In short, a temperature switch is an instrument that measures temperature and provides a required function (a switch opens or closes) at a programmed temperature.

One of the most common temperature switches is the thermostat switch in an electric radiator. You can set the thermostat to the required temperature and if the room is colder than the set temperature, the thermostat will switch the radiator on; if the room temperature is higher than required, the thermostat will switch the heating off.

In practice, there is a small difference between the set and reset points so that the control does not start to oscillate when the temperature reaches the set point. This difference is called hysteresis, or deadband. In the above radiator example this means that when the thermostat is turned to 20 °C (68 °F), the radiator may start heating when the temperature is below 19 °C (66 °F) and stop heating when the temperature is 21 °C (70 °F), showing a 2 °C (4 °F) deadband.

Naturally, there are many different applications for temperature switches in industry.

The main principle of temperature switch calibration

We will investigate the details of temperature switch calibration later in this article, but to start, let’s briefly summarize the main principle to remember when calibrating a temperature switch:

To calibrate a temperature switch you need to slowly ramp the temperature at the switch input (the temperature-sensing element) while simultaneously measuring the switch output to see at which temperature it changes its state. Then you need to ramp the temperature back to find the “reset” point, where the switch reverts back to its original state.

When the output changes state, you need to record the input temperature at that exact moment.

The switch output usually only has two states, e.g. open or closed.

Essential terminology

One term commonly discussed is whether a switch type is normally open (NO) (or closing), or normally closed (NC) (or opening). This indicates if the switch contacts are open or closed by default. Usually temperature switches are in their default position when measuring the environmental temperature.

Operating points may also be referred to as Set and Reset points, or On and Off points.

The temperature difference between the operation points is called deadband. Some difference is needed between the closing/opening operating points to prevent the switch from potentially oscillating on and off if they work at exactly the same temperature. For applications that require a very small deadband, additional logic is provided to prevent the switch from oscillating.

The switch outputs may be mechanical (open/close), electronic, or digital.

Dry/wet switches are also sometimes discussed. Dry means that the output is closed or open, while wet means that there is a different voltage level representing the two switch states.

Some switches have mains voltage over the contacts when the switch is open. This can be a safety issue for both people and test equipment, so it should be taken into account when testing the switch.