Psychrometrics

Psychrometrics (or psychrometry, from Greek ψυχρόν (psuchron) 'cold' and μέτρον (metron) 'means of measurement';[1][2] also called hygrometry) is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.

With the inventions of the hygrometer and thermometer, the theories of combining the two began to emerge during the sixteenth and seventeenth centuries.

In 1818, a German inventor, Ernst Ferdinand August (1795-1870), patented the term “psychrometer”, from the Greek language meaning “cold measure”.

[3] Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilation, and air-conditioning and meteorology.

Such substances include cotton, paper, cellulose, other wood products, sugar, calcium oxide (burned lime) and many chemicals and fertilizers.

Relative humidity is often controlled in manufacturing areas where flammable materials are handled, to avoid fires caused by the static electricity discharges that can occur in very dry air.

In many industrial applications it is important to avoid condensation that would ruin product or cause corrosion.

Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example).

A simplified definition is the temperature at which the water vapour turns into "dew" (Chamunoda Zambuko 2012).

Specific humidity is defined as the mass of water vapor as a proportion of the mass of the moist air sample (including both dry air and the water vapor); it is closely related to humidity ratio and always lower in value.

It may be evaluated with the following equation:[6][7] The psychrometric ratio is an important property in the area of psychrometry, as it relates the absolute humidity and saturation humidity to the difference between the dry bulb temperature and the adiabatic saturation temperature.

Mixtures of air and water vapor are the most common systems encountered in psychrometry.

The psychrometric ratio of air-water vapor mixtures is approximately unity, which implies that the difference between the adiabatic saturation temperature and wet bulb temperature of air-water vapor mixtures is small.

This property of air-water vapor systems simplifies drying and cooling calculations often performed using psychrometric relationships.

The "Mollier i-x" (Enthalpy – Humidity Mixing Ratio) diagram, developed by Richard Mollier in 1923,[17] is an alternative psychrometric chart, preferred by many users in Germany, Austria, Switzerland, the Netherlands, Belgium, France, Scandinavia, Eastern Europe, and Russia.