Instead of worrying about an almost meaningless number like relative humidity use an instrument like the one pictured in this post and keep track of the dry & wet bulb temperature.
How do Hygrometers Work?
Wet and dry bulb hygrometers or psychrometers are the most simple and common way of measuring humidity. This type of hygrometer uses two basic mercury thermometers, one with a wet bulb one with a dry bulb. Evaporation from the water soaked sock on the wet bulb thermometer causes its temperature reading to drop, causing it to show a lower temperature than the dry bulb.
Relative humidity is calculated by comparing the readings using a
calculation table that compares the
ambient temperature (the temperature given by the dry bulb) to the difference in temperatures between the two thermometers.
A mechanical hygrometer uses a slightly more complex system, based on one of the first hygrometers designed in 1783 by
Horace Bénédict de Saussure. This system uses an organic material (usually hair) that expands and contracts as a result of the surrounding humidity (that also explains why you always seem to have a bad hair day when it’s hot and humid!).
One problem with a mechanical hydrometer is that the temperature can affect the tension of the sensitive spring that attaches the needle to the hair or other organic material that expands or contracts with the moisture in the air.
I don't now, nor will I ever trust any hydrometer reading taken in a still-air incubator.
