How to Select the Right Differential Pressure Sensor for BMS Applications

Differential pressure (DP) transmitters appear on virtually every BMS project — from a single VAV box sensor to a hundred-point filter monitoring network. Yet they are also one of the most commonly mis-specified instruments. Selecting too large a range wastes resolution; selecting too small a range risks overrange damage. The wrong output signal adds unnecessary signal conditioning. And ignoring media compatibility causes premature failure.

This guide walks through the key decisions in order, so you can specify the right DP sensor first time.

What Differential Pressure Transmitters Actually Measure

A DP transmitter senses the pressure difference between two points — a high-pressure port and a low-pressure port. The output signal (4–20 mA, 0–10 V or Modbus) scales linearly with that difference. The transmitter does not know where it is installed; it simply measures ΔP.

In BMS applications this simple measurement supports three distinct use cases:

• Air-side DP: duct static pressure, filter differential, VAV box control
• Liquid-side DP: pump differential head, valve authority, coil ΔP
• Room pressurisation: isolation rooms, cleanrooms, theatres

Each use case has fundamentally different range requirements, so the first step is always to confirm which you are measuring.

Range Selection: The Most Important Decision

Selecting the correct DP range is critical. A general rule: choose a range where the expected operating point falls between 40% and 80% of full scale. Below 40% you waste resolution. Above 80% you risk saturation on transients.

Application	Typical DP Range	Notes
VAV box control	±50 to ±250 Pa	Bi-directional range preferred
AHU supply duct static	0–500 Pa to 0–1000 Pa	Unidirectional
Filter monitoring (G4–F8)	0–250 Pa to 0–500 Pa	Alarm at 80% of initial clean ΔP
Cleanroom / theatre	±5 Pa to ±25 Pa	Use auto-zero function
Pump DP (LPHW/CHW)	0–4 bar to 0–10 bar	Requires liquid-rated sensor
Coil ΔP (water side)	0–500 mbar to 0–2 bar	Stainless wetted parts recommended

Air-Side vs Liquid-Side: Very Different Sensors

This distinction matters more than most specifiers realise. An air-side DP transmitter uses a dry sensing element — typically a silicon MEMS or ceramic capacitive chip — behind barbed plastic pressure ports. It is not designed for liquid media. Introducing water into the pressure ports will destroy it.

Liquid-side DP transmitters use a wetted 316L SS diaphragm with silicone oil fill, a metal body and process connections (BSP or NPT). They handle liquids, steam and aggressive gases but typically start at 100 mbar ranges — far too coarse for air duct measurement.

The only correct approach: one sensor type for air, a completely different type for liquid.

Output Signal: 4–20 mA, 0–10 V or Modbus?

The right output depends entirely on what the BMS input card accepts:

Output	Wiring	Noise immunity	BMS compatibility
4–20 mA	2-wire loop powered	Excellent (current loop)	Universal — any analogue input card
0–10 V	3-wire (power + signal + common)	Good to ~50 m	Universal — most BMS AI cards
Modbus RTU	3-wire RS-485 (A, B, GND)	Excellent — digital	Requires RS-485 port on controller

For runs longer than 30 m, prefer 4–20 mA. Current loops reject interference that corrupts 0–10 V signals. For multi-sensor installations where running individual analogue cables is expensive, Modbus RTU on RS-485 allows up to 32 sensors on a single daisy-chain — dramatically reducing wiring cost.

Supply Voltage: Check Before Ordering

Most 4–20 mA DP transmitters are loop-powered: the BMS analogue input card provides 24 V DC, and the transmitter draws 4–20 mA from that loop. No separate power cable is needed.

0–10 V sensors require a separate power supply — usually 24 V DC. If your BMS input cards do not supply power, budget for a field power supply.

Modbus RTU sensors typically require 18–30 V DC supplied separately. This powers the RS-485 transceiver and processor in addition to the sensing element.

IP Rating and Housing Material

Most AHU and duct applications require IP54 minimum — splash-proof against condensation. Plant rooms and outdoor applications need IP65 (dust-tight, water jets). Submersible or wash-down applications need IP67 or IP68.

Avoid ABS plastic housings in plant rooms where water ingress is likely during maintenance. Polycarbonate or metal housings hold up better.

Auto-Zero for Low-Range Applications

Below ±50 Pa, thermal and mechanical drift becomes significant. As the sensor body changes temperature — from sunrise warming a wall, or from the HVAC system switching on — the zero point can shift by several Pa. This is normal behaviour for MEMS sensors; it is not a fault.

Transmitters with an auto-zero function compensate by periodically venting both pressure ports to atmosphere and re-zeroing the output. This is essential for cleanroom and isolation room monitoring where ±5 Pa ranges are typical. The auto-zero can be triggered by push-button on commissioning, or commanded via a BMS digital output.

Common Specification Mistakes

1. Specifying one range for everything — a 0–1000 Pa transmitter for a VAV box that operates at 30–80 Pa has 12× too much range. Specify purpose-matched ranges.
2. Using air-type sensors on water lines — guaranteed failure. Always check media compatibility.
3. Forgetting bi-directional ranges on VAV boxes — VAV dampers can produce reverse DP during system transients. A unidirectional sensor will saturate or give negative readings.
4. Long 0–10 V runs without cable screening — induced noise on a 0–10 V signal produces false readings. Use 4–20 mA for runs over 30 m, or switch to Modbus.
5. Ignoring access for commissioning — a zero/span push-button is worthless if the transmitter is buried in a ceiling void. Specify remote calibration via Modbus or install in accessible locations.

Recommended Models

For air-side BMS applications, the AG-DPT-100 covers ±50 Pa to ±5000 Pa with range selection via DIP switch — one model handles VAV, filter monitoring and duct static. For cleanroom and isolation room applications requiring ±5 Pa to ±500 Pa with auto-zero, the AG-LPT-100 is the correct choice. For liquid-side DP across coils, strainers and heat exchangers, the AG-DPT-200 with HART 7 covers 0–100 mbar to 0–70 bar on 316L SS wetted parts.