Introduction To Psychrometry & the Psychrometric Chart

Psychrometry is the field of engineering concerned with the physical and thermodynamic properties of gas-vapour mixtures.  Psychrometrics, and hygrometry are also common used terms for this field.

Air is defined as a mixture of gases and water vapour. Air is predominantly composed of Nitrogen (78% by volume), Oxygen (21%), with the remaining 1% being made up of Carbon dioxide and minute quantities of other gases. The basic composition of air, is practically identical everywhere. The amount of water vapour in the air however, varies greatly between locations and even the time of day.

All air in its natural state, always contains a certain quantity of water vapour, as such there is no such thing as “dry air” in its truest form. "Dry air" as a concept however, is useful in that it simplifies psychrometric calculations. The term “dry air” is normally used when referring to air without water vapour, whereas the terms “air” and “moist air” refer to the natural mixture of dry air and water vapour.


Dalton’s law, the statement that the total pressure of a mixture of gases, is equal to the sum of the partial pressures of the individual component gases. The partial pressure, is the pressure that each gas would exert, if it alone occupied the volume of the mixture at the same temperature.

Air being a mechanical mixture of gases and water vapour, follows Dalton’s law. Total barometric pressure, may therefore be considered to be the sum of the pressures exerted by the dry gases, and the partial pressure exerted by the water vapour.

Mixing Ratio or Humidity Ratio (w)

Humidity Ratio, defines the mass of water in the volume occupied by 1 kg of dry air. This can be further defined, as the amount of water required to be evaporated into 1 kg of dry air, in order to achieve a specific condition.

Humidity Ratio, is expressed as either kilograms per kilogram (kg/kg) or grams per kilogram (g/kg).

HVAC engineers as a rule, use the term Mixing/Humidity Ratio, as it is often found to be more practical than Absolute Humidity. Unlike Absolute Humidity, Mixing/Humidity Ratio, does not change with temperature, unless cooled below dewpoint.

In general, engineers and consultants specialising in the field of psychrometric design, now use the term “Moisture Content” instead of “Mixing or Humidity Ratio”.

Temperature °Cdb / °Cwb

Dry Bulb Temperature °Cdb

Temperature of air, as measured by a standard thermometer, with a dry sensing bulb.

Wet Bulb Temperature °Cwb

Temperature of air, as measured by a thermometer, using a sensing bulb covered by a wet wick. When air passes over a thermometer, water will evaporate from the wick, at a rate determined by the relative humidity of the surrounding air. As the water evaporates, latent heat is adsorbed from the wick, which lowers the bulb temperature. The drier the air, the more the thermometer cools and hence, the lower the wet-bulb reading.

At 100% RH there would no moisture evaporation from the wick, meaning the wet and dry bulb temperature would be the same.

Relative Humidity (RH)

Relative Humidity is the most commonly used psychrometric unit.

Relative humidity, is the relation between the amount of water vapour present, and the amount that is physically possible at that temperature.

%RH = 100% x (pw/pws) where pw is partial pressure of water vapour and pws is water vapour saturation pressure.

Relative Humidity (RH) expressed as a percentage, is the ratio of the actual partial pressure exerted by water vapour in air, to the maximum partial pressure, that would be exerted by the water vapour, if that air were saturated, at that temperature (100% RH).

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