Absolute Pressure Measurement

Gas flow is a function of upstream absolute pressure and the ratio of upstream to downstream pressures. Lohm testing done at The Lee Company is performed at an upstream pressure that is high enough so that downstream pressure does not affect the flow rate. To accurately determine the upstream absolute pressure, it is necessary to measure atmospheric pressure with a suitable barometer. This measurement will normally be in units of in. Hg, while the gauge pressure reading is in units of psig. Thus, the barometer reading must be converted to psia, and added to the gauge reading to get the value of pressure in psia.

EXAMPLE: What single-orifice restriction will permit a flow of 2.00 std L/min. of nitrogen at 70°F, with supply pressure at 10 psig, discharging to an atmospheric pressure of 29.5 in. Hg.

K = 276 (see tables in How to Calculate Flow Resistance in Pneumatic Systems section for more information on Units Constant “K”)

T₁ = 70°F, f_T = 1.00 (see How to Calculate Flow Resistance in Pneumatic Systems section for more information on Temperature Correction Factor f_T)

P₂ = 0.4912 x 29.5 = 14.5 psia

P₁ = 10.0 + 14.5 = 24.5 psia

P₁/P₂ = 24.5/14.5 = 1.69 (subsonic)

∆P = 24.5 – 14.5 = 10.0 psid

Q = 2.00 std L/min.