29/08/2025

How to Calibrate a Thermometer — 6 Easy Ways

It’s essential to check the accuracy of your thermometer regularly. Over time, readings tend to drift, making them less accurate. Doing accuracy checks mean you have more confidence in your readings. Plus, you’ll know when it’s time to send your thermometer to a laboratory for recalibration.

This guide will explore various calibration equipment options available in the market today. We will cover a range of tools — from simple, cost-effective solutions to more advanced equipment.

Whether you’re a professional in a scientific laboratory, a healthcare provider, or simply someone who wants to ensure the accuracy of their everyday thermometers, this guide is here to help you make informed decisions about the best temperature calibration equipment you can purchase.

How to Calibrate a Thermometer Using an Ice Bath

Making an accurate ice bath that is truly 0 °C can be slightly tricky. The ETI Ice Bath Mug is insulated and printed with instructions, making things easier.

How to make an ice bath:

Fill your container completely with ice. Crushed is better than cubed because there are less gaps between the ice.
Add cold water until you reach 13 mm below the top of the ice. Leave for one minute.
Stir your thermometer through the mixture, avoiding the bottom and sides of the container.
Check your thermometer reading. Compare with a high accuracy reference thermometer if possible.
If your thermometer readings are outside of the model’s specification, you should adjust it or sent it to a laboratory for recalibration.

Pros:

Budget-friendly

Cons:

Can be inaccurate if carried out incorrectly
Difficult if you have a lot of thermometers to calibrate

Shop the Ice Bath Mug.

How to Calibrate an Infrared Thermometer

Using an infrared thermometer accurately can be hard — accurate calibration tests are even harder. We recommend using either a Comparator Cup or a Black Body Calibrator.

How to use a comparator cup:

Wait for the cup and your infrared device to come to room temperature (around 22 °C). Using them at hot or cold temperatures increases the risk of moisture, which can skew the readings.
Insert a high accuracy reference probe into the hole in the base of the cup. Use this as your reference point.
If your infrared thermometer has adjustable emissivity, set it to 0.95. Point your infrared device directly into the top of the comparator and take a measurement. It should read within 1 °C of the reference thermometer, depending on your instrument’s accuracy specification.

Pros:

Budget-friendly

Cons:

Only suitable for ambient accuracy checks

How to use a black body calibrator:

Set your desired calibration temperature on the machine (from +50 to 500 °C). Wait for it to stabilise.
Point your infrared thermometer directly at the black body circle and take a measurement. It should read within 1 °C of the temperature on the machine, depending on your thermometer’s specified accuracy.
For increased accuracy, insert a reference thermometer probe into the calibration well.

Pros:

High accuracy infrared checks
Able to calibrate at temperatures up to 500 °C
Reference thermometer recommended but not required

Cons:

Cannot calibrate at temperatures below 50 °C

Shop the Comparator Cup or the Black Body Calibrator.

How to Use a Reference Thermometer

When doing calibration checks, a high accuracy reference thermometer is ideal for comparison. It will help you be more confident in the true temperature of your source (such as an ice bath), meaning you can clearly see how far your thermometer has deviated from the source.

Reference thermometers typically have very high accuracies such as ±0.03 °C, and come with UKAS calibration certificates at multiple points.

They have slow response times and delicate sensors, so they should only be used for calibration checks.

Shop the Reference Thermometer or Reference Thermapen.

How to Use Test Caps

Test caps are a fast, accurate and economical way of checking thermistor or PT100 instruments.

Available in a range of temperatures, they simply screw onto the instrument’s lumberg connector. Take a reading and compare with the test cap’s temperature — the difference should be within the instrument’s accuracy specification.

Test caps only check the accuracy of the device, not the probe. You will need to test your probes separately using a different method such as an ice bath or dry block calibrator.

Pros:

Fast, accurate and economical

Cons:

Thermistors only
Probes must be calibrated separately

Shop the Thermistor or the PT100 test caps.

How to Use a Microcal Calibrator

Similarly to thermistor and PT100 test caps, MicroCal calibrators enable quick and easy validation of thermometer instruments, but ones with thermocouple connectors.

MicroCal calibrators are available with adjustable temperature points, ideal for quickly switching between your relevant checkpoints.

They’re also compatible with multiple types of thermocouples (K, J, T, R, N, S and E).

Remember to calibrate your probes separately.

Pros:

Fast, accurate and economical

Cons:

Thermocouples only
Probes must be calibrated separately.

Shop the MicroCal 1 calibrator.

How to Use a Dry Block Calibrator

Dry block calibrators, also known as dry wells, create a stable temperature source for testing thermometer probes. These portable devices are a more accurate and reliable alternative to ice baths, enabling users to check large quantities of probes quickly and at desired checkpoints.

You can use a dry block calibrator on a wide variety of thermometer types, including thermocouples, thermistors and PT100. They’re also available with varying hole sizes for different probe diameters.

To use a dry block:

Switch on the machine and set your desired temperature. Wait for five minutes for it to stabilise.
Insert your thermometer probe into the correct size well. Compare the difference with the machine’s set temperature.

Pros:

Calibrate at hot, cold and ambient temperatures
Suitable for a wide variety of probe types
Quick and accurate
Portable

Cons:

Higher price point

Shop the cool or hot dry well calibrators.

How Often to Calibrate Food Temperature Probes

For food businesses, the general recommendation is to calibrate food temperature probes at least once a month.

How often you calibrate your food temperature probes depends on how often you use them, how accurate they need to be, and what guidelines are outlined in your food safety plan.

Remember to keep a record of calibration dates and results as EHO inspectors often request to see them.

What to Do After Calibrating a Thermometer?

If you have verified the accuracy of your thermometer and found it to be out of range, you’ll need to send it to a laboratory to be recalibrated.

At ETI, we have eight in-house calibration laboratories, including three UKAS laboratories and one specifically designed for infrared calibration.

Learn more about our calibration services.

Summary

The calibration method you choose depends on how many thermometers you need to calibrate, how often you need to calibrate them, and the time and budget you have allocated to this process.

Whichever one you go for, the most important thing to remember is to carry out each test with care and precision so that results are accurate and you can continue using your instruments with confidence.