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Empirical temperature scales

An empirical temperature scale uses a specific thermometric property (a property that varies with temperature, such as volume, electric resistance) of a certain material (e.g. mercury).

The points of reference and the number of divisions within an empirical scale are essentially arbitrary.

An empirical scale can be obtained by measuring the length of a mercury column at the boiling point and at the freezing point of water (the reference points). The range between these two reference points is then divided into smaller units. This was the basis of the original Celsius scale.

The range of empirical scales is limited by the property of the material used to calibrate them.

Ethanol boils at $$78^{\circ}\text{C}$$. An empirical scale based on the volume expansion of liquid ethanol thus cannot be used to measure temperatures above $$78^{\circ}\text{C}$$ accurately.

It can be difficult to convert between different empirical scales because the variance of the thermometric property of the material used in calibrating a particular scale differs from that of other materials.

Ethanol and mercury have different rates of expansion. Calibrating a scale according to the properties of ethanol and using the same scale on a mercury thermometer would give inaccurate temperature readings.