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Selecting suitable mechanical thermometers for a process is not an instant task. Bimetal thermometers or gas-actuated thermometers ? the decision on the sort of thermometer alone depends on several factors. Just about the most important selection criteria is the response time of the measuring Instrument.
In Profit of speed, there is 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 the two 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 used in the pointer via a movement.
Thermowells affect the response time
Much like other measuring instruments, in critical processes, mechanical thermometers must be combined with a thermowell. It ought to be considered that such a sleeve will also have an influence, to some extent significant, on the response time of the thermometer. To what extent, depends on 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 our articles
Selection criteria for mechanical thermometers (2): process temperature
Selection criteria for dial thermometers (3): Capillary