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Technical Instructions
Document No. CA1N1552E-P25
November 24, 2003
QBM81-…
Differential Pressure Switch
Description
Differential pressure switch for monitoring air pressure in HVAC systems.
Features
• Monitors air filters, airflow and fan belts
• Can be used to monitor pressure in clean rooms, kitchens, etc.
• Easy to mount
Application
The QBM81-... Differential Pressure Switches are used to monitor differential pressure,
underpressure and overpressure in HVAC installations. By measuring differential
pressure, they monitor the state of air filters, prevailing airflows, damaged fan belts and
overpressure in clean rooms, kitchens, etc. They are also used to monitor differential
pressure and positive and negative relative pressure in HVAC systems.
Product Numbers
There are three differential pressure switches available that measure various pressure
ranges:
Table 1. Product Numbers.
Ordering
Product Number
Pressure Range (Inches WC)
QBM81-3
0.08 to 1.20
QBM81-5
0.20 to 2.00
QBM81-10
0.40 to 4.00
If required, FK-PZ... duct probes must be ordered separately.
When placing an order, specify the quantity, product number and description.
Example:
1 QBM81-5 Differential Pressure Switch and one set of FK-PZ2 duct probes
Siemens Building Technologies, Inc.
Technical Instructions
Document Number CA1N1552E-P25
November 24, 2003
Operation
QBM81-… Differential Pressure Switch
The differential pressure between the two pressure connections deflects a springloaded diaphragm. This special diaphragm ensures long-term stability of switching
points.
Each type is engraved with individual scales for highly accurate adjustment. The
options for adjustment are shown in Figure 2.
Switch State
XD
1
2
3
SMIS0077R1
1
1
2
3
0
∆p
W
Figure 1. Operation Diagram.
Switching Points
Cut-in
Pressure
(in WC)
min.
1.20
Cut-in
Pressure
(in WC)
Switching
Differential
min.
2.00
max.
Cut-in
Pressure
(in WC)
max.
1.60
2.40
1)
1)
max.
3.20
1.20
0.80
min.
4.00
1)
0.80
1.60
0.40
0.80
Pressure Range
0.08 to 1.20" WC
∆p
in WC
Pressure Range
0.20 to 2.00" WC
0.24
0.16
0.08
0
0
0.16
0
0.08
∆p
in WC
0
0.12
0.08
0.04
0
0
SMIS0078R2
0.40
∆p
in WC
Pressure Range
0.40 to 4.00" WC
1. Factory Setting
Figure 2. Switching Points.
Mechanical Design
The QBM81-... Differential Pressure Switch consists of:
• Housing and cover
• Diaphragm
• One sheet-steel mounting bracket
Connection kit (supplied with each switch) consists of:
• Two duct adapters
• Four fixing screws
• 6.6 feet (2 m) tubing, ø 0.25 inch
Page 2
Siemens Building Technologies, Inc.
QBM81-… Differential Pressure Switch
Accessories
Technical Instructions
Document Number CA1N1552E-P25
November 24, 2003
For difficult conditions or cases where high-precision measurements are required,
two other kits are available:
FK-PZ1 Set of two duct probes (nickel-plated iron) with rubber grommet
FK-PZ2 Set of two duct probes (aluminum) with aluminum mounting rosettes and
four mounting screws
Mounting Notes
Mounting instructions are enclosed with the pressure switch.
NOTE:
Mounting positions other than vertical affect the cut-in pressure. See
Commissioning Notes.
The pressure switch is suitable for mounting on air ducts or walls. The recommended
orientation is vertical, but any orientation is acceptable. The pressure connection tubes
can be of any length, but the response time will increase if they are longer than 6.6 feet
(2 meters).
The pressure switch should be mounted so that it is above the pressure connection
points. To prevent the accumulation of condensation, the tubing must be routed so that
there is a gradual incline from the pressure connection points to the pressure switch (no
looping).
Commissioning
Notes
The required setpoint can be selected on the setpoint knob located under the cover.
See Dimensions, Item 5.
NOTE:
The setpoint scale provided with the unit is metric.
The pressure switch is factory-calibrated in the vertical position. If installed horizontally,
this will affect the switching point as follows:
Specifications
Electrical Interface
•
With cover facing upwards:
Switching point is 0.044" WC higher than scale.
•
With cover facing downwards:
Switching point is 0.044" WC lower than scale.
Type of switch
Contact rating
Operating voltage
Switching differential (∆p)*
Reset
Service life
Product Data
Measuring range
Repeatability
Range 0.08 to 1.20 in WC
Range 0.20 to 4.00 in WC
Maximum overload on one side
Admissible media
Siemens Building Technologies, Inc.
Single-pole change-over, multi-layer
contact
24 Vac/dc, >0.01 A
250 Vac
Maximum 5 A resistive
Maximum 3 A inductive, cos ϕ >0.6
(0.8 A starting current sixfold,
cos ϕ > 0.6
Maximum 250 Vac
Adjustable
Automatic
>1,000,000 switching operations
See Product Numbers
<± 0.10 in. WC
<± 0.02 in. WC
20.07 in. WC
Air and non-corrosive gases
Page 3
Technical Instructions
Document Number CA1N1552E-P25
November 24, 2003
QBM81-… Differential Pressure Switch
Housing
Cover
Diaphragm
Mounting bracket
Duct adapters
Tubing
Fiberglass reinforced polycarbonate
Polycarbonate
Silicone (low-swell rubber, no ABS)
Sheet-steel (galvanized)
ABS
PVC, soft
Connections
Electrical connection
Cable entry
Pressure connections
3 screw terminals
PG11 cable gland
Male, ø 0.24-inch
Weight and Dimensions
Weight (including packaging)
Dimensions
0.42 lb. with mounting bracket
See Figures 5 and 6
General Ambient
Conditions
Ambient temperature
Operation
Storage
Ambient humidity
–4°F to 185°F (–20°C to 85°C)
–40°F to 185°F (–40°C to 85°C)
<90% rh (non-condensing)
Mounting
Orientation
Any. See Commissioning Notes.
Agency Approvals
Protection class
Class 2
Protection standard
IP54 to IEC529
Combustion class
to UL94
Pressure casing and housing
V-0
Cover
HB
Plastic tubing
V-2
Duct adapters
HB
Conforms to CE requirements
* The switching differential is factory-set to a fixed value (See Figure 2), and the
adjustment screw is sealed with paint (approximately one turn counterclockwise from
the end-stop).
Specifications,
Continued
Materials
SMIS0079R1
Wiring Terminals
1
Ph
2
SWITCH CONNECTS 1-2 ON PRESSURE FALL
3
SWITCH CONNECTS 1-3 ON PRESSURE RISE
∆p
Figure 3. Wiring Terminals.
SMIS0080R1
Application
Examples
∆p
+
∆p
–
–
+
+ Pressure upstream of filter
– Pressure downstream of filter
+ Pressure downstream of filter
– Pressure upstream of fan on inlet
side or open to atmospheric
pressure. With radial fans, locate
at inlet center
Figure 4. Application Diagram.
Page 4
Siemens Building Technologies, Inc.
QBM81-… Differential Pressure Switch
Technical Instructions
Document Number CA1N1552E-P25
November 24, 2003
Dimensions
All dimensions in inches
(millimeters)
3.52
(89.4)
1.75
(44.4)
0.98
(25)
1.58
(40)
3.60
(91.4)
0.87
(22)
1
2.01
(51)
5
2
3
ø0.24
(ø6.2)
3.47 (88)
0.75
(19)
P2
ø3.19 (ø81)
P1
SMIS0081R1
4
Legend:
1 Mounting bracket
2 P1 connection, higher pressure
3 P2 connection, lower pressure
4 Pressure differential scale (factory-sealed with paint)
5 Setpoint knob
Figure 5. QBM81-… Differential Pressure Switch Dimensions.
ø0.26
(ø6.5)
3.09
(78.5)
2.36
(60)
SMIS0082R1
2 x ø0.13
(2 x ø3.4)
ø0.63
(ø16)
ø0.26
(ø6.5)
Figure 6. Duct Adapter Dimensions.
NOTE:
Two duct adapters are supplied with the pressure switch.
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. Other product or company names mentioned herein may be the trademarks of their respective owners.
© 2003 Siemens Building Technologies, Inc.
Siemens Building Technologies, Inc
1000 Deerfield Parkway
Buffalo Grove, IL 60089-4513
U.S.A.
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Document No. CA1N1552E-P25
Printed in the U.S.A.
Page 5