A differential-pressure flow measurement estimates fluid flow from a pressure difference produced by a known restriction or velocity probe. The pressure sensor does not directly count units of air or liquid. Converting Δp to velocity or volume flow requires density, geometry and an appropriate coefficient or instrument configuration.
Why low differential pressure becomes difficult to interpret
Pitot-style dynamic pressure rises approximately with the square of velocity. Halving velocity reduces dynamic pressure to roughly one-quarter, making zero offset, tubing leakage and probe alignment more important. Always check whether a device measures static difference, a calibrated flow element or Pitot differential before applying any velocity equation. For related guidance, see Pressure Measuring Instruments: Types, Selection and Differential-Pressure Testing.
What produces the pressure difference?
In a Pitot-static measurement, dynamic pressure relates to air velocity. An orifice plate or Venturi produces a differential pressure across a restriction; each arrangement needs its own flow equation and calibrated assumptions. A filter differential-pressure check is not automatically a flow measurement: filter loading also changes the pressure drop.
Simple Pitot relationship
For an idealized low-speed air measurement, velocity may be estimated as v = √(2Δp/ρ), where Δp is dynamic pressure in pascals, ρ is air density in kg/m³ and v is velocity in m/s. This is a teaching approximation; actual measurement requires the correct static/total pressure pickup, probe coefficient and traverse method.
Hypothetical: with Δp = 30 Pa and an assumed density of 1.20 kg/m³, the idealized velocity is √(60/1.20) = 7.07 m/s, about 1,391 ft/min. The estimate is not a documented field reading and is not a qualified duct traverse. For the practical limits of using a single duct reading, see why one air velocity reading is not enough for duct flow.
How a duct measurement is carried out
- Find a safe representative location with sufficient straight duct where feasible.
- Verify the Pitot tube alignment and the high/low tubing connections.
- Zero the differential-pressure instrument in the prescribed manner.
- Measure a suitable set of traverse points instead of treating the duct center as the mean.
- Convert readings using the appropriate density, area and traverse calculation; document the method.
Testo instrument choice
The testo 510i measures differential pressure and can support duct airflow work when used with an appropriate Pitot tube, which is not included with the standard probe. It also measures HVAC filter/coil pressure drop, but those readings should not be relabeled as duct flow without a valid model.
What undermines accuracy?
- Velocity too low for the differential-pressure sensor’s practical resolution.
- Disturbed flow after elbows, dampers or fans.
- Incorrect duct cross-section or density.
- Reversed pressure tubes, blockage or condensate in hoses.
- Missing correction factors and insufficient traverse points.
ASHRAE testing and balancing guidance treats Pitot traverses and correction factors as important for comparing field airflow methods.