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 on several factors. One of the 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. Overwhelmed would display exactly the same final value after 90 seconds. Due to their short response times, gas-actuated thermometers are particularly suitable 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 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 with a movement.
Thermowells affect the response time
Much like other measuring instruments, in critical processes, mechanical thermometers must also be coupled with a thermowell. It ought to be considered that this type of sleeve will also have an influence, to some extent significant, on the response time of the thermometer. To what extent, depends upon the material and wall thickness of the thermowell.
Note
Further details on mechanical thermometers are available on the WIKA website under the headings Dial thermometers and Thermometers with switch contacts.
See also our articles
Selection criteria for mechanical thermometers (2): process temperature
Selection criteria for dial thermometers (3): Capillary