How Pump Efficiency Is Actually Measured—and Why It Matters for Buyers
When a pump manufacturer claims an efficiency of 82%, what does that number actually mean? How was it measured? Under what conditions? With what uncertainty? HI 40.6, “Methods for Rotodynamic Pump Efficiency Testing,” answers these questions. Published in 2014 and revised in 2021, it establishes the minimum testing protocols that give buyers and manufacturers a common language for pump efficiency verification.
What HI 40.6 Measures
A pump efficiency test under HI 40.6 consists of three independent measurements:
- Flow rate: Measured with a calibrated flow meter (electromagnetic, ultrasonic, Coriolis, or differential pressure). The meter must have a current calibration certificate traceable to a national standard.
- Head: Calculated from suction and discharge pressure measurements, corrected for gauge elevation and velocity head difference. Pressure transducers or calibrated gauges are required; compound gauges that read both vacuum and positive pressure are permitted for suction measurement.
- Power input: Measured at the pump shaft (for bare pump efficiency) or at the motor terminals (for wire-to-water efficiency). A calibrated torque meter and speed sensor provide the most accurate shaft power measurement.
Efficiency is calculated as: η = (Q × H × SG) / (3,960 × P_shaft) in US units, where all measurements include their associated uncertainties.
Uncertainty: Why Every Efficiency Number Has a ± Band
Every measurement has uncertainty. The overall uncertainty of the calculated efficiency is the root-sum-square of the individual measurement uncertainties. HI 40.6 defines two grades of testing based on permissible uncertainty:
| Test Grade | Flow Uncertainty | Head Uncertainty | Power Uncertainty | Overall Efficiency Uncertainty |
|---|---|---|---|---|
| Grade 1 (highest accuracy) | ±1.5% | ±1.0% | ±1.0% | ±2.0% to ±2.5% |
| Grade 2 (standard accuracy) | ±2.5% | ±2.0% | ±2.0% | ±3.5% to ±4.5% |
What this means: if a manufacturer reports 82% efficiency from a Grade 2 test, the true efficiency lies between approximately 78.3% and 85.7% (82% ± 3.7%). For ANSI pump buyers, this uncertainty band is larger than many realize—and it reinforces the importance of specifying Grade 1 testing for high-energy or critical-service pumps.
The Connection to DOE Pump Efficiency Regulations
HI 40.6 is the test standard referenced by the U.S. Department of Energy’s Pump Energy Conservation Standard. The DOE standard uses a Pump Efficiency Index (PEI) calculated from HI 40.6 test data. Pumps sold in the U.S. for covered applications must meet minimum PEI values. When you specify that a pump must be tested per HI 40.6 Grade 1, you are ensuring the efficiency data used for DOE compliance is based on the most accurate measurement protocol available.
Key Takeaways
- HI 40.6 defines how pump efficiency is measured, calculated, and reported—creating a common standard for the industry.
- Efficiency measurements have inherent uncertainty (±2-4.5% depending on test grade). A reported 82% efficiency is really a range.
- Grade 1 testing (±2-2.5% overall uncertainty) is recommended for high-energy pumps, pumps above 100 hp, and any pump where efficiency guarantees are contractually specified.
- HI 40.6 forms the technical foundation for DOE pump efficiency regulations—compliance testing is conducted per this standard.