About thermometer calibration

Calibration procedures, calibration methods & examples.

About thermometer calibration


Temperature calibrations are important as over time thermometers can drift and frequent recalibrations over an instruments lifetime ensures accurate results which are fundamental to quality, safety and innovation of most products and services we use every day.

There are 2 main types of thermometer calibration methods; the Comparison Method or Fixed Point Method. Here at Brannan we use the comparison method as standard, but can use the fixed point method on request.


Comparison method


This method of calibration is the most widely used. It is based on comparing the thermometer under test to a higher accuracy standard thermometer. Here at Brannan, we use the comparison method using a series of high accuracy platinum resistance thermometers and liquid stirred baths with various liquids that are temperature dependant:

  • -60°C – +10°C we use denatured industrial alcohol
  • >10°C – 70°C we use a mixture of glycol and water
  • >70°C – 250°C we use oil

To carry out a thermometer calibration using this method, it is done in order of temperature, meaning the lowest temperature is calibrated first and we work up the thermometer to the highest temperature required. The temperature of the bath is set and is given time to stabilise. During this time, the thermometer is acclimatised to the ambient temperature and humidity within the laboratory. Once the desired temperature has been reached within the bath, the thermometer under test is inserted to the correct immersion and given time to stabilise. Once stabilised, the technician will take a series of measurements from the thermometer under test and the standard thermometers. These measurements will then be used to produce a calibration certificate.

At Brannan, we offer both UKAS and Traceable to National Standards temperature calibrations between -60 and 250°C and °F.


Fixed temperature point method


Temperature is one of the most widely measured physical quantities. But unlike other quantities, such as mass and time whose SI units are based on physical realisations, the temperature is defined on a theoretical set of conditions. The current working temperature scale is the International Temperature Scale of 1990 (ITS-90) and it is measured in degrees Celsius (°C).

A thermometer can be calibrated at a series of temperature fixed points (freezing/melting points, triple points or vapour pressure points of pure materials). This method involves inserting a thermometer in a fixed-point cell which provides the desired temperature point.

The water triple point, in which water is in equilibrium with all three states: liquid, gas and vapour, is the most important and accurately realisable of the fixed points. The apparatus used for the realisation is a glass flask partially filled with water and placed in a bucket of finely crushed ice. Once the triple point has been achieved the temperature is always 0.01°C. A thermometer under test will then be inserted into the flask and calibrated. The fixed points method is the most accurate calibration method, and it is used only in the highest quality calibrations.


Get a quote for thermometer calibration now by enquiring here.

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