A psychrometer is the device serving for definition of humidity. Psychrometer August (Fig. 1) consists of two identical thermometers, attached to the tripod. Mercury bulb one of the thermometers wrapped cloth moistened (gauze or Baptiste), the ends of which is omitted in a container with distilled water. When water evaporates from the surface of the fabric covering mercury bulb temperature mercury decreases the more the lower the humidity (see) air at the point of measurement. The difference in the readings of dry and wet thermometers is inversely proportional humidity. Calculation of results of definition of produce according to the formula Regnault or psychrometric table. To determine the humidity of the psychrometer put in the place, where they definition, and in 10-15 minutes record the testimony of dry and wet thermometers (mercury bulbs, thermometers shall be protected from the action of radiant energy of sunlight and heating appliances). In the calculation you enter a correction factor for the speed of air movement. In aspiration psychrometer of Assmann (Fig. 2)
mercury thermometer balls protected from the action of radiant energy of the cases with double walls. The device is equipped with a fan that provides a constant speed of air movement in mercury bulbs thermometers (2 m/s). Before defining the fabric covering mercury wet bulb, moistened with distilled water. Excess water is removed by shaking, after which include fan and the device is placed at the point where you need to make the determination. When the temperature of 15-20° reading the testimony of the thermometer is made in 4 minutes At temperatures below 15 degrees duration pulling air increased to 20-30 minutes (until then, until a constant temperature and wet bulb temperature). Calculation of results of definition of produce according to the formula of Sprung or in a special table.
the psychrometers

Fig. 1. Psychrometer August. Fig. 2. Psychrometer of Assmann: 1 - fan, 2 - psychrometric thermometers; 3 - eyedropper to wet wet thermometer.

Psychrometer (from the Greek. psychros - cold and metreo " measure") - the device for measurement of humidity and temperature of air.
The principle of the psychrometer is based on the difference in readings of dry and wet thermometers depending on the humidity of the air. Psychrometer consists of two identical mercury thermometers with graduated scale (up to +50 degrees and 25 degrees) with an interval of divisions of 0.2 degrees. The tank right thermometer wrapped Baptiste and before the beginning of measurements moistened with distilled, rain or snow water. The intensity of evaporation of water from the surface of the tank wet bulb will depend on the moisture content of the investigated air and its mobility. The drier the air, the more intense will evaporate water, as well as the evaporation of water is connected with the cooling of the body, from where it evaporates, then dipped thermometer will indicate a lower temperature than dry, and this difference readings will be greater, the drier the air. Dry thermometer will indicate the temperature of the surrounding air. According to the testimony of dry and wet thermometers define humidity, using the psychrometric tables, and temperature - indication dry bulb thermometer. Measuring range relative humidity from 10 degrees to 40 degrees.
In the USSR produced three types of psychrometers: the station, aspiration and presenilny. Of them more precise and easy to use is the aspiration psychrometer, which mainly use sanitary practices and the conduct of scientific research. The station psychrometer is less precise; it determines the humidity of the air in the immediate vicinity of the instrument; it has been adopted in the meteorological stations as the main instrument. Presenilny psychrometer in sanitary practice is not used.
In aspiration psychrometer (Fig) mercury thermometers is fixed in a special metal frame in the form of pipes, the outer surface of which is carefully polished and neklinovka. Tanks thermometers placed in double tube protection, which protects them from the heat of the sun. In the upper part of the instrument is winding mechanical or electrical mechanism with fan, designed for broaching investigated air at a constant speed through tubes surrounding the reservoirs thermometers. The plant mechanism - 8-10 minutes Aspiration psychrometer carried out of the premises in the summer for 15 minutes, in the winter for 30 minutes prior to the observation and hung in an open area on a pole or a tripod to the height of 2 m above the ground and away from any objects that are able to radiate heat. Moisten Baptiste on the tank thermometer in the summer for 4 minutes, and winter for 15 minutes before the beginning of the observation. Get the key (or by connecting to the mains) the fan mechanism almost to capacity and in 4 minutes quickly produce countdown evidence thermometers (first decimal degrees, and then the whole). The air flow rate affects the accuracy of the readings, so the winding mechanism of the fan should work with the same speed, not less than 2 m/s. A strong wind (over 4 m/s) at slots aspirator put on the windward side guard. Psychrometer allows you to define humidity at temperature of air not below -10 degrees.
Calculation of absolute and relative air humidity on the testimony of the psychrometer is produced by special psychrometric tables.
Example. Suppose that the testimony of the dry bulb temperature of 21.6 degrees; the amendment to the thermometer on the verification svidetelstvo 0.1 degree. Indications wet bulb 14,4 C; the amendment to the thermometer on the verification certificate -0,1 degrees. Fixed thermometers are: dry 21,6°+0.1 deg=21,7 degrees soaked 14,4°-0,1°= 14,3 degrees. Atmospheric pressure P = 1040 MB. On the first table in the column of t - 21,7° take line t1 = 14,3 degrees and in the first column, find the number of n-16. In another table when P = 1040 MB, n-16 (for aspiration psychrometer) find Δt1=+0,4 degrees; found value added to the testimony wet bulb temperature t1= 14,3°+0,4°= 14,7°, then in the first table on t=21,7 degrees and t1 = 14,7° we find the values of absolute humidity e= 11,2 MB and a relative humidity of "r"=43%.
Detailed instructions psychrometer and calculation of the results of observations, and the care of the psychrometer is contained in the instructions to the device (GOST 6353-52).

Aspiration psychrometer of Asman: 1 - thermometers: 2 - metallic tube; S - tubes for tanks thermometers; 4 - fan; 5 - the device for hanging; 6 - eyedropper to wet wet thermometer.