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Humidity and Thermal State Behaviour

Working Group 3.41

Hygrometers and their functional principles – an overview

To measure gas humidity, measuring instruments with different functional principles are used. The general term used for these instruments is "hygrometer"; recording instruments are also called "hygrographs". Description of the main functional principles:

  • A dew point hygrometer measures the dew or frost  point of a humid gas. The dew point is the temperature at which a humid gas is saturated with water at a given pressure. If saturation with ice is considered, one uses the term "frost point". To this end, the humid gas is conducted, in a dew point hygrometer, via a small mirror that is cooled down thermoelectrically. As soon as condensation (dew or frost formation) starts forming at the surface of the mirror, this condensation is detected via an optical system due to the change in the reflective properties of the mirror. By means of a feedback circuit, the temperature of the mirror is kept at exactly the level at which the water starts to condensate from the humid gas. This so-called "dew-point temperature" is a measure of the absolute moisture content of the gas. If this temperature is lower than 0 °C, the dew layer present on the mirror may, in principle, transform into a frost layer. In this case, it is referred to as the "frost-point temperature".

  • A polymer sensor uses the change in the properties of a special synthetic material (polymer) as a function of the ambient humidity. The measuring signal of such a sensor is always a measure of the relative humidity, i.e. of the percentage of the maximum possible humidity at a given temperature. Simple polymer sensors exploit the change in the electrical resistance of a polymer layer (resistive polymer sensors), whereas higher-end devices have metallic surfaces applied to the sides of a small polymer plate and measure the change in the dielectric constant of the polymer as the change in the capacitance of the arrangement (capacitive polymer sensors). If the measurement temperature is known, it is possible to convert relative humidity into absolute humidity.

  • In a spectrometer, the humid gas is led through a measuring cell into which light of an accurately defined wavelength is irradiated on the one side, whereas the decreasing intensity of the light is measured on the other side. The light sources used are primarily lasers or laser diodes. The wavelength of the light is chosen in such a way that the energy from the light excites the water molecules that are being present into oscillating, which leads to energy from the light being absorbed. The ratio of the intensity of the irradiated light to that of the light exiting the measuring cell is therefore a measure of the concentration of water molecules present in the measuring cell – and thus a measure of the absolute humidity.

  • A psychrometer is a conventional hygrometer that exploits the fact that a humid object (e.g. cotton, etc.) cools down due to evaporation when air is blown onto it. If the humidification of that object is ensured at all times, the resulting cooling is greater the lesser humidity the air used for blowing onto the object contains. The "psychrometric difference" is the difference between the temperature of the air measured with a dry thermometer and the temperature of the humid object. This difference is usually measured by pulling a wet cotton sock over a thermometer. A conventional Assmann psychrometer contains two mercury glass thermometers, one of which is kept humid by means of the above-mentioned cotton sock. The required airflow is generated by means of a ventilator. Today, thermometers are often designed as platinum resistance thermometers (PRTs). From the psychrometric difference, it is possible to calculate the humidity of the air flowing above the arrangement.

  • Hair hygrometers or twisted-fiber hygrometers hardly play a role today; they are still found in instruments for domestic use in the form of esthetically pleasant dial instruments. These instruments rely on synthetic fibers (horse or human hair was formerly used, hence the name) stretching with increasing relative humidity, this stretching being transmitted to a dial via a lever system.