3.3. RELATIVE HUMIDITY (RH)
Relative humidity is a measure of the ratio of the amount of water vapour present in an atmosphere to the amount of water vapour that atmosphere could hold, ie. Ihe humidity present relative to the humidity possible. The latter varies with temperature, hence RH measurements are related to their prevailing temperature. Air at 100% relative humidity is saturated and cannot absorb any more water. If air at 100% rh is heated, the amount of water the air can hold increases and therefore the %rh will drop unless more water is introduced. If air at 100% rh is cooled, water will be lost through condensation on to the cooling surface.
For each atmospheric RH/temperature relationship there is another relevant temperature, known as the dew point. Surfaces at or below this temperature chill the water vapour sufficiently to cause condensation.
3.3.1RELATIVE HUMIDITY DETERMINATION
Air with a relative humidity less than 100% flowing over a wet surface causes evaporation of water from that surface and causes a temperature drop at the surface. The actual drop in temperature compared with the temperature of a similar dry surface subjected to the same air flow can be used to measure and control relative humidity. The technique is known as "wet & dry bulb psychrometry" and the difference in temperature is referred to as the "wet bulb depression". Psychrometric tables give interrelated values for temperature, relative humidity and wet bulb depression.
In the cabinet, conditioned air flows over two resistance thermometer (RT) sensors, one dry and one kept permanently wet by a wick fed from a constant level water reservoir. The RT's are connected to the programmer. The relative humidity is determined from the psychrometric difference between the wet and dry RT's. The reference temperature for humidity control is compensated for the actual chamber temperature. Errors are corrected by actuating the vapour phase generator (vpg).
3.3.2RELATIVE HUMIDITY CONTROL
Relative humidity levels are maintained by balancing continuous, minimised, extraction of water by condensation with intermittent injection controlled by an electronic controller.
3.3.3RELATIVE HUMIDITY INCREASE
All cabinets are fitted with a vapour phase generator (VPG).
Vapour phase generation
Water vapour produced in the vpg is introduced into the treatment chamber and is absorbed by the air.
The vapour phase generator is provided humidification from 15% to 95% rh at 60°C to 50to95%at20°C.
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