14.2 Temperature scales

A thermometric property varies with temperature and can be calibrated to provide a temperature reading; examples include liquid volume, resistance or thermocouple emf.

A useful property should be monotonic over the range, reproducible and sensitive enough to resolve the required temperature change.

The resistance of a platinum sensor can be calibrated against fixed points and then used to infer an unknown temperature.

A property changing with temperature is not automatically a good thermometer; calibration and response range are essential.

The thermodynamic temperature scale is based on universal physical principles rather than a chosen material property; the kelvin is the SI unit.

A practical thermometer is calibrated to approximate this scale, but its raw property may be nonlinear or limited in range.

A gas, resistance and radiation thermometer can agree after calibration even though their measured properties differ.

The Celsius scale and a material’s expansion are convenient representations, not the fundamental definition of temperature.

Thermodynamic temperature T in kelvin relates to Celsius temperature θ by T=θ+273.15.

Kelvin is an absolute scale with the same degree size as Celsius but a different zero. Use kelvin in gas and thermodynamic equations unless instructed otherwise.

25 °C is 298.15 K; 0 °C is 273.15 K, not 0 K.

A temperature difference of 1 °C equals 1 K, but an absolute temperature of 1 °C is not 1 K.

Absolute zero is zero kelvin, the lowest limit of thermodynamic temperature; it corresponds to −273.15 °C.

It is a limiting state, not simply “no motion” in every quantum description. Use it as the zero of the absolute scale.

Cooling from 300 K to 150 K halves the absolute temperature even though Celsius readings do not behave as a ratio scale.

Negative Celsius temperatures can be physically valid, but temperatures below 0 K are not reached in the ordinary thermodynamic scale.