Relative Humidity (RH) is a derivation of Volumetric Humidity, sometimes seen as Absolute Humidity, and it is expressed as a ratio in terms of a percentage in common usage. It is called relative humidity because the vapour content of the air is dependent on the temperature and pressure, relative to the maximum possible content of water vapour for the same parameters.
RH is critical in industrial process applications, as water vapour will condense to liquid water when the temperature falls below the dew-point temperature. The dew-point is a calculation of the mass-basis absolute humidity, more commonly known as the moisture content of the air. This is problematic in situations where mould, mildew, corrosion, oxidation or rusting, and similar detrimental effects are exacerbated by the presence of liquid water.
The RH formula itself is rather trivial. In word form: the Relative Humidity is equal to the partial pressure of water vapour present divided by the saturation pressure of water vapour, the result multiplied by 100 and expressed as a percentage.
The measurement of RH is dependent on the application, the most common device being a psychrometer or hygrometer containing both wet-bulb and dry-bulb thermometers or temperature sensors. The resulting temperatures are generally recorded on strip charts on drums, or can be used as a humidistat with functions similar to a thermostat to initiate some process or action through associated electronic circuitry. Other measurement systems use infra-red radiation, taking advantage of water's characteristics of absorption and radiation of heat in a very predicable manner to trigger electronics similarly as a humidistat.
Humidity control by means of humidity measurement is fundamental to mitigate damage to delicate circuitry, rare or expensive woods, precision machinery including weapon systems, and other devices where moisture exposure (or lack of moisture) will deteriorate or impact an item's function or value. Humidity control can be complicated by its environment. In closed or enclosed situations, custom-designed enclosures such as a flexible storage solution for the protection of a mothballed armoured tank, all the air can be scrubbed dry with impunity. In buildings or enclosed areas where humans are likely to be present, some degree of humidity has to be allowed for survival. Physical size, dimensions, available power source options, and air-handling capacity must be taken into consideration; all are dependent on the practical application and must have sufficient ability to remove or control moisture content.
Humidity indicators, whether they are electronic digital readouts or analogue charts, give human operators an ability to precisely monitor humidity and initiate protective measures. Automated humidity control systems use humidity sensors to test humidity levels and can control processes, such as chemical dryers, required to maintain a specific relative humidity. The degree of humidity control is proportional to the value of the items being protected and their ability to withstand the effects of humidity exposure over time.