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Measurement of temperature and humidity within kilns is traditionally achieved using a wet and dry bulb hygrometer. A wet and dry hygrometer consists of an ordinary thermometer, the dry bulb, and a second thermometer, the bulb of which is enclosed by a fabric sleeve which is kept wet with distilled water from a reservoir (see figure below). By reading the temperatures of the dry and wet bulb, both thermometers being in direct contact with the circulating air of the kiln, a record of the amount of cooling due to evaporation of water from the wet bulb is obtained in terms of degrees depression or lowering of the wet bulb thermometer temperature below that of the dry bulb thermometer. Thus the higher the relative humidity the lower the wet bulb depression. At 100 % relative humidity the wet bulb depression will be zero. The relative humidities corresponding to various wet bulb depressions are given below. The equilibrium moisture contents of wood for these conditions are also shown. While wet and dry bulb hygrometers are the traditional means of measuring temperatures and humidities within kilns, other means are being increasingly used. Thus humidistats which measure humidity directly can be used to control relative humidity. Computer based systems can also be programmed to operate kilns according to a pre-determined schedule, while more advanced systems are able to control a sequence of kiln conditions on the basis of the moisture content of the timber which is monitored by remote sensors in the load.
 Wet and Dry Bulb Hygrometer.
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