The Use of Orifices to Control the Flow Rate of Gases

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Abstract
The flow rates of gases, at constant volume, that are commonly used in studies of photosynthesis, respiration and transpiration, can be obtained by using small orifices that are commercially available. When the ratio of the absolute pressure downstream of the orifice to that upstream of the orifice is less than 0.47 (the critical pressure ratio), the volume flow rate is independent of downstream pressure. A slight non-linearity in the relationship between volume flow rate and the absolute upstream pressure is due to deviation of the gas from ideal gas behavior, and to the fact that the theoretical relationship between flow pressure is not followed until the Reynolds'' number for flow through the orifice exceeds 30,000. The volume flow rate is proportional to the square root of the absolute temperature. The expected relationship between volume flow rate and orifice diameter (50-175 .mu.m) is here derived for orifices discharging to atmosphere, with upstream absolute pressures between 0.24 and 0.9 MPa.

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