Basic science

Absolute humidity is the mass of water vapour in a given volume of gas at a specified temperature and pressure. It is measured as mass per unit volume, for example g·m⁻³ or mg·l⁻¹.

Relative humidity is the ratio of the mass of water vapour present in a volume of gas to the mass required to fully saturate that same volume. It is expressed as a percentage.

Cooling or compressing a gas has no effect on absolute humidity, but causes a rise in relative humidity because cooler air holds less water vapour. Where the relative humidity reaches 100%, condensation occurs. This is known as the dew point.

Hygrometers are used to measure humidity. Depending on the device, it will either measure absolute or relative humidity.

The hair hygrometer consists of human or animal hair attached to a spring. As humidity rises, the length of the hair increases, which subsequently moves a pointer over a scale.

The psychrometer (wet and dry bulb hygrometer) consists of a wet wick enclosing a thermometer. Evaporation of water from the wick cools the thermometer. The rate of evaporation is directly related to the relative humidity. Relative humidity is determined by comparing the temperature of the wet thermometer to ambient temperature.

Regnault’s hygrometer bubbles air through a silver tube containing ether, which evaporates and cools the tube. Condensation on the tube indicates the dew point. Measuring the temperature at this point allows determination of the absolute humidity from reference tables.

The gravimetric hygrometer measures the mass of an air sample and compares this to the mass of an equal volume of dry air. It is very accurate and measures absolute humidity.

Humidity transducers measure the change in resistance or capacitance of a material such as lithium chloride when it absorbs moisture, and translate this into humidity.

Other methods include mass spectrometry, Raman spectroscopy and absorption spectrophotometry.