measuring the relative humidity. Provided that ne extreme temperatures and very low humidities ( <20% ) occur, these instruments meet the general requirements of accuracy (+3 to 5%). Hair hygrographs are the most frequently used standard recording instruments for humidity measurements in international networks. However, the following disadvantages have to be noted nonlinear response to humidity changes, changes in zero point require frequent cleaning and recalibration; sensitivity to destruction or errors through chemical pollution (e.g. ammonia gases, low response rate at low temperatures. - There are simple hygrometers for direct readings and hygro graphs with adjustable mechanisms for direet recording on cylindric drums driven by clockwork. Similar hygrometric sensors have been constructed, where the change in length of the hair bundle operates an electrical potentiometer, which allows for remote control. Suppliers AB Psychrometers - Psychrometers basically consist of two thermometers one of which is measuring the air temperature (dry bulb thermometer) while the other is covered and kept moist to measure the "wet bulb temperature. For accurate measurements, provision has to be made - for appropriate protection aganst radiation - to avoid error due to conductive heat • - to ensure adequate wetness of the wet bulb thermometer - that the thermometers used have approximately the same lag coefficient. Distinction is made between simple psychrometers without ventilation and artificially ventilated psychrometers. Nonventilated psychrometers are still in general use at climatological stations, but as ventilation (up toms) affects the results considerably, the accuracy of these instruments vary with the conditions of natural ventilation. The error will usually be about 5% relative humidity but can reach 10$ in dry air.
10 Artificially ventilated psychrometers, such as the Assmann type, the aspirated screen type, and the whirling type, are desiged to eliminate these errors to a large extent by providing forced ventilation of at least 2.5 ms. (The sling or whirling type, however, needs particular attention to avoid errors caused by radiation. The general accuracy of these psychrometers depend mainly on the accuracy of the thermometers and the wet bulb reading. (An error of 0.5 degrees centigrade in the wet bulb reading will cause an error of Min relaltive humidity at -25 degrees centigrade but only 2% in relative humidity at +15 degree centigrade air temperature. Psychrometers using liquid in glass thermometers allow for instant readings only. Recording psychrometers have been designed by using electrical temperature sensors and providing for permanent water supply for the "wet bulb" thermometer. Modern microprocessors, by applying the psychrometric formula immediately to the temperature (dry and wet bulb) readings of such psychrometers, allow for continuous recording of air humidity in technical units. (SuppliersrA 1-5, 5, BC M) 2.2.223 Absorption method - Electrolytes which change their electrical resistance with relative humidity are used for measuring and recording the vapour content of the atmosphere. Lithium chloride or sulphanated polystyrene layers are most commonly used for these purposes. The nonlinear resistance/humidity response of these sensors has to be considered as a disadvantage, especially in very dry or very humid conditions. The fact that these electrolytical sensors are easily contaminated by gas, smoke or oil vapours contained in the air necessitates great care in handling. Suppliers AH, B 5) - There are also small humidity sensors which change electrical capacity with humidity. Their size (less than 1 cm ) and fast response (approx. sis of particular interest for agricultural meteorological applications. With an appropriate microelectronic circuit, a linear (approx. 1$) DC. voltage output can be achieved. Suppliers CC Humidity sensors using the condensation methods - Dew point hygrometers indicate dew point temperature rather than relative humidity. Using i the i pr incl pl e of cooling an air sample - which has to be delivered without changing its watervapour content to a measuring unit - until condensation takes place, these instruments are, in general, more complicated, more expensive but more accurate. One such instrument, an illuminated condensation mirror, is alternately cooled and heated by a circuit energised by a photocell relay which maintains the mirror at dew point temperature.