Selecting suitable mechanical thermometers for a process is not a quick task. Bimetal thermometers or gas-actuated thermometers ? the decision on the type of thermometer alone depends upon several factors. One of the most important selection criteria is the response time of the measuring Instrument.
With regards to speed, there is absolutely no way around gas-actuated thermometers. As a rule of thumb, these thermometers take about 30 seconds for a temperature adjustment from 10�C to 90�C. A bimetal thermometer would display exactly the same final value after 90 seconds. Because of their short response times, gas-actuated thermometers are particularly ideal for integration into switching requirements.
The difference in speed is explained by the various functional principles of both thermometer types ? unlike the bimetal thermometer (which uses the principle of different expansion coefficients), the measuring system of a gas-actuated thermometer is filled up with helium under great pressure. Any change in temperature causes an immediate change in internal pressure in the stem. The pressure deforms the measuring tube and the deflection is used in the pointer via a movement.
Thermowells affect the response time
As with other measuring instruments, in critical processes, mechanical thermometers must also be coupled with a thermowell. It should be considered that such a sleeve may also have an influence, to some degree significant, on the response time of the thermometer. From what extent, depends upon the material and wall thickness of the thermowell.
Note
Further information on mechanical thermometers can be found on the WIKA website under the headings Dial thermometers and Thermometers with switch contacts.
See also Highest for mechanical thermometers (2): process temperature
Selection criteria for dial thermometers (3): Capillary