Jcl28: Measurement Temperature

Front 1

Which responds more quickly to a change in temperature; small or large mass sensor?

Back 1

Small mass.

Front 2

What happens to a thermowell when the natural frequency and wake frequency coincide ? (Not stiff enough)

Back 2

The well vibrates to the point where is breaks off

Front 3

In flowing liquids what is the recommended immersion length of a thermowell?

Back 3

6x the outside diameter of the protecting tube

Front 4

When installing a thermowell in a small pipe, what are the 2 mounting options that you can use ?

Back 4

Mount at 45 degree angle

Mount on an elbow with tip towards flow

Front 5

3 common variations of glass stem thermometers and their immersion requirements.

Back 5

Partial immersion; insert to immersion line

Total immersion; insert to meniscus level

Complete immersion; insert entire thermometer

Front 6

Describe the principle of operation for a glass stem thermometer.

Back 6

Thermal expansion

As temperature increases

Volume increases

Front 7

List the 4 classifications of thermal filled systems and their fill types.

Back 7

Class 1 - liquid other than mercury

Class 2- liquid vapour interface

Class 3 - gas

Class 5- mercury

Front 8

Which thermal filled system classification does not require case or ambient compensation?

Back 8

Class 2- liquid vapour filled systems

Front 9

What letter is places behind the class designation number to state that is is case compensation only?

Back 9

B

Front 10

In thermal filled systems, as the bulb becomes smaller or the capillary becomes longer; what happens to the error due to ambient temperature?

Back 10

Error due to ambient temperature increases

Front 11

List the 4 classifications within the class 2 family and their measurement limitations.

Back 11

Class 2A; measures temp above ambient temp

Class 2B; measures temp below ambient temp

Class 2C; measures temp above and below ambient temp (cross ambient)

Class 2D; does it all! measures temp above, below, and through the ambient temp crossover.

Front 12

Define full compensation for a filled thermal system.

Back 12

Used to compensate for changes in ambient temp in both the capillary and the instrument case.

Front 13

Define case compensation.

Back 13

Compensate for ambient temp change in the instrument case only. Used for short capillaries.

Front 14

What is the purpose of a thermowell?

Back 14

Protect from chemical and mechanical damage.

Easy maintenance

Front 15

What condition makes it possible to have an incorrect reading on a thermometer?

A it is not vertical

B no correction is made for atm pressure

C depth of immersion is incorrect

Back 15

C depth of immersion in incorrect

Front 16

Name the major disadvantage of a thermowell.

Back 16

Slows response time

Front 17

What is transmission lag? As it applies to filled thermal systems

Back 17

The time it takes for a change in process temp to register as movement in the expansion element

Front 18

Define temperature.

Back 18

The average kinetic energy of the particles in an object

Front 19

Why is solving for temerpatures using thermocouple tables more accurate than using seeback coefficient?

Back 19

B/c temperature and voltage relationship is non-linear. Tables are adjusted for this

Front 20

What is the fundamental property that governs how a thermocouple operates ? Explain.

Back 20

Seeback effect ; the redistribution of electrons when one end of the conductor is heated. Negative charge at one end equal positive charge at the other end generates voltage- which is the seeback voltage.

Front 21

How do u calculate thermocouple sensitivity?

Back 21

The difference between the two seeback coefficients of the metals.

Front 22

True or false? There is a voltage produced at the measurement junction on a thermocouple.

Back 22

False

Front 23

Which thermocouple provides the most linear response to a change in temperature ?

Back 23

Type K

Front 24

What is the law of intermediate metals? (Thermocouples)

Back 24

The introduction of a third metal into the circuit has no effect upon Eo if the junctions created by the third metal are the same temperature.

Front 25

Why must we compensate for reference junction temperature changes in a thermocouple ?

Back 25

Because when the reference junction temperature changes, the value of Eo changes, even though the measurement junction temperature does not change. This causes measurement errors if we do not compensate.

Front 26

What are 3 methods of reference junction compensation, and a limitation of each?

Back 26

Constant temperature chamber- (keeps reference junction constant) high cost

Software compensation- (microprocessor in transmitter) most versatile b/c allows for any type of thermocouple but extra time is required to calculate temperature

Hardware compensation- (changes it's voltage in response to change in reference) each circuit has restrictions to particular thermocouple type

Front 27

What is the purpose of an extension wire ?

Back 27

Extends circuit to make sure the reference junction is at the transmitter to avoid errors.

*extension wires must be same type as thermocouple

Front 28

What is the difference between a thermocouple grade wire and an extension grade wire?

Back 28

Thermocouple grade wire had a higher purity and higher temperature limit than extension grade wire