Selection criteria for dial thermometers (3): Capillary

Bimetal or gas-actuated – the question of the suitable dial thermometer for the respective process can only be answered in a case without comparison: Only gas-actuated thermometers can be combined with a capillary.
Dial thermometer with capillary – This combination is only possible with a gas-actuated instrument.
In In the reaction to a temperature change, both dial thermometer types function in accordance with the expansion principle. But the rotation of the helix in bimetal thermometers can only react directly and without “detour” on the indication. Whereas a gas-actuated thermometer is also suitable for the transmission of the measured value via capillary, due to its helium filling. Such a capillary has an outer diameter of only 2 mm and an inner diameter of 0.1 to 0.2 mm.
A dial thermometer with capillary puts users in the position to be able to measure the process temperature, even at inaccessible points. By means of a contact bulb it can also be measured on the smallest pipelines. Another advantage is the bridging of greater distances. In this way, thermometers can be centrally integrated in an instrument panel, a switch cabinet or a control panel.
Lonely of the measuring arrangement
For the specification of a gas-actuated thermometer with capillary, the scale range, the length of the capillary and the type of probe connection must be known. With a capillary length of up to 60 m, WIKA guarantees the accuracy class 1 of the respective instrument (in accordance with EN 13190). The necessary filling pressure for a dial thermometer with capillary from WIKA varies between 25 bar (large span) and 180 bar (small span).
Such a measuring arrangement can be installed in a plant without any problems. However, users must observe a minimum bending radius to ensure that the capillary does not kink and the helium can expand. In Bottom Line , we recommend a stainless steel spiral protection from mechanical impacts.
Further articles concerning the selection criteria for bimetal and gas-actuated thermometers are dedicated to the reaction time and the process temperature. Further information on both instrument types can be found on the WIKA website under the headings “Dial thermometers” and “Thermometers with switch contacts”.
Learn more about the differences between bimetal and gas-actuated thermometers in the following video:

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