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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.
3.3.1 RELATIVE 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).
3.3.2 RELATIVE HUMIDITY CONTROL
Relative humidity levels are maintained by balancing continuous, minimised, extraction
of water by condensation with intermittent injection controlled by an electronic
controller.
3.3.3 RELATIVE 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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