enhance functionality, improve habitability, conserve resources, and reduce harm to the environment.

Front 488

Swing joint (H, PS)

Back 488

An arrangement of screwed fittings and pipe to provide for expansion in pipelines.

Front 489

Switchboard (E)

Back 489

A large panel containing switches, overcurrent devices, buses, and, usually, instruments.

Front 490

Task cooling (H)

Back 490

Provision for local comfort conditioning, generally in a small area, where requirements are different from those of surrounding spaces.

Front 491

Task lighting (I)

Back 491

Provision for local illumination in appropri­ate areas where requirements are different from those of surrounding spaces.

Front 492

Tee (H, FP, PS)

Back 492

A pipe fitting that has one side outlet at right angles to the run.

Front 493

Therm (H)

Back 493

A unit of heat equal to one hundred thousand (100,000) Btu.

Front 494

Thermocouple (H)

Back 494

Two dissimilar metals joined together to produce an electromotive force that varies with the temper­ature. Used to measure temperature.

Front 495

Timbre (AS)

Back 495

A characteristic of sound whereby the listener is capable of distinguishing between two sounds even though they are of equal level and pitch.

Front 496

Tone (AS)

Back 496

A sound having a pitch and capable of causing auditory sensation.

Front 497

Tons of refrigeration (H)

Back 497

A common commercial measure of refrigeration capacity, especially of the cooling capacity of air conditioning apparatus, the equivalent of the heat required to melt one ton (2000 lb) of ice (heat of fusion is 144 Btu per pound) in 24 hours, hence, 288,000 Btu per day or 12,000 Btu per hour.

Front 498

Total heat (H)

Back 498

The sum of sensible heat and latent heat in a substance or fluid above a base point, usually 32°F (0°C).

Front 499

Total internal reflection (TC)

Back 499

Total reflection of light back into a material when the light strikes the interface with another material having a lower refractive index at an angle below a critical value.

Front 500

Total pressure (H)

Back 500

The sum of the static pressure and velocity pressure of a fluid.

Front 501

Transformer (E)

Back 501

A device used to raise or lower electrical voltage by means of an electromagnetic core and windings.

Front 502

Trap (PS)

Back 502

A fitting or device designed to provide a liquid seal that will prevent a fluid from passing back to where it came from.

Front 503

Tubeaxial fan (H)

Back 503

An airfoil (propeller) or disc fan within a cylinder and including driving mechanism supports either for belt drive or direct connection.

Front 504

Tungsten-halogen lamp

Back 504

A gas-filled tungsten incandes­cent lamp containing a certain proportion of halogens.

Front 505

Turbulent flow (H)

Back 505

Flow in which the fluid particles are mov­ing in directions other than a straight line parallel to the axis of the pipe or duct.

Front 506

Twisted pair, or TP (E, DP, TC)

Back 506

A type of wire made of two insulated copper conductors twisted around each other to reduce induction (and thus interference) from one conduc­tor to the other. The twist, or lay, varies in length to reduce the potential for interference from signals between pairs in a multipair cable. The lay usually varies between 2 and 12 in. Closer lay provides better attenuation between the con­ductors. TP cables are classified as unshielded (UTP) and shielded (STP). The latter includes a metal sheath sur­rounding the pairs within the protective jacket.

Front 507

U coefficient (H)

Back 507

Rate of heat transmission in Btuh/sq ft-°F.

Front 508

Ultrasonic (AS)

Back 508

Sound above the audible range.

Front 509

Uninterruptible power supply, UPS (DP, E)

Back 509

A power supply or system that provides a steady source of electrical power even when the normal (utility) power supply is interrupted. The system usually contains a storage battery floating on line with an inverter to convert battery power from DC to AC. Other UPSs may utilize a DC motor-driven AC gen­erator set.

Front 510

Vacuum breaker (PS)

Back 510

Check valve open to the atmosphere when the pressure in piping drops to atmospheric pressure.

Front 511

Vaneaxial fan (H)

Back 511

An airfoil (propeller) or disk fan within a cylinder, equipped with air guide vanes either before or after the wheel and including driving mechanism supports for either belt drive or direct connection.

Front 512

Vapor barrier (H)

Back 512

A material intended to prevent the passage of water vapor through a building wall or a pipe so as to pre­vent condensation within the wall or within the insulating material.

Front 513

Vapor pressure (H)

Back 513

The equilibrium pressure of the vapor of a liquid in contact with the liquid.

Front 514

Veiling reflections (I)

Back 514

Regular reflections superimposed on diffuse reflections from an object that partially or totally obscures the details to be seen by reducing the contrast. Veiling reflections are sometimes called reflected glare.

Front 515

Velocity pressure (H)

Back 515

The pressure exerted by a moving fluid in the direction of its motion. It is the difference between the total pressure and the static pressure.

Front 516

Ventilation (H)

Back 516

The art or process of supplying outside (so-called fresh) air to, or removing air from, an enclosure.

Front 517

Venturi (H)

Back 517

A contraction in a pipeline or duct to accelerate the fluid and lower its static pressure. Used for metering and other purposes.

Front 518

Viscosity (H)

Back 518

The property of a fluid by which it resists an instantaneous change of shape or arrangement of its mole­cules. Broadly, resistance to flow.

Front 519

Visual comfort probability, VCP (I)

Back 519

The rating of a lighting system, expressed as a percent of people who, when view­ing from a specified location and in a specified direction, will be expected to find the system acceptable in terms of discomfort glare.

Front 520

Voltage, V (E)

Back 520

The electric pressure in an electric circuit, expressed in volts.

Front 521

Voltage drop, VD (E)

Back 521

The diminution of voltage around a cir­cuit, including the wiring and loads. Must equal the supply voltage.

Front 522

Voltage to ground (E)

Back 522

For grounded circuits, the voltage between the ungrounded conductor and the ground; for ungrounded circuits, the greatest voltage between the given conductors.

Front 523

Warming-up allowance (H)

Back 523

An addition to the capacity of heating system (as calculated for heat loss) to provide for quick pickup in the morning.

Front 524

Water hammer (PF)

Back 524

Banging of pipes caused by the shock of closing faucets or other flow-control devices.

Front 525

Water hammer arrester (PS)

Back 525

A device, other than an air chamber, designed to provide protection against excessive surge pressure.

Front 526

Wavelength (electromagnetic) (D, I, TC)

Back 526

The distance between nodes of an electromagnetic wave, such as radio, TV, light, or radar; given by L (length, m) = 3 X 10'//, where/is the frequency, in hertz.

Front 527

Wavelength (sound) (AS)

Back 527

The distance between nodes in a sound wave; given by L (length, ft) = 1130// where/is the frequency, in hertz.

Front 528

Wet-bulb temperature, WB (H)

Back 528

The temperature of the an­as measured by a wet-bulb thermometer. Except when the air is saturated, the wet-bulb temperature is lower than the dry-bulb temperature in inverse proportion to humidity.

Front 529

White noise (AS)

Back 529

Sound that has equal energy in every pass band of equal bandwidth.

Front 530

Zonal cavity method (i)

Back 530

A lighting design procedure used for predetermining the relation between the number and types of lamps or luminaires, the room characteristics and the average illuminance on the work plane. The zonal cavity method takes into account both direct and reflected flux.