QFM81 Series
Technical Instructions
Document No. 155-707
April 21, 2010
Duct Hygrostats
QFM81.21
QFM81.2
Siemens Industry, Inc.
Description
On / off hygrostat with microswitch, and temperature-compensated humidity sensor for
temperature-independent humidity measurements.
Features
•
Stabilized sensing strip (good linearity, very stable even at high humidity, insensitive
to dust and contaminated air).
•
Can be mounted in ventilating ducts or rooms.
Application
For controlling humidification equipment and dehumidification equipment.
Product Numbers
Product
Number
Setpoint Range
(W
h
)
Switching
Differential
(X
d
)
Degree of
Protection
Setpoint
Adjustment
QFM81.2 IP
30
Externally
QFM81.21
15 to 95% r.h.
Approx. 4% rh
IP 55
Internally
NOTE:
Includes a mounting flange (for duct or wall mounting) and a sealing ring (for
duct mounting).
Mode of Operation
The hygrostat acquires the relative humidity of the air with its humidity sensor, 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).