Formula used
- Direct difference:
ΔP = P1 − P2 - Fluid column:
ΔP = ρ g Δh(hydrostatic pressure change) - Restriction estimate:
ΔP = (ρ/2) · (Q / (Cd·A))² - Dynamic / Pitot:
ΔP = ½ ρ (v2² − v1²)
How to use this calculator
- Select a calculation method matching your setup.
- Enter values and choose their units in each field.
- Set the output unit you want for reporting.
- Click the calculate button to display the result.
- Use the CSV or PDF buttons to export your run.
Example data table
| Scenario | Inputs | Computed ΔP |
|---|---|---|
| Direct difference | P1 = 120 kPa, P2 = 95 kPa | 25 kPa |
| Fluid column | ρ = 1000 kg/m³, g = 9.80665 m/s², Δh = 0.35 m | 3432 Pa |
| Dynamic / Pitot | ρ = 1.225 kg/m³, v2 = 25 m/s, v1 = 0 m/s | 383 Pa |
| Restriction estimate | ρ = 998 kg/m³, Q = 0.006 m³/s, A = 0.00025 m², Cd = 0.62 | ~ 1.85×106 Pa |
Differential pressure in practical measurements
1) What differential pressure represents
Differential pressure (ΔP) is the pressure difference between two points in a system. In piping and ducts, it is commonly used to infer flow, detect filter loading, or quantify losses across equipment. Because ΔP can be positive or negative, always define a clear “high” and “low” side before recording data.
2) Typical unit ranges you may encounter
Instrumentation spans wide ranges. HVAC work often uses inches of water (inH2O) or Pascals, where 0.5 inH2O ≈ 125 Pa. Process piping frequently reports kPa or bar, and restriction devices can reach hundreds of kPa under high flow. This calculator supports common reporting units for quick comparison.
3) Fluid-column method and manometer data
For a static fluid column, ΔP = ρ g Δh. With water at ρ ≈ 1000 kg/m³ and g ≈ 9.80665 m/s², each 1 cm height difference corresponds to about 98.1 Pa. For mercury (ρ ≈ 13,595 kg/m³), 1 mm corresponds to roughly 133 Pa, matching the mmHg convention.
4) Dynamic pressure and Pitot-style readings
In gases and liquids, a Pitot tube relates velocity to dynamic pressure: ΔP = ½ ρ v². For air at ρ = 1.225 kg/m³ and v = 25 m/s, dynamic pressure is about 383 Pa. If you compare two velocities, the calculator uses ½ρ(v2² − v1²) to isolate the change.
5) Restriction estimates from flow and geometry
When you know flow rate Q, an effective flow area A, and a discharge coefficient Cd, the estimate ΔP = (ρ/2)(Q/(Cd·A))² provides a fast check. Small areas make ΔP rise sharply because velocity scales as Q/A. Use realistic Cd values (often 0.6–1.0) to avoid overestimating.
6) Interpreting sign, direction, and reference points
Direct subtraction uses ΔP = P1 − P2, so the sign depends on which pressure you place in P1. For hydrostatic setups, the sign depends on how you define Δh. In reporting, document the tap locations and elevation direction so results are reproducible.
7) Data quality, uncertainty, and calibration
Accuracy depends on sensor range and resolution. A ±0.5% full-scale transmitter on a 10 kPa range yields ±50 Pa uncertainty, which is significant for low-pressure air systems but minor for high-pressure liquid lines. Zeroing, temperature effects, and impulse line condition can dominate errors.
8) Choosing the right method for your scenario
Use the direct method for two known pressures, the fluid-column method for manometers, the dynamic method for velocity-related measurements, and the restriction estimate for quick engineering checks. For design-critical work, validate with standards and include additional losses from fittings and roughness.
FAQs
1) Can ΔP be negative?
Yes. A negative value simply means the “low” side is higher than the “high” side based on your input order. Swap P1 and P2 if you want a positive convention.
2) Which method should I use for a U-tube manometer?
Use the fluid-column method (ρ g Δh). Enter the manometer fluid density and the measured height difference, then select your preferred output unit for reporting.
3) Why does the restriction method sometimes give very large values?
ΔP grows with the square of velocity. If area A is small, Q/(Cd·A) becomes large, producing a large ΔP. Double-check geometry, Cd, and that Q is in the correct unit.
4) How do I use the Pitot option?
Enter fluid density and velocity. If you have a baseline velocity, enter it as v1; otherwise keep v1 at zero. The result represents the dynamic pressure change.
5) Does this include friction losses in pipes?
No. The calculator focuses on core ΔP relationships and simple estimates. For pipe friction and minor losses, use a dedicated head-loss model and sum losses across components.
6) What density should I use for gases?
Use density at operating conditions. For air, 1.225 kg/m³ is a common sea-level reference, but temperature, humidity, and pressure can change density and therefore ΔP predictions.
7) Why do my readings drift near zero?
Near-zero ΔP is sensitive to sensor zero offset, temperature changes, and vibration. Re-zero the transmitter, inspect impulse lines, and ensure the selected range gives adequate resolution.