As a temperature switch needs to measure temperature, it needs to have a temperature sensing element, in other words a temperature sensor.

In some cases the temperature sensor is a separate instrument and can be removed from the switch, while in others the sensor is fixed to the switch so they cannot be separated.

These two different scenarios require very different methods to calibrate the switch.

As explained above, you need to provide a slowly changing temperature for the switch input. This is very different depending on if the switch has a fixed temperature sensor or if the sensor can be removed.

Let’s look at these two different scenarios next.

#1 - Temperature switch with a separate/removable temperature sensor

In some cases, you can remove the temperature sensor from the temperature switch. The sensor will often be a common standard sensor, such as a Pt100 sensor (or a thermocouple). In these cases you can calibrate the switch without the temperature sensor by using a simulator or calibrator to simulate the Pt100 sensor signal, generating a slow temperature ramp (or a series of very small steps) as the input to the switch.

Naturally you also need to calibrate the temperature sensor, but that can be calibrated using normal temperature sensor calibration at fixed temperature set points, without needing to slowly ramp the temperature, which makes the sensor calibration much easier (and with less uncertainty).

In accurate applications, the switch may be compensating for RTD sensor error by using correction coefficients, such as ITS-90 or Callendar van Dusen, so when simulating the temperature sensor your sensor simulator should be able to take this into account.

You can calibrate the sensor and switch together as a loop; you don’t have to calibrate them separately. But if you don’t have a system that generates a slow, controlled temperature ramp, it is easier to calibrate them separately.

If the removable temperature sensor is a not a standard sensor type (neither an RTD nor a thermocouple), then you can’t really calibrate the sensor and switch separately as you can neither measure nor simulate the signal of the non-standard sensor. In that case you need to calibrate them as one instrument when they are connected.

#2 - Temperature switch with an integrated/fixed temperature sensor

If your temperature sensor is fixed to your temperature switch and cannot be removed, you need to calibrate it all as one instrument. In that case you need to generate a temperature ramp with a temperature source that you insert the temperature sensor into.

How to calibrate temperature switches

Before calibration

As with any process instrument calibration, before starting, isolate the measurement from the process, communicate with the control room, and make sure the calibration will not cause any alarms or unwanted consequences.

Visually check the switch to ensure it is not damaged and all connections look ok.

If the sensor is dirty, it should be cleaned before inserting it into the temperature block.

Generate a slow temperature ramp as input

If you are calibrating the temperature switch and its temperature sensor together, you need to generate a slow enough temperature ramp in the temperature source where you install the switch's temperature sensor.

This means you need to have a temperature source that can generate a controlled temperature ramp at a constant speed, as slow as the application requires.

In practice you can quickly reach a temperature set point close to the calibration range, let the temperature fully stabilize, and then start slowly ramping the temperature across the calibration range. After the calibration you can quickly return back to room temperature.

A temperature ramp like this is most commonly generated with a temperature dry block. Not all dry blocks are able to generate a suitably slow ramp. And you also need to be able to measure the generated temperature very accurately, while at the same time being able to measure the switch output signal. In addition, the calibration system should have the capability to automatically capture the input temperature at the exact moment when the switch output changes its state.

Use an external reference temperature sensor – don’t use the internal one!

Temperature dry blocks always have an internal reference sensor, but do not use this when calibrating temperature switches!

The internal reference sensor is located in the bottom part of the temperature block, which is heated and/or cooled. The internal reference sensor is also usually close to the heating/cooling elements and responds quickly to any temperature changes.

From that temperature block, the temperature will transfer to the insert and from the insert it will transfer to the actual temperature sensor. This means that there is always a significant delay (lag) between the internal reference sensor and the sensor being calibrated, located in the hole in the insert.

In a normal sensor calibration, done at fixed temperature points, this delay is not so critical, because you can wait for the temperatures to stabilize. But for temperature switch calibration this delay has a huge impact and will cause significant error in the calibration result!

Instead of using the internal reference sensor, you should use an external reference sensor that is installed in the insert together with the switch’s sensor to be calibrated. The external reference sensor should have similar characteristics to the temperature switch sensor in order for them to behave the same way, with a similar lag.

At the very least make sure that the dimensions of the reference sensor and temperature switch sensor are as similar as possible (e.g. similar length and diameter). Ensuring that the sensors have the same length means they will go equally deep into the insert, with the same immersion depth. Different immersion depths will cause error and uncertainty in the calibration.

Naturally the reference temperature sensor also needs to be measured with an accurate measurement device.

Measuring the switch output

Once you have the input temperature ramp figured out, you also need to measure the switch output terminals and their state.

With a traditional open/close switch, you need to have a device that can measure if the switch contacts are open or closed.

If the switch is more modern with an electrical output, you need to be able to measure that. That may be current measurement for an mA signal, or voltage measurement for a voltage signal.

Anyhow, as the switch output has two states, you need to have a device that can measure and recognize both.

Capturing the operation points

To calibrate manually you need to start the temperature ramp and monitor the switch output. When the switch’s status changes, you need to read what the input temperature is, i.e. what the reference temperature sensor is reading. That is the operating point of the temperature switch. Usually you want to calibrate both operation points (the “set” and “reset” points) with increasing and decreasing temperatures to see the difference between them, which is the hysteresis (deadband).

If you don’t want to do that manually, then you need a system that can perform all of the required functions automatically, i.e. it needs to:

• Generate the temperature ramp, going up and down at the required speed, within the required temperature range for the switch in question

• Measure the switch’s output state (open/close, on/off)

• Measure the reference temperature sensor inserted in the temperature source

• Capture the temperature when the switch changes state