Siemens QFM81.2 DUCT HYGROSTAT WITH EXTERNAL SETPOINT, 30 - 90% RHSiemens QFM81. 2 Duct Hygrostat with External Setpoint, 30 90% RH Hygrostats are used for controlling and monitoring relative humidity in facilities under HVAC control. They ensure duct humidity control within the selectable range of 15 to 95% relative humidity. The hygrostats do this by controlling humidification or dehumidification equipment. Duct hygrostats can be used as minimum or maximum limiters in the supply air duct. Additional Information