The critical orifice revisited: a novel low pressure drop critical orifice.

The ability to maintain a constant air flow rate under varying load conditions is of basic importance in air sampling. Critical orifice flow devices are often used to accomplish this. A major disadvantage of most critical orifice designs is that a pressure drop in excess of 350 mm Hg (14 in. Hg) is required to ensure stable flow. It is possible, however, to design a flow restricting device which will function as a critical orifice at pressure drops significantly less than those required for conventional designs. Presented here is an inexpensive and convenient method for controlling flow in the range of 20-90 L/min. Pressure drop versus flow rate data demonstrate that a vacuum of 150 mm Hg (6 in. Hg) or less is required to reach critical flow conditions using this design. Thus the convenience of unattended constant flow rate control with a substantial reduction in vacuum pump capacity and cost is achieved.