Siemens QFM81 Series Owner's manual

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
(Wh) Switching
Differential
(Xd)
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 Xdand 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).

Technical Instructions QFM81 Series Duct Hygrostats
Document Number 155-707
April 21, 2010
Page 2 Siemens Industry, Inc.
X
d
t
t
13
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
dSwitching 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 Xd, 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.

QFM81 Series Duct Hygrostats Technical Instructions
Document Number 155-707
April 21, 2010
Siemens Industry, Inc. Page 3
Mounting Notes
Air Duct Mounting
If the duct hygrostat is used for control, it must be mounted in the exhaust air duct after
the room to be controlled.
If the duct hygrostat is used for monitoring the maximum or minimum humidity level, it
imust be mounted in the supply air duct.
Mounting Positions The immersion sensor stem must be mounted either horizontally or vertically with the
stem pointing downward. Do not mount with the stem pointing upward.
To ensure accurate humidity measurements, it must be made certain that the air to be
measured is sufficiently mixed at the point of measurement.
SEN0508R1
SEN0509R1
<5m/s
>5m/s
Figure 2. Acceptable Mounting Positions.
The mounting orientation is dependent on the air velocity in the ductwork: at <5 m/s, the
holes in the stem must face the airflow; at >5 m/s, they must be perpendicular to the
direction of airflow (see Figure 2).
Minimum immersion
length When mounting the hygrostat in air ducts, the minimum immersion length of the stem 5.12 inches (130 mm) must be observed. The mounting flange supplied with the unit
allows the immersion length to be adjusted between 5.12 inches (130 mm) and 6.14
inches (156 mm).
Wall Mounting The hygrostat should be mounted on an inner wall approximately 4.9 feet (1.5 m) above
the floor and at least 1.6 feet (0.5 m) from the closest wall.
Mount the unit where there is a natural circulation of room air (do not mount near drafts,
in corners, behind curtains, too close to doors and windows, or on an outer wall).
Sources of heat and refrigeration (radiators, computers, televisions, concealed heating
pipes, hot or cold water pipes) must be at an adequate distance.
The hygrostat should not be exposed to direct sunlight.
For wall mounting, the mounting flange supplied with the hygrostat must be used.

Technical Instructions QFM81 Series Duct Hygrostats
Document Number 155-707
April 21, 2010
Page 4 Siemens Industry, Inc.
Specifications
Setpoint range 15 to 95% rh
Control mode On/off
Switching differential Approximately 4% rh (fixed)
Type of switch Potential-free microswitch (SPDT)
Contact rating
Maximum 5 (3) A, 24 Vac/Vdc
Minimum 100 mA, 24 Vac/Vdc
Temperature influence Compensated
Long-term stability Approximately −1.5% rh/a
Balancing At 55% rh, 73°F (23°C)
Time constant (v = 0.2 m/s) Approximately 3 minutes
Permissible air velocity 10 m/s
Permissible ambient temperature
Operation 32°F to 158°F (0°C to 70°C)
Storage/transport –22°F to 158°F (−30 to 70°C)
Degree of protection
QFM81.2 IP 30 to EN 60 529
QFM81.21 IP 55 to EN 60 529
Safety class II to EN 60 730
conformity to low-voltage directive 73/23/EEC
UL UL873
cUL Canadian Standard C22.2 No. 24-93
Connection terminals for 20 AWG. minimum
2 × 16 AWGmaximum
Materials
Sensing element Polymer
Casing with stem PPS, Fortron 1140L6, fiberglass
reinforced
Cover PC Lexan 940
Transparent cover PC Makrolon 2014R, transparent
(only with QFM81.21)
Weight Approximately 12 ounces (0.34 kg)
Maintenance Maintenance-free, can be recalibrated
Wiring Diagram
SEN0510R1

1
23
1 – 2 Humidification
1 – 3 Dehumidification
Figure 3. Wiring Connections.

QFM81 Series Duct Hygrostats Technical Instructions
Document Number 155-707
April 21, 2010
Information in this publication is based on current specifications. The company reserves the right to make changes in specifications and models as
design improvements are introduced. Product or company names mentioned herein may be the trademarks of their respective owners.
© 2010 Siemens Industry, Inc.
Siemens Industry, Inc.
Building Technologies Division
1000 Deerfield Parkway
Buffalo Grove, IL 60089-4513
U.S.A.
Your feedback is important to us. If you have
comments about this document, please send them
to SBT_technical.editor.[email protected]
Dimensions
SEN0511R1
0.39
(10)
0.65
(16,5)
ø 1.18
(ø 30)
1.64
(41,7)
5.51
(140)
2.87
(73)
2.52
(64)
5.12...6.14
(130...156)
0.83 (21)
A B
Pg 11
9.69
(246)
0.22
(5,5)
1.06
(27)
0.22
(5,5) 2.13
(54)
A - B
2.24
(57)
1.12
(28,5)
0.24
(6) 1.10
(28)
2.67
(68) 1.65
(42)
2.99 (76)
2.13
(54)
Figure 4. Dimensions in Inches (Millimeters).
Document No. 155-707
Printed in the U.S.A.
Page 5