Technical Instructions
QFM81 Series Duct Hygrostats
Document Number 155-707
April 21, 2010
Page 2
Siemens Industry, Inc.
X
d
t
t
1
3
2
w
x
S1
r. h. [%]
SEN0507R1
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
If the relative humidity exceeds the adjusted setpoint, the potential-free contact of the
microswitch will change over from 1
−
2 to 1
−
3. If the relative humidity falls by the amount of
the fixed switching differential X
d
, the contact will return to the position 1
−
2.
Mechanical
Design
QFM81.2
The hygrostat consists of a base with immersion sensor stem and cover. The cover is
secured to the base with a screw.
The stem accommodates the temperature-compensated humidity sensing element
(stabilized plastic strip). The strip is mechanically linked to the microswitch via a transfer
lever. Transfer lever, microswitch, setpoint setting element and connection terminals for
connecting the humidification or dehumidification equipment are mounted on a printed
circuit board inside the base. The connection terminals are protected by a hinged cover to
avoid direct access when the cover is removed.
The cover has a hole for the setpoint knob. The hygrostat is designed for mounting in air
ducts, but can also be mounted on a wall. For both mounting methods, a mounting flange is
required, which is supplied with the unit.
QFM81.21
Same design as the QFM81.2, but with an additional transparent cover on the hole for the
setpoint knob, cable gland Pg 11, and seal under the unit cover.
Setting Elements
Setpoint knob
On both units, the setpoint is adjusted with the setpoint knob. The setting scale is on the
unit cover.
With the QFM81.21, the setpoint can be adjusted only when the cover is removed.