Selecting suitable mechanical thermometers for an activity is not an instant task. Bimetal thermometers or gas-actuated thermometers ? your choice on the sort of thermometer alone depends upon several factors. Probably the most important selection criteria may be the response time of the measuring Instrument.
When it comes to speed, there is absolutely no way around gas-actuated thermometers. Generally of thumb, these thermometers take about 30 seconds for a temperature adjustment from 10�C to 90�C. A bimetal thermometer would display the same final value after 90 seconds. Due to their short response times, gas-actuated thermometers are particularly ideal for integration into switching requirements.
The difference in speed is explained by the different 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 instantaneous change in internal pressure in the stem. The pressure deforms the measuring tube and the deflection is transferred to the pointer with a movement.
Thermowells affect the response time
Much like other measuring instruments, in critical processes, mechanical thermometers must be combined with a thermowell. It must be considered that this type of sleeve will also have an influence, to some extent significant, on the response time of the thermometer. From what extent, depends upon the material and wall thickness of the thermowell.
Note
Further details on mechanical thermometers can be found on the WIKA website under the headings Dial thermometers and Thermometers with switch contacts.
See also Optimistic for mechanical thermometers (2): process temperature
Selection criteria for dial thermometers (3): Capillary