Problems with Low Pressure Measurements
Problems with Low Pressure Measurements
(OP)
We are currently using a manometer (DWYER SERIES 475 MARK III) to measure pressure loss in our current product. We’re measuring in ranges of 2(in. WC) thru 3.5(in. WC) and not loose more than 0.35(in. WC) /min. We’re have a so-called sealed unit which we use to evaluate are measuring system. But have had issues, different readings, when measuring in different rooms. Currently we make measurements in an environmental chamber where there is limited air flow. And our repeatability as far as leak measurement has been fairly good, but there is day to day variation. Currently we estimate +/-0.1(in. WC) in our reading. This is a problem when we have a leak at 0.3(in. WC). Any suggestions on having more accurate results?





RE: Problems with Low Pressure Measurements
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StoneCold
RE: Problems with Low Pressure Measurements
RE: Problems with Low Pressure Measurements
RE: Problems with Low Pressure Measurements
RE: Problems with Low Pressure Measurements
But your question is how to estimate what the pressure change would be...
Start with PV = mRT (ideal gas law)
For a leak test, you put a fixed amount of gas (mass = m) into a device of fixed volume (V), and those two parameters stay (pretty much) constant, i.e. the density does not change significantly (m/V). So, the equation
P = ρRT
is what you want to use. Here, P is absolute pressure in Pascal (N/m^2), rho is in kg/m^3, R = 287 J/kg-K for air, and T is absolute temperature in Kelvins.
To convert from inH20 to Pa, multiply by 247; i.e.
a 0.1 inH20 change = 24.7 Pa.
A little calculus: take dP/dT = ρR = 344.4. To get a 24.7 Pa change in pressure takes a temperature change of dT = 24.7/344.4 or about 0.07 degrees C. Note that this value is tied to rho, which will change (a little) depending on your actual absolute pressure and temperature during the test.
RE: Problems with Low Pressure Measurements