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28 Cards in this Set
- Front
- Back
Which responds more quickly to a change in temperature; small or large mass sensor? |
Small mass. |
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What happens to a thermowell when the natural frequency and wake frequency coincide ? (Not stiff enough) |
The well vibrates to the point where is breaks off |
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In flowing liquids what is the recommended immersion length of a thermowell? |
6x the outside diameter of the protecting tube |
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When installing a thermowell in a small pipe, what are the 2 mounting options that you can use ? |
Mount at 45 degree angle Mount on an elbow with tip towards flow |
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3 common variations of glass stem thermometers and their immersion requirements. |
Partial immersion; insert to immersion line Total immersion; insert to meniscus level Complete immersion; insert entire thermometer |
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Describe the principle of operation for a glass stem thermometer. |
Thermal expansion As temperature increases Volume increases |
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List the 4 classifications of thermal filled systems and their fill types. |
Class 1 - liquid other than mercury Class 2- liquid vapour interface Class 3 - gas Class 5- mercury |
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Which thermal filled system classification does not require case or ambient compensation? |
Class 2- liquid vapour filled systems |
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What letter is places behind the class designation number to state that is is case compensation only? |
B |
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In thermal filled systems, as the bulb becomes smaller or the capillary becomes longer; what happens to the error due to ambient temperature? |
Error due to ambient temperature increases |
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List the 4 classifications within the class 2 family and their measurement limitations. |
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. |
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Define full compensation for a filled thermal system. |
Used to compensate for changes in ambient temp in both the capillary and the instrument case. |
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Define case compensation. |
Compensate for ambient temp change in the instrument case only. Used for short capillaries. |
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What is the purpose of a thermowell? |
Protect from chemical and mechanical damage. Easy maintenance |
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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 |
C depth of immersion in incorrect |
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Name the major disadvantage of a thermowell. |
Slows response time |
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What is transmission lag? As it applies to filled thermal systems |
The time it takes for a change in process temp to register as movement in the expansion element |
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Define temperature. |
The average kinetic energy of the particles in an object |
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Why is solving for temerpatures using thermocouple tables more accurate than using seeback coefficient? |
B/c temperature and voltage relationship is non-linear. Tables are adjusted for this |
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What is the fundamental property that governs how a thermocouple operates ? Explain. |
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. |
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How do u calculate thermocouple sensitivity? |
The difference between the two seeback coefficients of the metals. |
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True or false? There is a voltage produced at the measurement junction on a thermocouple. |
False |
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Which thermocouple provides the most linear response to a change in temperature ? |
Type K |
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What is the law of intermediate metals? (Thermocouples) |
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. |
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Why must we compensate for reference junction temperature changes in a thermocouple ? |
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. |
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What are 3 methods of reference junction compensation, and a limitation of each? |
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 |
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What is the purpose of an extension wire ? |
Extends circuit to make sure the reference junction is at the transmitter to avoid errors. *extension wires must be same type as thermocouple |
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What is the difference between a thermocouple grade wire and an extension grade wire? |
Thermocouple grade wire had a higher purity and higher temperature limit than extension grade wire |