310201b Thermometers And Filled Thermal Systems Flashcards
What are the 3 common immersion variations for glass stem thermometers?
- Partial immersion (insert to immersion markl
- Total immersion (immersion depth is at the meniscus level)
- Complete or full immersion (immerse the entire thermometer)
Describe the principle of operation for a glass stem thermometer
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.
Regarding the proper care and use of glass stem thermometers list two examples of actions that damage the instrument
- Tapping the thermometer against a hard surface
- Exposing a glass thermometer to extreme temperature changes, such as moving it from boiling water immediately to an ice bath
List three common variations of glass stem thermometers and describe their immersion requirements
- Partial immersion (insert to immersion mark)
- Total immersion (immersion depth is at the meniscus level)
- Complete/full immersion (immerse the entire thermometer)
Describe the principle of operation for a bimetallic thermometer
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
List the three basic components in most filled thermal systems
- Bulb
- Capillary
- Expansion element
List the 4 classifications of filled thermal systems that SQMA outlines and their fill types
Class 1 - liquid other than Mercury
Class 2 - liquid vapor interface
Class 3 - gas
Class 5 - Mercury
Name the filled thermal system classification that does not require case or ambient compensation.
Class 2 liquid vapor interface
List the 4 classifications of thermal filled systems within the class 2 family, and describe their measurement limitations or applications.
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.
Define a volatile liquid
A volatile liquid readily vaporizes at a relatively low temperature
Define full and case compensation and explain when to use them
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
List three applications where you can use a filled thermal system
- Heat trace controllers
- Temperature measurement for mechanical chart record
- Pneumatic temperature controllers
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.
- Insert the bulb into the ice bath, giving time to adjust to the temperature change
- Adjust the zero to produce 20 kPa, 3psi or 4 mA
- Insert the bulb into the dry block, giving time to adjust to the temperature change
- Adjust the span for an output of 100kPa, 15psi or 20mA
- Recheck zero and span readjust if required.
Describe type A full compensation for class 1 and 3 systems.
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
Define overrange limits and explain why most filled thermal systems have this featured?
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.
Name the filled thermal system that has the greatest overrange capability
Class 3, A or B
What causes errors due to head elevation when you use filled thermal systems?
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
Describe how to compensate for errors due to head elevation
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
When would thermowells be used in filled thermal systems?
Use thermowells when the measured medium is under pressure, or when it can harm the bulb material.
Name a major disadvantage to using the thermowell
Using a thermowell slows down the response time of the combined measurement system.
Define transmission leg as it applies to filled thermal systems
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.
What type of compensation is not required on all class 2 filled systems?
Ambient temperature compensation
When is head elevation compensation required on a fluid filled system?
If there is liquid in the in the capillary
What are case and full compensation used for?
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
What classes of fluid filled thermal systems are affected by static head (elevation) pressure?
Class 1
Class 2A
Class 2C
Class 2D
What is the most effective way of removing error created by static head pressure?
Install the bulb and gauge at the same height
Which class of fluid filled system must have the bulb and capillary installed at the same height?
Class 2C
What factors effect dead time on a temperature loop?
Distance and speed of the process