PRESSURE CHANGE EFFECTS ON HYPODERMIC NEEDLE CRITICAL ORIFICE AIR-FLOW RATES

Hypodermic needles are often used as critical orifices to provide a known and constant air sampling flow rate. A study of the effect of ambient air pressure changes on critical flow rates showed that comparative theoretical pressure change calculations gave excessively low flow rates for large pressure (altitude) changes. As the ambient air pressure drops, the discharge coefficients change, keeping critical flow rates high relative to orifice plate theory but lower than ideal mass flow theory. The discharge coefficient changes are not linear with pressure drop, but a linear approximation can be made with small error. Calculated critical flow rates with an average of 2.2% and a maximum 5% error off the measured flow rates were obtained by using the equation: Qa-max=Qc-max—CSAP, and specific adjustment constants (Cs) ranging from 1.2 to 0.125 mL min-1 mmHg-1 pressure change (AP) for needle gage no.'s 18 to 27. © 1979, American Chemical Society. All rights reserved.