Green Air THC-1 Total Humidity Controller 

Control humidity automatically and accurately with Green Air Products THC-1 Total Humidity Controller. Switches 120V at 15 amp maximum. Sensor allows a 7% (differential) comfort zone between "on" and "off" functions. 

The THC can be switched between humidify and dehumidify for removing unwanted atmospheric moisture accumulating within an enclosed area or to add needed moisture to the air by means of humidification.

The THC-1 humidity switch controls humidity equipment by providing power when humidity varies from the adjustable set point.

In the dehumidifying mode high humidity conditions will activate fans or other dehumidifying apparatus until level drops approximately 7% in relative humidity. At this point dehumidifying equipment is disabled until humidity increases again 7%.

In the humidifying mode this control operates fogging or other humidifying equipment by activating switches, motors, valves or pumps. Humidifiers can be precisely controlled according to atmospheric humidity.

More info from the Manufacturer (PDF format - Lots of detail.)