The absolute amount of water vapour in a gas expressed in either mg/L of gas mixture or mmHg (partial pressure).
Amount of water vapour in a gas expressed as a percentage of that which could be held by the gas if it were fully saturated at the same temperature, i.e.
R.H. = Actual water content / Water content fully saturated %, or
R.H. = Actual vapour pressure / Saturated vapour pressure %
Measurement of humidity
Wet and dry bulb hygrometer
A system using two thermometers, one with a wet and the other a dry bulb. Air movement over the wet bulb causes evaporative cooling, generating a difference in temperature readings. This difference relates to the rate of airflow over the wet bulb and the relative humidity. Tables are used to look up the relative humidity from the two temperatures.
Regnault’s (dewpoint) hygrometer
Using a precisely cooled shiny plate, the user observes the temperature at which condensation first occurs. At this temperature, the gas is fully saturated with water, hence both the water content and the relative humidity at any other temperature can be ascertained from a vapour pressure table.
Mass spectroscopy can be very accurate but only if condensation does not occur in the sample line. This is the best technique for assessing "in-circuit" humidity, as it can assess breath by breath changes.
Transducers are available in which the electrical conductivity of a membrane changes with water vapour pressure.