How does the humidistat work on a power fan?

The function of the humidistat is based on the expansion and contraction of a band of nylon in the humidistat.

The function of the humidistat is based on the expansion and contraction of a band of nylon in the humidistat. As the nylon band becomes wet through higher humidity air, it begins to expand a given amount for each level of increase in relative humidity. As the air dries, the nylon band will begin to shrink. The nylon band is visible through the holes in the cover of the controller but it is easier to see with the cover off. The humidistat operates by using a spring-loaded switch which keeps a little tension on the nylon band. By turning the control knob, a contact is moved closer or further from the nylon band. Increasing the distance between the nylon band and contact will require the nylon band to expand more to make contact, thus it will need more humidity. Decreasing the distance will mean the nylon band needs to shorten to lose contact, thus dry out. Whether wetting or drying, there is going to be a significant adjustment time for this nylon band to be dried or wetted. If the power vent is not on and the contractor turns the knob to make contact, it may take 1 or 2 hours for the humidistat to dry out and turn off. Another factor affecting humidistat reaction time is the humidistat's accuracy range which is 7% Relative Humidity. Thus, if the humidistat were set to 80% the attic may need to rise to 87% to turn the sensor on; then as the attic dried it may need to drop to 73% to turn off. The 7% accuracy variation is completely independent of the time it takes for the nylon band to react. The most accurate relative humidity sensors are accurate to 1% RH. Actually, they do not measure the relative humidity. They measure the dew point of the air, the temperature of the air and then calculate the relative humidity from those two measurements. The drawback for the accuracy is  cost.