Technical Instructions
QFA1000, QFA1001 Room Hygrostats
Document Number 155-708
July 19, 2010
Page 2
Siemens Industry, Inc.
Mode of Operation
The hygrostat acquires the relative humidity of the ambient air with its humidity
measuring element, which is a stabilized, plastic strip. The strip actuates a microswitch
with a fixed switching differential X
d
and a potential-free contact output (SPDT),
depending on the relative humidity of the air. If the actual humidity deviates from the
adjusted setpoint, the hygrostat switches the associated humidification or
dehumidification equipment on or off as shown in the following function diagram
(Figure 1).
Figure 1. Function Diagram.
rh
Relative humidity in %
S1
Microswitch
1
−
2 Humidification
1
−
3 Dehumidification
w
Setpoint
x
Actual
value
X
d
Switching
differential
t Time
Due to the measuring element’s aging effect, the switching point drifts slowly and
constantly. For this reason, recalibration may be required in due time.
At temperatures other than the calibration temperature, the switching point drifts
systematically (temperature influence). Also, in the case of fast humidity changes, the
switching point will temporarily be shifted.
Mechanical Design
QFA1001
The room hygrostat is designed for wall mounting, and fits on most commercially
available recessed conduit boxes. The cables are introduced either from the rear
(recessed conduit boxes) or from above (surface-run wires), after removing the knock-
outs.
The unit consists of base and cover that can be separated (snap-on connection).
The base accommodates the humidity measuring element, setpoint setting element
with setting spindle, scale, microswitch, and screw terminals.
The cover carries the removable setpoint knob with its imprinted scale.
QFA1000
This model has the same basic design as the QFA1001, but without an external
setpoint knob. The setpoint can only be adjusted when the cover is removed.
