Appendix E: Gossary - ECD SmartDRY SD Series Setup And Operation Manual

APPENDIX E: GOSSARY

Relative Humidity (%RH):
The amount of water vapor currently in the air divided by the maximum amount of water vapor the air could
hold at that same temperature multiplied by 100, expressed in percent. Cold air cannot hold very much water
vapor, and hot air can hold much more. In fact, 5°C air at 100% relative humidity holds only 6.8 grams. This
means that at 5°C, air will go from 0% RH to 100% RH by adding (evaporating) only 6.8 grams if water. At 25°C
it takes 23 grams of water to reach 100% RH. So, the hotter the air the more water it takes to reach 100% RH,
or saturation. This also means that if relative humidity needs to be lowered quickly and there is no access to a
dehumidifier, raising the temperature will produce this effect. You can lower the room's RH from 70% at 20°C
to 55% simply by increasing the temperature to 25°C.
At any given temperature, 100% RH is defined as the point where the rate at which water is evaporating from
liquid (or ice, AKA: sublimation) to vapor (gas) and the rate at which water is condensing from vapor (gas) back
to liquid (or ice, AKA: deposition) has become equal. This means that relative humidity is valid below freezing
because the water (or ice) has the unique ability to go from solid to gas and back, maintaining the equilibrium
necessary to achieve 100% RH. In part, this is also the reason that relative humidity measurements have no
meaning over the boiling point of water, because the water (or ice) has all gone to vapor. The water is still in
the air as
vapor (gas), however, the small amount of vapor in the air at temperatures over the boiling point typically cal-
culates to a relative humidity value less than 2%. So it is more meaningful to measure absolute humidity, when
measuring moisture content in the air above 100°C.
SD Series Dry Cabinets Setup and Operation Manual | A55-7379-00 Rev: 2.0.0 Page 88