Most of the above humidity generation methods utilise some form of negative feedback to control the relative or absolute humidity in the wet air they output. In order for such a system to work, it is necessary to accurately measure the air's humidity. Humidity can either be measured directly; or indirectly, by measuring the dew-point and the temperature, and thus inferring the humidity. Some methods, such as the two-pressure method, assume saturation and evaluate resulting humidities using measurements of just temperature and pressure. Temperature and pressure measurements can be performed by any standard apparatus including thermometer or pyrometer, manometer or Piezoelectric respectively.
The simplest type of hygrometer is a Psychrometer. This constitutes a pair of temperature sensors. These are placed in a wet air stream. One of the thermometers (the wet bulb) has a wet wick around it; evaporation causes this thermometer to read a lower temperature than the other. The difference in temperatures, and the absolute temperature of the wet thermometer can be used to calculate the relative humidity of the air.
· A is the psychrometer constant (typically, values of A, for Tw above 0°C, are around 5×10-4 to 10-3)
Alternatively, for a given Psychrometer, the values obtained can be looked-up in a reference table.
Mechanical hygrometers utilise the change in dimensions of various porous materials (such as wet paper and hair) as they absorb/exude water vapour. This change in dimensions can be used to move a needle or dial; which, when calibrated, will give the relative humidity.
Electrical impedance sensors measure the changes in electrical capacitance or resistance of a hygroscopic material. The material will absorb or desorb water depending on the partial vapour pressure in the atmosphere around it, thus changing its electrical properties. These sensors measure relative humidity. While capacitive hygrometers can withstand condensation, resistive ones usually cannot.
Dew-point sensors measure the temperature of the surrounding air, and the dew-point of a small sample thereof. The humidity of the air is then inferred from the dew-point temperature and the initial air temperature. Optical sensors cool a surface (usually a mirror) until condensation starts to form. This indicates that the surface is at the air's dew-point. Other methods of measuring dew-point include measuring the oscillation rate of a quartz crystal. When condensation forms on the crystal, this rate will change.
Other sensors utilize changes in other properties, such as colour, thermal conductivity and acoustic transmission, to determine relative or absolute humidity. Electrolytic sensors relate the current flowing through a desiccant (usually phosphorus pentoxide) to the volumetric concentration of water vapour. Spectroscopic sensors use the emission spectra peaks of water to determine absolute humidity.