Direct animal calorimeters measure the total heat generated inside them and partition the heat into its two components, evaporative and non-evaporative. Non-evaporative or sensible heat is heat given off from an animal by radiation to surrounding surfaces, by convection to the surrounding air and by conduction to any objects with which the animal is in contact. Evaporative heat loss occurs because the conversion of liquid water into vapour requires heat energy. The latent heat of vaporisation is the heat required to vaporise unit mass of water. When water is vaporised in the respiratory passages during normal respiration and panting, or at the skin surface during perspiration, the latent heat of vaporisation is derived mainly from the animal. This heat loss by the animal is transferred to the air in the form of increased humidity; the enthalpy of the air (which is a measure of its energy content and depends on temperature, humidity and pressure) is increased.

The psychrometric chart

The relationships between temperature, humidity and enthalpy of air are often illustrated in the form of a psychrometric chart. Fig. 5.1 is a scaled down version of a psychrometric chart for standard barometric pressure. The heavy line is a plot against temperature of the mixing ratio (i.e. humidity expressed as weight of water per weight of dry air) for air saturated with water vapour; it represents the maximum amount of vaporised moisture that air may contain.