The Control Of The Testing Room Atmosphere.

n the previous section the instrument described are those used in the measurement  of the percentage R.H but it is often to control the atmospheric condition of a work room or testing laboratories.Some of the instrument described can be modified so that am air conditioning plant is controlled by them.Such control instruments are referred to by the makers as "hygrostats" or humidodtats. One controlled system is shown in a simplified from in which represent the system used in the testing laboratory of the Bolton technical college.The thermostats and hygrometer control valves operates by compressed air.The air input to the conditioning plant may come from outside or be recirculated air, or a mixture of both.In all instance it it filtered before heating and conditioning .A fan blows the conditioned air in to the laboratory through special ducts with louvered outlets in the ceiling.All the windows are double glazed used to prevent unwanted air coming in from the corridor when people pass in and out of the laboratory.Where a large air conditioning plant is not justified a self conditioned unit can be installed.In the Reynolds and Branson automatic unit two "towers" are used,one for adding moisture and one for subtracting moisture from the air.Each tower draws in air from near floor level  by an exhaust fan and discharges the air in an upwards direction.The wetting unit consists of a double walled cylinder which contact porcelain ring.A sprinkler fed from an automatic siphoning tank keeps the unit  saturated with water.The drying unit contains silica gel for the drying agent .

The automatic control of the humidity is accomplished by means of a sensitive hair suspension which, in conjunction with vacuum relays, operates the wetting and drying fans.Temperature control is provided bu thermostatically controlled heater of 1 Kw .The room temperature is brought to within a few degree of the desired controlled temperature,preferably by a space heating system which will ensure a reasonably constant temperature.The 1 Kw heater will then raise the temperature to, and maintain it at, a cabinet can be given automatically controlled humidity and temperature.