Hygrometers are instruments used to measure the relative humidity of the air. This is defined as the ratio between absolute humidity, i.e. the amount of water vapour present in the atmosphere at a given time, and saturation humidity, i.e. the maximum amount of water vapour that can be present at a given pressure and temperature. This measurement is a function of other measurements, such as temperature, pressure, mass or a mechanical or electrical change in a substance when moisture is absorbed. Modern instruments use condensation temperature or changes in capacitance or electrical resistance to measure differences in moisture. Even in the laboratory, it is essential for some processes to calculate humidity. For this, hygrometers are common instruments in chemistry.
Because there is also a relationship with temperature in the calculation of relative humidity, it is possible to speak of thermohygrometers. These hydrometers are sophisticated instruments that can be digital or analogue. Digital models are equipped with LCD liquid crystal displays that allow the simultaneous reading of indoor or outdoor temperature and relative humidity. They also allow the storage of minimum and maximum temperature with the possibility of recall. The various models of digital thermo-hygrometers are characterised by their possible functions and measuring ranges. In addition to the obvious physical differences between models, such as weight and size, some have, for example, a temperature range of -40° +70°. Or a further factor is the accuracy of the temperature measurement or the humidity range covered. There are also 'XS' digital models. These have a more convenient design than standard digital models, as they are used when repeatability and speed are required.
Analogue models, on the other hand, measure temperature and humidity by means of hands, like a clock. They can be made of plastic material with a brass ring.
They are also made of plastic with a brass ring.
Hygrometers and hygrographs
It is important to emphasise the difference between hygrometers and hygrographs. The latter are in fact instruments similar to the former, but used to calculate the change in humidity over a given time interval. Hygrographs consist of a hygrometer, which measures the relative humidity of the air, connected to a recorder. Hygrographs can be digital or mechanical, usually paper-based. Like thermo-hygrometers, thermo-hygrographs are hygrographs that also take temperature measurement into account. They are made of a stainless steel case, painted with baked epoxy powder.
The barometric station is an instrument that combines several measuring instruments. It may consist of a hygrometer-thermometer and a barometer. The latter is a device that calculates atmospheric pressure.