Humidity is defined as the water vapour content in air or other gases. Humidity is usually measured in terms of absolute humidity (the ratio of the mass of water vapour to the volume of air or gas), dew point (the temperature and pressure at which a gas begins to condense into a liquid), and relative humidity, or RH (the ratio of the moisture content of air compared to the saturated moisture level at the same temperature or pressure). (Wilson, 2005)

A hygrometer is an instrument used to sense humidity. Simple hygrometers are usually of the “wet bulb” or hair varieties. A “wet bulb” hygrometer contains two thermometers, with the bulb of one thermometer kept wet so that evaporation lowers its temperature - for temperatures above freezing, comparing the output of the two thermometers yields information on the ambient humidity. Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart.

A “hair” hygrometer measures the relative length of a stretched human or animal hair, which changes relative to the ambient humidity.

Capacitive humidity sensors sense water by measuring the change in capacitive between two plates. An example is the BCcomponents 2322 691 90001, in which a non-conductive foil is covered on both sides with a layer of gold. The dielectric constant of the foil changes as a function of ambient humidity, altering the capacitance value of the sensor. (BCcomponents) The incremental change in the dielectric constant of a capacitive humidity sensor is nearly directly proportional to the relative humidity of the surrounding environment. The change in capacitance is typically 0.2–0.5 pF for a 1% RH change.

Resistive humidity sensors use membranes that change conductivity in relation to water absorption. The impedance change is typically an inverse exponential relationship to humidity. The sensor absorbs the water vapour and ionic functional groups are dissociated, resulting in an increase in electrical conductivity.

Humidity sensors with integrated circuits for measuring relative humidity are available - in increasing order of durability - as SIP (Single Inline Package), SMD (Surface Mount Device), and TO-5 can devices. Additionally, there are analog and capacitive outputs as well as sensors measure both humidity and temperature. With regards to interchangeability of sensors, Honeywell reports 5% accuracy error. [source: Honeywell International 2008]

Humidity sensors have been used to detect finger contact at shakuhachi holes (Kanamori et al. ICMC 1993).

• Hygrometer
• Honeywell Humidity Sensors PTM
• Honeywell Humidity Sensors Line Guide
• Sensirion Digital Humidity Sensors (RH&T) - Overview