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The Design and Performance of a Second-Generation Phi Meter



Ryan Falkenstein-Smith, Thomas Cleary


This report documents the design and performance of a second-generation phi meter used in multi-scale experimental applications. A constructed second-generation phi meter is implemented in bench and 2/5th scale experiments to demonstrate the instrument's ability to conduct global and local equivalence ratio measurements and gas species concentrations of combustion products (i.e., carbon dioxide, water vapor, and oxygen). Bench-scale experiments evaluated the second-generation phi meter's functionality by introducing known fuel-air ratios into the instrument and verifying measurements via the carbon to hydrogen ratio and mass balance. The bench-scale experiments selected methane, propane, and propene as the fuels of interest. The 2/5th scale experiments implemented two second-generation phi meters in a series of 25 kW methane backdraft experiments. Backdraft experiments provided an ideal setting to test the phi meter's performance by subjecting the instrument to transient heterogeneous gas mixture. The phi meters' sampling location was positioned such that the extracted gas samples fed into the instrument were unknown. Global and local equivalence ratio measurements made in the 2/5th scale compartment were compared to independent gas analyzer and gas chromatograph measurements. In the bench-scale experiments, the global equivalence ratio, water vapor concentration, and carbon to hydrogen ratio measurements are in fair agreement with their expected values. Real-time global and local equivalence ratio and carbon to hydrogen ratio measurements obtained during a backdraft experiment were observed to increase as the fuel concentration within the 2/5th scale compartment increased. Real-time global and local equivalence ratios were verified by the independent instrumentation.
Technical Note (NIST TN) - 2184
Report Number


Equivalence ratio, Gas extraction sampling, Phi meter.


Falkenstein-Smith, R. and Cleary, T. (2022), The Design and Performance of a Second-Generation Phi Meter, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online],, (Accessed May 28, 2024)


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Created January 5, 2022, Updated November 29, 2022