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3. Measurements of Humidity
The right measurement of air humidity
To determine the relative humidity content of the air a variety of measuring methods, using corresponding test instrumentation, can be applied:
1. Hair hygrometer
The hair hygrometer is the classic instrument for measuring relative air humidity. The measurement principle is based on the hygroscopic properties of the human hair, of which the changes in length owing to humidity absorption (= elongation) and discharge (= shortening) are transferred to a pointer by way of a lever system. The corresponding relative air humidity is read from a scale in percent. Today, in place of human hairs specially developed synthetic or cotton fibres are also used as measuring elements.
As the elongation elements need to be kept constantly under spring tension for the pointer movement, fatigue may occur in the spring and in the elongation elements even after just a short time. External influences, such as direct sunlight and heat, play an even greater role in damaging the elongation elements. Measurements with the hair hygrometer should therefore be generally regarded with caution, and are actually only acceptable in conjunction with regular regeneration (such as by winding a wet cloth around it) accompanied by calibration with an aspiration psychrometer as described below.
2. Thermo hygrograph
The thermo hygrograph is a variant of the classic hair hygrometer. It is able to represent not only the relative air humidity, but also the temperature fluctuations, in graph form on a cylinder dial (see diagram below). This is essential for recording and evaluation of long-time values. In place of a pointer, two coloured pens are moved over a uniformly rotating cylinder dial. One pen draws the curve for the relative air humidity, the other the one for the temperature. The temperature is measured by a sensitive bi-metal ring, which controls the pen by way of its excursion on the cylinder dial.
As the mode of functioning of the thermo hygrograph is identical to that of the classic hair hygrometer, the same points of criticism also apply to it.
3. Aspiration psychrometer
The aspiration psychrometer is considered to be a highly precise instrument for measuring air humidity. A ventilator circulates the room air around two precision thermometers. One thermometer measures the normal room air temperature, the other the so-called ''dew point temperature'' (= latent heat). To do so, this second thermometer is moistened with demineralised water shortly before the measurement is taken. Based on the two temperatures and a psychrometric scale, the relative air humidity can be determined very accurately. State-of-the-art aspiration psychrometers perform the temperature measurements and the psychrometric conversion fully electronically using a microprocessor.
4. Electronic sensor measurement
A digital measuring device measures the relative air humidity electronically by means of a sensitive sensor technique. In this method, an electrolyte with varying relative air humidity changes its electrical conductivity, which is then electronically converted and represented in digital form as a readout on the display (see diagram above). Temperature fluctuations, dirty sensors and interference-sensitive electronics may, however, cause even these instruments to deliver false readouts.
Highly specified, but very expensive, electronic measuring devices are able to perform a so-called ''sensor check'' to verify the reliability of the measurement. In this process, the measuring cell is inserted into a reference cell before and after each measurement and the accuracy of the measurement is checked. The accuracy is to within 1 - 2 % relative humidity.
In view of the complexity and necessary handling expertise involved in the sensor check, use of an aspiration psychrometer to measure the room air humidity is preferable, because it is easier to operate and each measurement represents a calibration. To measure humidity in a stack of paper only electronic measurement using an electronic plunger hygrometer is possible. In this case the calibration is performed by way of a sensor check.
