The Siemens QBM81-3 is part of the Symaro QBM81 Differential Pressure Switch family - a precision Swiss-engineered differential pressure switch for monitoring air filter loading, airflow, fan-belt condition, and overpressure in clean rooms, commercial kitchens, and HVAC systems. Configured for a field-adjustable set-point range of 0.08-1.2 inWC (0.2-3 mbar) with <+/-0.10 inWC repeatability (low-range model).
The differential pressure between the two pressure connections (P1 = high, P2 = low) deflects a spring-loaded silicone diaphragm with proprietary long-term stability characteristics; an engraved scale under the cover provides direct dial-set adjustment with high accuracy - the set point can be read directly off the scale without iterating against an external manometer. SPDT change-over contact with automatic reset, 5A resistive / 3A inductive at 250 VAC, and over 1,000,000 switching operations service life. The fiberglass-reinforced polycarbonate housing carries IP54 protection (IEC 529), UL94 V-0 flammability classification, and Class 2 electrical protection. The switch is CE marked under EU Low Voltage Directive 2014/35/EU and certified per DIN EN 1854 (differential manostat for air and non-aggressive gases) with DVGW EU type examination certificate. Manufactured in Switzerland by Siemens Schweiz AG; distributed in North America by Siemens Building Technologies, Inc.
This SKU is best suited for low-pressure HVAC and laboratory applications: residential and light commercial filter loading, low-static fan-coil airflow verification, clean-room and isolation-room envelope monitoring, BSC/fume-hood face-velocity verification, and any application where the working differential is between roughly 0.1 and 1 inWC. The Siemens Symaro line is preferred for spec-engineer documentation and projects where European manufacturing pedigree, DIN EN 1854 compliance, or compatibility with broader Siemens building automation systems matter. The engraved scale and direct dial-set adjustment make commissioning faster than switches that require iterative manometer-and-turn calibration. Per Siemens's published applications: monitoring air filters, airflow, fan belts, and pressure in clean rooms, kitchens, and other HVAC installations; positive and negative relative pressure monitoring.
Key Features
The QBM81's signature feature: an individual engraved scale on each unit allows direct dial-set adjustment of the trip pressure - no iterative manometer-and-turn commissioning required. The set-point knob is located under the cover; turn to the desired pressure on the engraved scale and the trip point matches. Switching differential is factory-set and sealed with paint (approximately one turn counterclockwise from end-stop) to preserve the calibration. The factory scale on this SKU is calibrated for the 0.08-1.2 inWC (0.2-3 mbar) range.
A spring-loaded silicone (low-swell) diaphragm separates the two pressure chambers. The silicone formulation is specifically engineered for long-term stability of switching points - minimizes set-point drift over the service life. Free of ABS contamination, compatible with air and non-corrosive gases. The LSR (liquid silicone rubber) diaphragm is emission-free and meets RoHS Directive 2011/65/EU.
A single-pole change-over multi-layer contact provides three screw-terminal connections: 1 (NC), 2 (COM), 3 (NO). On pressure rise above the set point, the switch connects terminals 2-3; on pressure fall, it connects terminals 1-2. Same SKU wires for prove-flow or prove-no-flow logic. The multi-layer contact construction handles both low-voltage signaling and full-load switching without contact oxidation.
Versatile contact rating across both signaling and switching voltages: 24 Vac/dc, >0.01A for BAS controller inputs; maximum 5A resistive / 3A inductive (cos phi > 0.6) at 250 VAC for interposing relays and pilot loads. Starting current up to 6x the inductive rating (0.8A starting) accommodates relay-coil and contactor-coil inrush. Maximum operating voltage 250 VAC.
Over one million switching operations mechanical service life. Well-suited for applications with frequent cycling: filter blockage alarms that trip and clear daily, intermittent fan-status verification, demand-controlled ventilation interlocks, and any process where the switch sees high duty-cycle action.
IP54 protection per IEC 529 - protected against falling dust and water splash from any direction. Suitable for indoor mechanical-room installations, panel mounting, and protected outdoor locations. Housing is fiberglass-reinforced polycarbonate (PC GF 10); cover is polycarbonate; mounting bracket is galvanized sheet steel. Class 2 electrical protection and UL94 flammability classification (housing V-0, cover HB, plastic tubing V-2).
Mounting orientation is not restricted - any orientation is acceptable, though vertical is recommended for the factory-calibrated scale to read accurately. If installed horizontally, the switching point shifts +/-0.044 inWC (+/-11 Pa): with cover up, scale reads 0.044 inWC higher than actual; with cover down, scale reads 0.044 inWC lower. Mount the switch above the pressure connection points with tubing on a gradual incline to drain condensate.
Three screw terminals for permanent wiring connections, accessed by removing the safety cover. Cable entry through a PG11 cable gland. Pressure connections are male 0.24-inch (6 mm) diameter for push-on flexible tubing. The included 6.6 ft (2 m) of 0.25-inch tubing handles the typical short-run sample lines; pressure tubes longer than 6.6 ft will increase response time.
Operating temperature range -4 to 185 deg F (-20 to 85 deg C); storage -40 to 185 deg F (-40 to 85 deg C). Ambient humidity <90% RH non-condensing. Maximum overload on one side is 20.07 inWC (50 mbar) - well above the operating range, providing headroom for transient pressure spikes.
QBM81 Range Selection Guide
All three QBM81 variants share identical mechanical, electrical, and enclosure specifications - they differ only in pressure range and the precision of factory repeatability calibration. Pick the variant whose range bracket includes your working set point. Note: QBM81-5 and QBM81-10 deliver tighter <+/-0.02 inWC repeatability while the lower-range QBM81-3 is rated to <+/-0.10 inWC - if accuracy matters and your trip point can fit a wider-range model, the higher-range SKUs deliver better precision.
| Model | Pressure Range | Repeatability |
|---|---|---|
| QBM81-3 ← this SKU | 0.08-1.2 inWC (0.2-3 mbar) | <+/-0.10 inWC |
| QBM81-5 | 0.2-2 inWC (0.5-5 mbar) | <+/-0.02 inWC |
| QBM81-10 | 0.4-4 inWC (1-10 mbar) | <+/-0.02 inWC |
Technical Specifications
| Specification | Detail |
|---|---|
| Product Type | Differential pressure switch (Siemens Symaro line) |
| Pressure Range (Set Point) | 0.08-1.2 inWC (0.2-3 mbar) |
| Repeatability | <+/-0.10 inWC (low-range model) |
| Switching Differential | factory-fixed, sealed with paint - varies with set point across approximately 0.04 to 0.12 inWC |
| Contact Type | Single-pole change-over, multi-layer contact (SPDT) |
| Contact Rating | 24 Vac/dc, >0.01 A (signaling); maximum 5 A resistive, 3 A inductive (cos phi > 0.6) at 250 VAC; starting current sixfold (0.8 A starting, cos phi > 0.6) |
| Operating Voltage | Maximum 250 VAC |
| Reset Style | Automatic |
| Service Life | Over 1,000,000 switching operations |
| Maximum Overload (One Side) | 20.07 inWC (50 mbar / 5 kPa) |
| Admissible Media | Air and non-corrosive gases (non-aggressive) |
| Diaphragm Material | Silicone LSR (low-swell rubber, no ABS, emission-free) |
| Housing Material | Polycarbonate, fiberglass reinforced (PC GF 10) |
| Cover Material | Polycarbonate |
| Mounting Bracket | Sheet steel, galvanized |
| Duct Adapters (Included) | ABS, halogen-free |
| Tubing (Included) | Soft PVC, 6.6 ft (2 m) of 0.25-inch (6 mm) ID |
| Operating Temperature | -4 to 185 deg F (-20 to 85 deg C) |
| Storage Temperature | -40 to 185 deg F (-40 to 85 deg C) |
| Ambient Humidity | <90% RH non-condensing |
| Mounting Orientation | Any (vertical recommended for scale accuracy); horizontal mounting shifts switching point +/-0.044 inWC (+/-11 Pa) depending on cover direction |
| Enclosure Rating | IP54 per IEC 529 / IEC 60529 |
| Protection Class | Class 2 (per IEC) |
| Flammability (UL94) | Housing V-0, cover HB, plastic tubing V-2, duct adapters HB |
| Agency Approvals | CE (EU Low Voltage Directive 2014/35/EU); DVGW EU type examination (CE-0085CL0280); DIN EN 1854 certified; RoHS Directive 2011/65/EU compliant |
| Electrical Connection | 3 screw terminals (1 NC, 2 COM, 3 NO) |
| Cable Entry | PG11 cable gland |
| Pressure Connections | Male 0.24-inch (6 mm) diameter, P1 (higher) and P2 (lower) |
| Dimensions | 3.19 in dia x 3.47 in deep (81 x 88 mm); mounting bracket footprint 3.52 x 1.75 in (89.4 x 44.4 mm) |
| Weight | 0.42 lb (137 g) with mounting bracket and packaging |
| Manufacturer | Siemens Schweiz AG, Building Technologies Division (Zug, Switzerland) |
| Country of Origin | Switzerland |
| North American Distributor | Siemens Building Technologies, Inc., Buffalo Grove, IL |
| Manufacturer Part Number | QBM81-3 |
What's In the Box
- Siemens QBM81 Differential Pressure Switch with factory-engraved scale
- Galvanized sheet-steel mounting bracket (pre-attached)
- 2 x ABS duct adapters (halogen-free) for in-duct pressure pickup
- 4 x fixing screws for duct-adapter mounting
- 6.6 ft (2 m) of soft PVC tubing, 0.25 inch (6 mm) ID
- Cover with PG11 cable gland
- Mounting instructions (Siemens doc 53150)
- FK-PZ1 - Set of 2 duct probes (nickel-plated iron) with rubber grommet (for cases requiring more robust pickup than the included ABS duct adapters)
- FK-PZ2 - Set of 2 duct probes (aluminum) with aluminum mounting rosettes and 4 mounting screws (for high-precision installations)
- Wire (16-22 AWG appropriate for the contact load) routed through PG11 cable gland
- Manometer for setpoint verification during commissioning (engraved scale is direct-read but a check is recommended)
- Additional tubing if sample-line runs exceed 6.6 ft (note: runs over 2 m increase response time)
Documentation
Frequently Asked Questions
What is the QBM81-3 used for?
The QBM81-3 is a Siemens Symaro differential pressure switch used in HVAC and Energy Management applications for monitoring air filter loading, airflow, fan-belt condition, and overpressure in clean rooms, commercial kitchens, and process spaces. The SPDT change-over contact transfers when the differential pressure across the diaphragm crosses the dial-set trip point - giving a binary pressure-OK/not-OK signal to a BAS controller, alarm panel, or interlock circuit. Suitable for sensing positive pressure, vacuum, or true differential. This SKU is field-adjustable across 0.08-1.2 inWC (0.2-3 mbar).
How do I adjust the setpoint?
Remove power before adjusting. Remove the cover to expose the setpoint knob and the engraved pressure-differential scale. Turn the knob until the indicator aligns with your desired trip pressure on the scale - this is a direct read, no manometer iteration required. Replace the cover and secure it before re-energizing. The factory scale on this SKU is calibrated for 0.08-1.2 inWC. Important: do NOT attempt to adjust the switching-differential screw - it is sealed with paint at the factory (approximately one turn counterclockwise from the end-stop), and breaking the seal voids the calibration.
Why does mounting orientation matter?
The QBM81 is factory-calibrated in the vertical orientation. Any orientation is mechanically acceptable, but horizontal mounting shifts the actual switching point relative to the engraved scale: with cover facing up, the switching point is 0.044 inWC (11 Pa) higher than the scale reads; with cover facing down, it's 0.044 inWC lower. For applications with set points below 0.3 mbar (0.12 inWC), DVGW specifically requires vertical mounting or horizontal mounting with electrical connections downward. Mount the switch above the pressure connection points and route tubing with a gradual incline to prevent condensate from pooling in the sample lines or being forced into the switch.
How do I wire the SPDT contact?
Three screw terminals: 1 (NC, normally closed), 2 (COM, common), and 3 (NO, normally open). Cable enters through a PG11 cable gland. On pressure rise above the trip point, terminals 2 and 3 connect (the NO circuit closes); on pressure fall below the trip point, terminals 1 and 2 connect (the NC circuit closes). For prove-flow logic (alarm or signal when pressure is present), wire 2-3. For prove-no-flow logic (alarm when pressure is absent), wire 2-1. Maximum load is 5 A resistive / 3 A inductive at 250 VAC; low-voltage signaling at 24 Vac/dc requires >0.01 A minimum current.
Which port is high and which is low?
P1 = high pressure, P2 = low pressure. For filter monitoring: upstream (clean-air side) to P1, downstream (loaded side) to P2 - the switch trips as the filter loads up. For fan status: downstream of fan to P1, upstream (inlet side or open to atmospheric) to P2 - or for radial fans, locate the inlet probe at the inlet center. For clean-room or kitchen overpressure: monitored space to P1 if you want overpressure trip; or to P2 if you want underpressure trip. The pressure tubes can be any length, but response time increases for runs longer than 6.6 ft (2 m).
What accuracy does this SKU deliver?
<+/-0.10 inWC repeatability (low-range model). The three QBM81 variants have different repeatability ratings: the higher-range QBM81-5 (0.20-2.00 inWC) and QBM81-10 (0.40-4.00 inWC) deliver <+/-0.02 inWC repeatability, while the lower-range QBM81-3 (0.08-1.20 inWC) is rated <+/-0.10 inWC (the looser tolerance accommodates the very small absolute pressures at the bottom of the range). If your trip point falls in an overlap zone where multiple SKUs would work, prefer the higher-range model for tighter repeatability.
What accessories do I need for installation?
For most installations, nothing extra is needed - every QBM81 ships with 2 ABS duct adapters, 4 fixing screws, and 6.6 ft (2 m) of 0.25-inch PVC tubing. For more demanding pickup conditions: FK-PZ1 is a set of two nickel-plated iron duct probes with rubber grommets (rugged in-duct mounting for high-velocity or contaminated airstreams); FK-PZ2 is a set of two aluminum duct probes with aluminum mounting rosettes and 4 mounting screws (high-precision applications, clean rooms). Wire is the only other consumable - 16-22 AWG, routed through the PG11 cable gland.
Is the QBM81 UL listed?
The QBM81 carries CE marking under EU Low Voltage Directive 2014/35/EU, DIN EN 1854 certification for differential manostats, and DVGW EU type examination certificate (CE-0085CL0280). It is Class 2 electrical protection and UL94 flammability classified (housing V-0). However, UL94 is a fire-rating classification, not the same as a UL listing for North American electrical safety - the QBM81 is NOT UL- or CSA-listed. For applications requiring UL or CSA listing (most North American life-safety and code-compliance applications), specify a UL/CSA-listed alternative; for general HVAC monitoring applications, the QBM81 is widely used in North America through Siemens Building Technologies' distribution and is compatible with most BAS systems.
What media can the QBM81 handle?
Air and non-corrosive (non-aggressive) gases: clean ambient air, nitrogen, argon, dry CO2, dry instrument air. Also rated under the DVGW certification for flue gas monitoring in EU gas-appliance applications (per EU Gas Appliances Regulation EU/2016/426, Manostat for gas, air, and flue gas - product category 4107). The silicone diaphragm is NOT compatible with: liquids, refrigerants, corrosive gases (chlorine, ammonia, hydrogen sulfide, acid fumes), or media that degrade silicone. Maximum overload on one side is 20.07 inWC (50 mbar).
How does the QBM81 compare to US-made HVAC pressure switches?
QBM81 strengths: direct-read engraved scale (faster commissioning than switches requiring manometer-and-turn iteration), European manufacturing pedigree with DIN EN 1854 certification, 1,000,000+ cycle life, IP54 with Class 2 protection, integrates into broader Siemens building automation systems. Trade-offs vs. US-made alternatives: NOT UL/CSA listed (carries CE only), 5A maximum rating (comparable to BAPI BA/SW and BA/APSW; lower than ACI AFS-460 at 15A), no IP65 outdoor option (ACI's DBL and BAPI's BA/APSW-65 are better for true outdoor use), made in Switzerland (longer supply chain than US-made alternatives). Choose QBM81 for projects specifying Siemens automation, EU certification requirements, or where the engraved-scale commissioning advantage matters.
Where is the QBM81 made?
Manufactured in Switzerland by Siemens Schweiz AG, Building Technologies Division (Gubelstrasse 22, CH-6301 Zug, Switzerland). The manufacturer operates under ISO 9001 (Quality, certified 1986) and ISO 14001 (Environmental, certified 1998) management systems. North American distribution is handled by Siemens Building Technologies, Inc. (Buffalo Grove, IL). ZOT Supply ships from US inventory for fast nationwide delivery.
- Series:
- QBM81
- Pressure Range:
- 0.08-1.2 inWC
- Range Class:
- Low
- Contact Style:
- SPDT change-over
- Reset Style:
- Automatic
- Sample Line Fitting:
- 0.24 in (6 mm) male pressure connections
- Maximum Pressure:
- 20.07 inWC (50 mbar) overload
- Approvals:
- CE, DIN EN 1854, DVGW, UL94, IP54 / IEC 529
- Mounting:
- Any orientation (vertical recommended)
- Made in USA:
- No
