WIKA

Bimetal thermometer

Bimetal thermometers by WIKA

In industrial temperature measurement, primarily, three types of dial thermometers are used: bimetal thermometers, gas-actuated thermometers and expansion thermometers. Which measurement principle should be used depends on a number of different factors, such as response time, temperature limits & application range.

What is a bimetal thermometer and how does a bimetal thermometer measure?

Bimetal thermometers are thermometers based on the functional principle that metals expand differently depending on the change in temperature. A bimetal thermometer always consists of two different metal strips that have a different thermal expansion coefficient. The two strips are joined together inseparably and thus form the bimetal strip. When the temperature changes, the different metals expand to different degrees, which leads to a mechanical deformation of the bimetal strip. This mechanical deformation can be detected in a rotary movement. The measuring system works in the form of a helical or spiral tube. This movement is transmitted to the pointer of the thermometer via the pointer shaft, which thus enables the temperature to be measured.

In which applications are bimetal thermometers used?

Bimetal thermometers are used in many different industrial applications, e.g. in machine building, in heating technology and in air conditioning and refrigeration technology. The wide variety of models of WIKA bimetallic thermometers offer a large selection of thermometers in a wide range of applications.

The model 52 bimetallic thermometer, with its case and stem made of stainless steel, represents the entry-level model among the process thermometers, but is also used for demanding applications in machine building.

The model A48 bimetal thermometer has been specifically developed for use in air-conditioning and refrigeration technology to measure the temperature in air ducts, whereas the model A43 bimetal thermometer was developed as a standard version for simple temperature indication in heating technology. As you can see, WIKA offers a suitable thermometer for every application. Our specialists will be happy to advise you on your selection.

What are the advantages and disadvantages of bimetal thermometers?

Compared to gas-actuated thermometers, bimetal thermometers are much cheaper in terms of price. This price advantage is due to their simpler construction.

The scale ranges of bimetal thermometers are between -70 and +600 °C with accuracy classes 1 and 2 in accordance with EN 13190. For applications in which high vibrations occur, thermometers with liquid filling are used. For the filling, silicone oil is used, which damps the vibration and limits the maximum temperature to +250°C.

When the temperature changes, all types of dial thermometers react in accordance with the same principle; the expansion principle. The difference, however, is that with the bimetal thermometer the rotary movement of the bimetal can only be shown directly on the display. With the gas-actuated thermometer, on the other hand, the measured value can easily be transmitted via capillary due to its inert gas filling. Remote capillaries can even be used here to install gas-actuated thermometers up to 60 metres away from the measuring location. Expansion thermometers can also be installed via remote capillaries, but, instead of an inert gas, a filling medium is used for the measuring system, e.g. xylene or silicone oil.

The final decision on which dial thermometer to use depends on a number of different factors. Our specialists will be happy to advise you in the selection of the right thermometer.

FAQ
FAQ

What mechanical influences, other than the actuation of switch contacts, can cause measuring errors in bimetal thermometers?

In helical bimetal spring designs, a stroke movement of the pointer may occur, which may lead to the pointer touching on the dial or the window.  With the assistance of modern design and manufacturing techniques, such errors are avoided nowadays.