Accurate and reproducible efficiency measurement is a prerequisite for effective energy performance policy. A new EMSA Policy Brief describes the robustness of current methods for determining the isentropic efficiency of small, packaged air compressors.
Key findings
A round-robin exercise across three laboratories (Australia, Denmark, Germany) identified several critical issues:
- Calculation ambiguity in ISO 1217: Two valid approaches (corrected vs. uncorrected values for isentropic power and input power) yield different isentropic efficiencies from identical datasets.
- Reproducibility is not guaranteed: While results for the isentropic efficiency of one compressor showed good agreement, another compressor exhibited a significantly wider spread across the labs, despite similar average values.
- Influence of ambient conditions: Existing correction factors do not fully compensate for variations in inlet pressure and temperature, affecting inter-laboratory comparability.
- Flow measurement as source of uncertainty: Measurement of air flow is a challenging parameter, particularly at low flow rates, with limited guidance from the test standard.
Methodological contribution
The EMSA study delivers:
- A step-by-step measurement guide aligned with ISO 1217
- The EMSA CompressorCalc calculation tool for two alternative isentropic efficiency calculations
Policy relevance
For regulators and standardisation bodies, the findings highlight:
- The need for an unambiguous calculation method
- Improved specification of test conditions and correction methods
- Greater clarity on flow measurement requirements
These aspects are critical to ensure that policy frameworks are robust, enforceable, and comparable across jurisdictions.
Conclusion
The results provide direct input to the ongoing revision of ISO 1217 and support the development of more reliable efficiency metrics for air compressors.
