310201b Thermometers And Filled Thermal Systems

Question 1

What are the 3 common immersion variations for glass stem thermometers?

Answer 1

1. Partial immersion (insert to immersion markl
2. Total immersion (immersion depth is at the meniscus level)
3. Complete or full immersion (immerse the entire thermometer)

Question 2

Describe the principle of operation for a glass stem thermometer

Answer 2

Glass stem thermometers operate on the principle of thermal expansion of a liquid.
As the temperature of a liquid increases, the volume that the liquid occupies also increases, you see this as a higher level on the stem of the thermometer.

Question 3

Regarding the proper care and use of glass stem thermometers list two examples of actions that damage the instrument

Answer 3

1. Tapping the thermometer against a hard surface
2. Exposing a glass thermometer to extreme temperature changes, such as moving it from boiling water immediately to an ice bath

Question 4

List three common variations of glass stem thermometers and describe their immersion requirements

Answer 4

1. Partial immersion (insert to immersion mark)
2. Total immersion (immersion depth is at the meniscus level)
3. Complete/full immersion (immerse the entire thermometer)

Question 5

Describe the principle of operation for a bimetallic thermometer

Answer 5

Bimetallic thermometers consist of 2 dissimilar metal strips that are bonded together
The different coefficient of expansion rates of these materials causes bending to occur when you apply or remove heat
This bending is proportional to the temperature change, the square of the length and is inversely proportional to the thickness of the metals

Question 6

List the three basic components in most filled thermal systems

Answer 6

1. Bulb
2. Capillary
3. Expansion element

Question 7

List the 4 classifications of filled thermal systems that SQMA outlines and their fill types

Answer 7

Class 1 - liquid other than Mercury
Class 2 - liquid vapor interface
Class 3 - gas
Class 5 - Mercury

Question 8

Name the filled thermal system classification that does not require case or ambient compensation.

Answer 8

Class 2 liquid vapor interface

Question 9

List the 4 classifications of thermal filled systems within the class 2 family, and describe their measurement limitations or applications.

Answer 9

Class 2A - use where measurement temperatures are above the instrument case temperature
Class 2B - use where measurement temperatures are below the instrument case temperature.
Class 2C - use for measurement temperatures above and below the instrument case temperature, but where an accurate reading at the ambient temperature is not critical
Class 2D - use for measurement temperatures above below and through the ambient temperature crossover.

Question 10

Define a volatile liquid

Answer 10

A volatile liquid readily vaporizes at a relatively low temperature

Question 11

Define full and case compensation and explain when to use them

Answer 11

Full compensation is used to compensate for changes in ambient temperature along the capillary. Used when capillary lines are relatively long

Case compensation is used to compensate for changes in ambient temperature.In the instrument case and expansion element. Used when the capillary is relatively short

Question 12

List three applications where you can use a filled thermal system

Answer 12

1. Heat trace controllers
2. Temperature measurement for mechanical chart record
3. Pneumatic temperature controllers

Question 13

Describe the calibration procedure for a filled thermal system using an ice bath and dry block calibrator, the range is 0°C to a 100°C.

Answer 13

1. Insert the bulb into the ice bath, giving time to adjust to the temperature change
2. Adjust the zero to produce 20 kPa, 3psi or 4 mA
3. Insert the bulb into the dry block, giving time to adjust to the temperature change
4. Adjust the span for an output of 100kPa, 15psi or 20mA
5. Recheck zero and span readjust if required.

Question 14

Describe type A full compensation for class 1 and 3 systems.

Answer 14

These full compensation systems use a second complete filled thermal system, which consists of a capillary that runs adjacent to the measurement capillary
It compensates for all temperature influences aside from the process measurement
Use these full compensation systems when capillaries are relatively long

Question 15

Define overrange limits and explain why most filled thermal systems have this featured?

Answer 15

Overranged, limits are the temperature limits that the filled system can withstand.
Filled thermal systems may have exposure to process and ambient temperatures in excess of the designed range. To prevent damage to the instrument, manufacturer’s design overrange features into the instrument.

Question 16

Name the filled thermal system that has the greatest overrange capability

Answer 16

Class 3, A or B

Question 17

What causes errors due to head elevation when you use filled thermal systems?

Answer 17

Errors due to head elevation are caused when the fill in the capillary and expansion element is liquid and a difference in level between the bulb and the expansion element in the indicating instrument occurs

Question 18

Describe how to compensate for errors due to head elevation

Answer 18

Most instruments have a zero adjustment that you can use to correct for pressure errors due to the effect of elevation differences between the bulb and the expansion element

Question 19

When would thermowells be used in filled thermal systems?

Answer 19

Use thermowells when the measured medium is under pressure, or when it can harm the bulb material.

Question 20

Name a major disadvantage to using the thermowell

Answer 20

Using a thermowell slows down the response time of the combined measurement system.

Question 21

Define transmission leg as it applies to filled thermal systems

Answer 21

Transmission lag is the time takes for a change in the temperature of the process medium to register as a pressure change in the instrument element.

Question 22

What type of compensation is not required on all class 2 filled systems?

Answer 22

Ambient temperature compensation

Question 23

When is head elevation compensation required on a fluid filled system?

Answer 23

If there is liquid in the in the capillary

Question 24

What are case and full compensation used for?

Answer 24

They are used for ambient temperature compensation

Case compensation is used when capillary lines are short

Full compensation is used when capillary lines are long

Question 25

What classes of fluid filled thermal systems are affected by static head (elevation) pressure?

Answer 25

Class 1
Class 2A
Class 2C
Class 2D

Question 26

What is the most effective way of removing error created by static head pressure?

Answer 26

Install the bulb and gauge at the same height

Question 27

Which class of fluid filled system must have the bulb and capillary installed at the same height?

Answer 27

Class 2C

Question 28

What factors effect dead time on a temperature loop?

Answer 28

Distance and speed of the process