What is Comparison Calibration?

With comparison calibration, a thermometer with known characteristics is compared to the thermometer we want to calibrate, relying on the “Zeroth Law of Thermodynamics”.

There are four “Laws of Thermodynamics”, the Zeroth Law was only formulated in 1931 by Fowler – it is more fundamental than the other three existing laws so it became known, not as the fourth law, but the “Zeroth” law.

The law states that if two systems are in thermal equilibrium, each having the same temperature as a third system, the two systems have the same temperature as each other.

This can be rephrased to explain comparison calibration: “If a calibrated thermometer is at the same temperature as a calibration bath, and a thermometer under test is at the same temperature as that calibration bath, then the calibrated thermometer and the thermometer under test are at the same temperature”.