Compare measured liquid flow with a square-root flow prediction from reference and measured pressure drop.
Why it matters: Flow through a fixed valve opening or resistance varies approximately with the square root of differential pressure.
ΣCALCULATE · VERIFY · INTERPRET
Calculated result
Engineering evaluation
Entered flow-deviation criterion met
Measured flow deviation from prediction (δQ)-3.333 %
Predicted flow (Qp)
150 gpm
Measured flow (Qm)
145 gpm
Engineering interpretation
Predicted flow is 150 gpm; measured flow differs by -3.333% versus the entered 10% screening tolerance.
Important limitations
Valve characteristic, authority, fluid density, position, piping changes, meter uncertainty, and non-turbulent behavior can invalidate the simple square-root relationship.
Confirm valve position and resistance path match the reference condition.
Scale reference flow by the square root of pressure-drop ratio.
Compare absolute flow deviation with the user-entered screening tolerance.
Equation legend
Inputs and calculated quantities
Qr
Reference flow — gpm or L/s
ΔPr
Reference pressure drop — psi or kPa
ΔPm
Measured pressure drop — psi or kPa
Qm
Measured flow — gpm or L/s
Tol
Entered allowable flow deviation — %
δQ
Measured flow deviation from prediction — %
Qp
Predicted flow — gpm
Qm
Measured flow — gpm
Applications
How this calculation is used
Building design
Document reference flow and pressure drop.
Construction
Check valve, flow-meter, and pressure-tap installation.
Commissioning
Compare measured flow with the differential-pressure prediction.
Quality controls
Assumptions and limitations
Assumptions
Entered data describe one stable operating condition and use consistent measurement boundaries.
Time, energy, and performance values are representative of the stated interval.
Limitations
This is a preliminary evaluation and commissioning-support tool, not an automatic acceptance determination.
Project sequences, manufacturer data, tariffs, codes, and professional review remain controlling.
Field use
Verification procedure
Confirm the operating mode, test boundary, and instrument calibration.
Collect simultaneous readings or a documented trend interval.
Compare the result with the approved sequence, submittal, and project criteria.
Common engineering mistakes
Combining measurements from different operating modes or time intervals.
Ignoring sensor uncertainty, trend sampling, or the defined equipment boundary.
Technical basis
References and source standards
ASHRAE Handbook—HVAC Systems and EquipmentHVAC equipment, central-plant, air-system, and control-performance context; verify the licensed current edition during professional review.
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
