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# Absolute Pressure Measurement

Gas flow is a function of upstream absolute pressure, and of the ratio of upstream to downstream pressures. Lohm testing done at The Lee Company is performed at an upstream pressure which 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 atmosphere 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 value of pressure in psia.

Pres. (psia) = Pres. (psig) + 0.4912 x Pres. (in. Hg @ 32 deg F)

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

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 K = 276 (click here for details) T1 = 70 deg F, fT = 1.00 (click here for details) 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) ΔP = 24.5 - 14.5 = 10.0 psid Q = 2.00 std L/min 