PhysicsMeasurement of humidity

Humidity defines the amount of water present in the vapour phase and is measured by hygrometers. Absolute humidity (HA) is the mass of water present in a volume of gas. Relative humidity (HR) is the ratio of water vapour partial pressure to saturated vapour pressure at a stated temperature, usually quoted as a percentage. The dew point (Td) is the temperature at which condensation first occurs. Conversion between these measurements is possible if the ambient temperature is known. Controlled humidity levels in operating theatres provide a comfortable working environment and minimize patient heat loss and the accumulation of static charges. Instruments which measure relative humidity include the hair hygrometer, wet and dry bulb hygrometer, and polymer sensors. Other hygrometers measure dew point temperature. Automated devices using light sensors to detect condensation on a mirror have superseded manually operated types such as Regnault's hygrometer. Absolute humidity is measured by weighing all moisture removed from a gas by cooling or chemical desiccants. This method is extremely accurate but takes many hours and is mainly used to calibrate other hygrometers.