Humidity. Methods of measuring humidity. Hygrometer
Absolute and relative humidity. The main methods for measuring humidity. Dew point hygrometer, electric hygrometer, psychrometer and others sensors.
Air surrounding us has a certain amount of water vapor (water molecules). The maximum amount of water vapor in the air (air saturation with water vapor) depends on the temperature. The number of molecules in a saturating water vapor increases with temperature. Saturated with water vapor is produced, for example, the surface of the water with equal water and air temperatures - an equilibrium: the number of evaporating water molecules equal to the amount of condensable molecules.
Most ordinary air at a given temperature does not contain saturated with water vapor. But when the air temperature (decreasing, for example, at night), unsatisfying water vapor can pass into the saturation - the dew point. Dew point is the temperature at which water vapor in the air to become saturated.
The level of water vapor the air is characterized by high humidity. Absolute humidity ρ - the amount of water vapor contained in 1 cubic meter. meter at a given temperature. Relative humidity f - ratio of absolute humidity to the amount of vapor ρî, which is necessary to saturate the 1 cubic meter of air at a given temperature (ie the number that corresponds to the condition of the dew point temperature. ρî can be found in the tables).
f = (ρ/ρî)100%
Instruments for measuring humidity is called a hygrometer. Now mostly used the following methods and means of measuring humidity.
Dew-point hygrometer. Cooling (for example, using a Peltier element) small mirror placed in the sample gas stream. With photocell, fixing education mirror dew, is governed by its temperature. Measuring the temperature, determine the dew point. There hygrometers that use mirrors instead of the electrodes, the conductivity of which changes as a result of sweating.
Psychrometer. The device consists of two thermometers - wet and normal. Analyzing the difference between the readings of thermometers determine the dew point. The lower the saturation of the air with water vapor, the more intense the water will evaporate from the surface of the wet bulb and the lower the temperature and the greater the difference between the two readings of thermometers.
Electrolysis hygrometer. Measured air flow (gas) is passed around the platinum electrodes with phosphorus pentoxide, which absorbs water. When voltage is applied to the electrodes of water decomposes into hydrogen and oxygen. Electrolysis current is proportional to the amount of absorbed water, ie absolute humidity.
Hygrometer infrared. By measuring the absorption of infrared radiation is the absolute humidity.
Electric hygrometers with a conductive film. Depending on the amount of absorbed water vapor changes the electrical resistance of the conductive film. Calibrate the film can be further studied to determine the absolute humidity of the gas.
Hair hygrometer. In such hygrometers will measure fat human hair extensions and defined humidity. Elongation under the influence of moisture content to be 2.5%.
Hygrometer with bimorph element. The basis of such a hygrometer is a metal coil spring, covered on one side moisture absorbing material. Under the influence of a substance absorbed water expands and changes the geometric parameters of the spring. Calibrated can be defined relative humidity (gas).