Relative Humidity and Controlled Environments
Humidity is defined as the water vapor content of a gas.
Absolute Humidity (AH) = (mass of water vapor / volume of air mixture)
Relative humidity is the amount of water vapor in air as a percentage of the amount needed for saturation at the same temperature.
Relative Humidity (RH) = (actual vapor density / saturation vapor density) x 100%
In most situations, both research and non-research, humidity is discussed in terms of relative humidity as absolute humidity is difficult to apply to most situations. For controlled environments, relative humidity impacts both the environment’s ability to control temperature and the life and growth of the contents. Relative humidity and temperature are closely tied in the small volume of a growth chamber as even small temperature changes cause significant changes in the relative humidity level. When humidity levels are important to the research being conducted, it is even more important that the temperature control is both stable and accurate.
Two Components to Controlling Humidity
There are two components to controlling or maintaining relative humidity levels in an incubator or growth chamber. The first component, adding humidity to the environment, is relatively straight-forward and can be accomplished in a variety of ways based upon the size of the environment, the required temperature range, the light intensity levels and the needed relative humidity percentage. The most common ways are:
1. Pan-type or reservoir humidifier – the simplest system which consists of a pan/reservoir of water and, in most cases, some type of heater to assist with evaporative humidification. Usually it is controlled by a humidity sensor.
2. Ultrasonic humidifier – a more advanced system that uses a transducer oscillating at a high frequency to produce ultrasonic waves which are directed to the water surface, atomizing the water to an extremely fine mist. Again, generally controlled by a humidity sensor.
3. Spray nozzle / spray mist – a forced-air system that uses a valve / nozzle to spray mist into the supply air of the chamber, the mist then being circulated with the conditioned air.
The second component to controlling relative humidity is dehumidification. This is most commonly achieved in a controlled environment by the use of a refrigeration / cooling coil dehumidifier. This system consists of an electrical heater and one dehumidifying evaporator that is separate from the chamber cooling evaporator. When dehumidification is required, the temperature of the evaporator will drop to the point where the chamber moisture will condense on the coil. The heater acts to reheat the dried air back to the programmed temperature before the air is returned to the chamber environment.