When the FM systems belong to a group, humidity is managed using dewpoint control. Humidity readings and setpoints are shown in %RH but are converted to dewpoint internally for control. Dew point is a function of both temperature and humidity so you must take temperature into account when calculating humidification demand.
The dew point temperature setpoint for humidification is calculated from the humidification setpoint and the reheat setpoint (use cooling setpoint if reheat is not present). The dew point of the room is calculated from the return humidity and the return temperature.
The dew point temperature setpoint for dehumidification is calculated from the dehumidification setpoint and the cooling setpoint.
For example, if the reheat setpoint is 68 F and the humidification setpoint is 50%, the dew point setpoint is 49 F. If the room temperature is 74 F and the return humidity is 40% the dew point of the room is 48 F.
The humidifier demand is calculated as the difference between the actual room dew point and the desired room dew point divided by the sensitivity.
So in this example; Demand = (49 - 48) / 2 = 0.5 or 50% demand. So the humidifier output will be about 50%.
There is one other note. Versions of the FM controller firmware v6.7.0 and earlier do not calculate low and high humidity alarms correctly. This has been corrected in version 6.11.0 which has just been released. I'm in the process of putting the new version on the firmware download page of apc.com, but if you can't wait, send me an e-mail and I'll forward it to you.
COOL SETPOINT / DEHUMID SETPOINT = DEWPOINT HIGH LEVEL
REHEAT SETPOINT / HUMID SETPOINT = DEWPOINT LOW LEVEL