Siemens QFM81.21 DUCT HYGROSTAT, 30 TO 90% RHSiemens QFM81. 21 Duct Hygrostat, 30 to 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 Degree of