(1) When one uses only one test pressure, then the test results can be expressed with just one number, the “flow coefficient”. This is very simple and avoids the need for any calculations. You are suggesting testing with multiple pressures so that the results can be plotted on a log-log scatterplot, and the best-fit line is a power function with two coefficients, the “flow coefficient” and the “flow exponent”. This is unnecessarily complicated.
(2) Experience tells us that ducts with good airtightness (i.e. that complies with the requirements in Scandinavia, normally Class B or C) have a pretty uniform flow exponent. Ducts with poor airtightness can have a significantly lower flow exponent, but then again such duct systems fail the leakage requirements anyway. Thus there is no real need to measure the flow exponent.
For more information you can check-out the following:
- Information Paper P187 “Duct System Air Leakage — How Scandinavia tackled the problem” by P.G.Schild & J.Railio, on the BUILDUP website
- Webevent presentation “Duct System Leakage Testing — Methods, requirements, handing over” by P.G.Schild, available on the website www.asiepi.eu