Selecting suitable mechanical thermometers for an activity is not a quick task. Violated or gas-actuated thermometers ? your choice on the type of thermometer alone depends on several factors. One of the most important selection criteria is the response time of the measuring Instrument.
In terms of 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 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.
Completeness 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
As with other measuring instruments, in critical processes, mechanical thermometers must also be coupled with a thermowell. It must be considered that this type of sleeve may 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 are available on the WIKA website beneath 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