How humidity control works

We see this question submitted at our website often (in this case, word for word). Can you control only humidity? Not with any accuracy or repeatability. If you just want to get as dry or humid as possible, then skipping temperature control is acceptable. But you still need to measure temperature to calculate relative humidity. Relative humidity is actually relative to the temperature. The higher the temperature, the more moisture the air can hold at a given temperature. Relative humidity is the ratio of the amount of moisture in the air vs. how much it can hold. And herein lies the best reason to have temperature control: If the temperature changes, the relative humidity changes. So a stable temperature helps ensure a stable humidity level.

Here are two other reasons why temperature control can help humidity control

1. You will need a mechanical system to dry or “dehumidify” the chamber. A refrigeration system is ideal for doing this. A cold cooling-coil in the chamber attracts moisture in the air, causing it to condense into water, which can then be drained away. So the refrigeration system helps control the temperature AND the humidity. Chemical desiccant materials (like silica, which you may find in little packets with items shipped from overseas) would need to be repeatedly replaced or dried. There are mechanical systems that allow you to continuously use a chemical desiccant by drying a portion of the chemicals while the others dry the test air.
2. Humidification can change the temperature. Steam humidification heats water to create vapor. Warm vapor heats the air, increasing the overall temperature. There are systems that spray (or atomize) the water to create water vapor. As the water particles become a vapor, they absorb energy, cooling the air. If the air isn't warm enough to vaporize the water, it will create a mist, instead. We at ESPEC think temperature control is so important to humidity control that in our Platinous and benchtop models, we actually keep the refrigeration system on over the entire humidity-controllable range. (This also avoids sudden spikes of cold when refrigeration is called for (in on/off systems), which attracts moisture, causing sudden drops in humidity.) Then we use heat, which is easier to turn on and off, to control the temperature. And if the temperature is being controlled accurately, it is much easier to control the humidity level.