Although a gas or vapor has many properties, only six are of particular importance in the study of refrigeration. These are pressure, temperature, volume, enthalpy, internal energy, and entropy. Pressure, temperature, and volume are called measurable properties because they can actually be measured. Enthalpy, internal energy, and entropy cannot be measured. They must be calculated and are therefore known as calculated properties.
Pressure, temperature, volume, and internal energy have already been discussed to some extent. A discussion of enthalpy and entropy follows.