DPT — Differential Pressure Transmitter
Human Analogy
Blow through a straw — you feel resistance. Now blow through a clean straw and then through a partially blocked straw. The partially blocked one requires more effort — more pressure to push the same amount of air through. The difference in pressure between the two sides of the blockage tells you how blocked it is.
A DPT measures this pressure difference — between two points — continuously and precisely.
What Is Differential Pressure?
Differential pressure is the difference in pressure between two points. Not the absolute pressure at either point — the difference between them.
Pressure at Point A = 250 Pa
Pressure at Point B = 180 Pa
Differential Pressure = 250 - 180 = 70 PaWhy Differential and Not Absolute?
Because many BMS applications care about the difference — not the actual value. A filter is either clean or dirty based on how much resistance it adds. That resistance is measured as differential pressure across the filter — not the absolute pressure at either side.
DPT Applications in Buildings:
FILTER MONITORING (most common):
High pressure side: before filter
Low pressure side: after filter
Normal DP: 20–50 Pa (clean filter)
High DP: 150+ Pa (filter choked — needs replacement)
BMS raises alarm when DP exceeds limit
Engineer knows to change filter before airflow is compromised
DUCT STATIC PRESSURE (AHU fan control):
Measures pressure in supply air duct
BMS controls AHU supply fan speed via VFD
Maintain constant duct static pressure as VAV boxes open and close
VAV boxes closing → duct pressure rises → BMS slows fan → energy saved
ROOM PRESSURE (pharma/hospital):
Measures pressure difference between room and corridor
Positive room: +10 to +15 Pa (clean room, OT)
Negative room: -10 to -15 Pa (isolation room)
BMS adjusts supply/extract fan balance to maintain
Signal Output: 4–20mA (analog) → AI on controller
DPT vs DPS — Important Distinction:
DPT: Differential Pressure Transmitter → continuous analog output (4–20mA)
DPS: Differential Pressure Switch → ON/OFF digital output only
Use DPT when you need to know how high the pressure is (trending, control).
Use DPS when you only need to know if pressure crossed a threshold (alarm only).
Memory Hook:
DPT = Differential Pressure Transmitter
Two pipes connected to two points
Measures the difference
Continuous reading (4–20mA)
Used for: filter status, duct pressure, room pressureRelated Topics
- What is BMS integration? — how a BMS connects with VFDs, energy meters, BACnet/Modbus devices and other building systems
- How to design a BMS system step by step — the complete BMS design methodology covering site survey, IO list, controller selection, sequence of operations
- What is a Building Management System (BMS)? — fundamentals of BMS controls and architecture for HVAC, lighting, energy and access
- What is BMS commissioning? — the disciplined commissioning process that turns a BMS install into a working building brain
- Browse all Sensors topics — more from this section of the EnSmart BMS Library