Inputs
Example data table
| Use case | Typical target differential | Notes |
|---|---|---|
| Clean-to-less-clean room cascade | 10–25 Pa | Maintains directional airflow through door cracks. |
| Stairwell pressurization (smoke control) | 25–50 Pa | Check door opening force requirements. |
| Dust control for temporary enclosures | 5–15 Pa | Negative pressure helps contain particulates. |
| Lab exhaust vs. corridor | -5 to -15 Pa | Negative sign indicates the lab is lower pressure. |
Use these as starting points. Always confirm project specifications and local code requirements.
Formula used
- Direct difference: ΔP = P2 − P1
- Manometer column: ΔP = ρfluid · g · h
- Opening / leakage: ΔP = (Q / (Cd·A))² · (ρ/2)
- Duct loss: ΔP = (f·L/Dh + ΣK) · (ρ·V²/2) + ρ·g·Δz
Symbols: ρ is air density, V is velocity, Dh is hydraulic diameter, and g is gravitational acceleration.
How to use this calculator
- Select the calculation method that matches your measurement setup.
- Choose the output unit required for your deliverable.
- Enter temperature and altitude when using flow-based methods.
- Fill the method-specific inputs; keep units consistent.
- Press Calculate to show results above the form.
- Download CSV for logs, or PDF for a quick report.
Pressure differentials in construction environments
Pressure differences drive air movement through doors, shafts, and small leakage paths. On active sites, controlling ΔP supports dust containment, smoke control commissioning, temporary infection isolation, and cleanroom-like fit-out work. Even a 10–25 Pa cascade can maintain directional flow when doors open briefly and partitions are not perfectly sealed.
Practical targets and what they mean
Common project targets range from 5–15 Pa for negative-pressure enclosures used for demolition dust, to 25–50 Pa for stairwell pressurization systems where door opening force must still remain acceptable. For reporting, remember 1 in.w.g is about 249 Pa, so a 0.10 in.w.g test result corresponds to roughly 25 Pa.
Measurement data and field checks
Field teams typically log P1 and P2 readings with handheld meters or derive ΔP from a manometer column. The manometer method uses ΔP = ρ·g·h, where fluid specific gravity and height difference set the result. Repeat readings at multiple door positions and record temperature and altitude if you later compare results to airflow-based calculations. Aim for stable readings over at least 60 seconds.
Using airflow to back-calculate ΔP
When you know leakage or transfer flow, the opening method estimates ΔP using an orifice relationship. The discharge coefficient (often 0.60–0.65 for sharp-edged openings) and the effective area strongly influence the estimate. Small errors in area can cause large ΔP changes because pressure varies with the square of flow velocity.
Duct loss planning and documentation
For temporary ventilation, duct losses can dominate delivered pressure. The duct method combines friction loss and fitting losses, then adds elevation effects. Use measured duct sizes, realistic friction factors (often 0.015–0.03 for smooth duct), and a summed K for bends, transitions, and dampers. Exporting CSV and PDF outputs supports commissioning records and helps teams compare scenarios during site changes.
Q1. What sign convention does the calculator use?
Direct difference reports ΔP = P2 − P1. A positive value means Zone 2 is at higher pressure than Zone 1. Keep the same convention across drawings, test sheets, and commissioning reports.
Q2. Which method should I choose for a site survey?
Use Direct difference when you have two pressure readings. Use Manometer when you only have a fluid column height. Use Opening or Duct when you want an estimated ΔP from airflow and geometry.
Q3. Why do temperature and altitude matter?
Air density changes with temperature and elevation. Lower density reduces the ΔP predicted by flow-based formulas for the same Q, Cd, and area. For direct instrument readings, temperature mainly affects sensor accuracy, not the physics.
Q4. What discharge coefficient should I enter?
For sharp-edged openings, 0.60–0.65 is commonly used. For smoother, well-rounded inlets, Cd can be higher. If you are unsure, use 0.62 and document the assumption in your report.
Q5. How do I estimate total fittings coefficient ΣK?
Add K-values for each elbow, tee, transition, damper, and diffuser in the run. Manufacturer data and HVAC references provide typical K values. Use a conservative sum when fittings are unknown or temporary.
Q6. Why is the duct friction factor limited in the tool?
Extreme f values can produce unrealistic losses. The calculator clamps f to a practical range to prevent accidental unit errors. If you have a tested value outside the range, adjust inputs and verify the output manually.
Q7. Can I use this for smoke control commissioning?
Yes, for preliminary checks and documentation. Confirm required targets, door forces, and measurement locations from the project design and applicable codes. Use multiple readings and record conditions so the commissioning team can replicate tests.