Hygrometers are available to purchase through lab suppliers in digital and analogue form and the most basic versions are generally known as psychrometers or wet/dry hygrometers. These usually contain two thermometers, one dry and one with a wet bulb.
In the wet/dry type, evaporation from the wet bulb of the thermometer usually gives a lower temperature reading. This is known as the 'wet bulb temperature'. The dry bulb gives the 'dry bulb temperature' and the ambient humidity is calculated using the temperature values from both thermometers, using a psychrometric or hygrometric table.
Very accurate humidity readings can be taken from wet/dry hygrometers so long as the device has been calibrated precisely. A calibration guide is usually supplied with the device or can be obtained from the manufacturer.
Hygrometers have in the past been calibrated by placing the device into a plastic bag containing a mug of just-boiled water, adding salt to the water and leaving for several hours. The device should eventually read 75% and can be adjusted if the reading is not this figure. Newer devices are more accurate and so we recommend following the calibration guidelines for the particular model you have.
Humidity is normally measured as relative humidity (RH). RH is a percentage that indicates the amount of moisture in the air relative to the maximum amount the air can hold at that temperature. For example, when air at a given temperature contains all the water vapour it can hold at that temperature, it has a RH of 100 per cent. If the humidity exceeds 100 per cent, moisture will begin to condense from the air. If the air contains only half the water it can hold at that temperature, the RH is 50 per cent.
Whirling hygrometers consist of two thermometers; one wet and one dry but are held in a bracket which can be whirled much like a football rattle. The whirling hygrometer is whirled rapidly for thirty seconds, after which time a reading of the two thermometers is taken, with the wet bulb thermometer normally read first. This is repeated until 2 consecutive readings are obtained which are comparable. Relative humidity can then be determined by correlating the readings with those on the hygrometric table supplied with each instrument.
Hygrometers should not be placed near radiators or sources of heat as these environments may affect the humidity readings.
Fig 1 shows a manual wet/dry hygrometer. Fig 2 shows a digital version.