Negative pressure means one measured location is at a lower static pressure than the location used as the reference. Measure the difference with the high and low ports connected consistently. A negative display alone does not identify a leak, establish room containment or prove that a ventilation system meets its design criteria.
A reference location changes the meaning of a negative reading
A room may register -5 Pa relative to an adjacent corridor yet show a different value relative to outdoors because wind and building pressure vary. Record which port is attached to which zone, close or open doors as required by the test plan, and document fan states. The pressure differential is a relationship, not a property of the room in isolation.
Define the reference before quoting a value
A room at −5 Pa relative to an adjacent corridor is different from a duct at −5 Pa relative to outdoors. Note reference location, fan/door operation, connected zones and units. Use an instrument with appropriate low-pressure sensitivity and a repeatable measurement setup.
Procedure for qualified HVAC personnel
- Confirm safe and permitted pressure test locations.
- Choose a stable reference point outside localized jets or drafts.
- Zero the differential-pressure meter according to its instructions.
- Connect the measurement and reference lines consistently. For room-to-room checks, make sure tubing routing does not itself hold a door open and change the pressure.
- Record the sign and magnitude at defined ventilation and door states.
- Repeat across relevant conditions and compare with the building’s documented design or operational criteria.
Worked example: the importance of sign
Hypothetical: a room measures −7 Pa relative to a corridor. That is approximately −0.028 inH₂O. Reversing the pressure connections could display +7 Pa: the magnitude is the same but the sign is reversed. Neither result is an acceptable/unacceptable judgment without an applicable design target and verified tubing orientation. For a guide to instrument types and differential-pressure testing, see Pressure Measuring Instruments and Differential-Pressure Testing.
What a pressure test cannot determine alone
| Observation | Why further checks are needed |
|---|---|
| Pressure changes when a door opens | Door state changes the airflow path |
| Steady negative room pressure | Does not directly measure airflow volume or contaminant migration |
| Large filter differential pressure | Depends on airflow as well as filter condition |
| Unexpectedly positive reading | Reference ports may be reversed or ventilation conditions changed |
Testo differential-pressure instrument
The testo 510i Smart Probe measures differential pressure and can be used for static pressure and filter pressure-drop checks. Its supported units and measurement range must match the expected magnitude. It is not a substitute for a properly designed continuous room-pressure monitoring system when alarms and logging are required.
Safety and documentation
In healthcare, laboratories and controlled environments, pressure relationships can support critical containment purposes. Do not alter dampers, doors or safety systems based on one spot reading. Record the layout and ask responsible site specialists to evaluate the applicable standards and acceptance tests. For a related HVAC example of why a single reading may not represent actual system conditions, see why one air velocity reading is not enough.