Measurements · Meteorology
Differential Pressure
Also known as DP, delta P, pressure drop
Differential pressure between two spaces or across an air filter, used to verify room pressurization (cleanrooms, isolation rooms, operator cabs) and track filter pressure drop.
What it is
What is differential pressure?
Differential pressure is the difference between the air pressures at two points: between two rooms, or across a filter, a duct section, a coil, or any element that resists airflow. Rather than an absolute pressure referenced to vacuum or the atmosphere, it is a difference, reported here in pascals (building engineers also use inches of water gauge). Across a component in an airflow, differential pressure is a direct measure of resistance: the more restricted the path, the greater the pressure the air loses passing through it. Between two spaces, it measures which is at higher pressure and by how much.
Because a restriction shows up as a rising pressure drop, differential pressure is the standard way to track filter loading, element fouling, and containment in pressurized spaces. The same measurement across a fuel or hydraulic filter vessel is its own parameter, fluid differential pressure.
The most common use is monitoring filters. A clean filter has a low pressure drop that climbs as it collects dirt, so differential pressure tells an operator when a filter is loaded and due for change: early enough to protect the system, but not so early that filter life is wasted. In HVAC and cleanrooms it also verifies room pressurization, confirming that a clean or containment space stays positive or negative relative to its surroundings so that air flows the right way through doorways.
For facilities and critical-environment operators, differential pressure is therefore both a maintenance signal and a safety verification, telling them that filters and containment are doing their job.
Limits
Differential Pressure Limit Values
The published values, by averaging period; each links to the standard that sets it.
Minimum, operating room (positive)
Minimum, airborne infection isolation room (negative)
Minimum, protective environment room (positive)
Minimum, buffer room (positive)
Negative range, containment room (C-SEC)
Minimum, sustained
Maximum
- ASHRAE 170 2.5 Pa: 0.01 in. w.g.
- ASHRAE 170 2.5 Pa: 0.01 in. w.g.
- ASHRAE 170 2.5 Pa: 0.01 in. w.g.
- USP 797 5 Pa: 0.020 in. w.c. relative to adjacent less clean areas
- USP 800 2.5–7.5 Pa: 0.01 to 0.03 in. w.c. negative relative to all adjacent areas, at all times
- ISO 23875 20 Pa: cab pressurization with the machine's electrical system on
- ISO 23875 200 Pa: cab pressurization
Differential pressure is reported in pascals (Pa) and interpreted against set thresholds: a change-out limit at which a loaded filter should be replaced and, for pressurization, a minimum differential the space must maintain. Those thresholds appear in the table on this page. Read the trend as much as the value. A steadily rising drop across a filter charts its loading and predicts when it will reach the limit, while a sudden fall can mean a ruptured or bypassed element.
How it is measured
How Differential Pressure Is Measured
Differential pressure is measured with a differential pressure transducer or gauge connected to two pressure tappings, one either side of the element, which senses the difference between them directly. The Aethair PRO carries a differential-pressure cartridge in its expansion socket for room-to-room monitoring over 0–10 kPa, as part of the unit. Correct connection of the high and low ports, appropriate range selection, and keeping the sensing lines free of blockage and condensate are what keep the reading trustworthy.
Aethair PRO cartridge1
A pre-calibrated cartridge in one of Aethair PRO's sockets: Aethair Diff. Pressure.
Placing differential pressure sensors
The two tappings are placed immediately upstream and downstream of the element whose drop is to be measured, so the reading reflects that element and not the losses of intervening pipework or ducting. For room pressurization, one port senses the controlled space and the other the reference space across the boundary. Sensing lines are routed to drain any condensate away from the transducer and kept short and unobstructed, and the sensor is zeroed at no-flow so that the measured difference genuinely represents the pressure drop across the element.
Every device and option that measures Differential Pressure 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 Aethair Diff. Pressure 0d08 | Expansion cartridge | Aethair PRO | Differential PressureDew PointHeat IndexRelative HumidityIlluminance / LightParticle Count (by size)PM1 |
Where it is monitored
Solutions That Monitor Differential Pressure
Each one links to the solution page showing what that industry measures it for.
Cleanrooms, Labs & Manufacturing
Core parameterRoom-to-room differential pressure is what keeps a cleanroom clean, maintaining the cascade that stops contaminated air flowing into critical spaces.
Healthcare Air Monitoring
Core parameterRoom-to-room pressure relationships keep contaminants where they belong in a hospital: positive pressure protects operating rooms and immunocompromised patients, while negative pressure contains airborne infection in isolation rooms.
HVAC & Building Management
Core parameterDifferential pressure is the number that tells an HVAC system how its filters and airflow are performing: as filters load, the pressure drop across them rises, and duct and room pressures reveal whether air is moving as designed.
Article Library
Reading on Differential Pressure
ASHRAE 170 Requirements and Which Edition Applies to You
ASHRAE 170 requirements explained: what the standard sets, which edition your facility is held to, what changed in 2025, and how each value gets verified.
Hospital Air Quality Monitoring: Standards by Space
Hospital air quality standards from ASHRAE 170 and The Joint Commission explained, plus what to measure in operating rooms, isolation rooms, and labs.
How to Choose an Air Quality Monitor: PRO, IAQ, or Thiamis
Compare Aethair PRO, IAQ, and Thiamis to match the right device to your site, from commercial interiors to industrial and fenceline monitoring.
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Questions
Differential Pressure FAQ
What does differential pressure tell you about a filter?
It measures the pressure the fluid loses passing through the filter, which rises as the filter collects dirt. A low, stable drop means a clean filter; a rising drop charts its loading and signals when it is due for change, while a sudden drop can indicate a torn or bypassed element.
How is differential pressure used for room pressurization?
By sensing the pressure of a controlled space against an adjacent reference space, differential pressure confirms the room stays positive or negative as intended, so air flows the right way through doors and gaps. This keeps contamination out of a clean space, or inside a containment space, and is monitored continuously in cleanrooms and hospitals.
How do I monitor differential pressure?
The Aethair PRO takes a differential-pressure cartridge in its expansion socket, alongside its built-in particle, temperature, and humidity sensors. Use the configurator to combine differential pressure with the other parameters your cleanroom, isolation-room, or containment monitoring requires.
Monitor Differential Pressure on the Record
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