The gas throughput generated by a constant pressure gas flowmeter is determined as the amount of gas displaced during a known time interval divided by the time interval. The volume of the flowmeter is reduced (intermittently or continuously) during this time interval so that a) the instant gas pressure deviates only lowly from a mean value and b) the pressure values in the initial instant and the final instant are equal. If the condition b) is accurately met, the displaced amount of gas is given as product of the initial pressure and the total volume displacement. But the pressure values in the initial instant and the final instant can be equal only with some uncertainty. This uncertainty causes an additional uncertainty of the displaced gas amount and thus also an uncertainty of the generated gas throughput.
Even if the uncertainty of the pressure equality is small the corresponding sensitivity coefficient in the gas throughput uncertainty budget could be great. This sensitivity coefficient has been computed. It depends on the ratio of the final volume of the flowmeter and the displaced volume. Practical consequences are:
- This additional uncertainty of the generated gas throughput can be reduced by proper design of the flowmeter. The ratio initial volume to final volume has to be as high as possible.
- This additional uncertainty of the generated gas throughput can be readily made negligible at the flowmeters for high gas throughputs but it increases when very small throughputs are to be generated. It is one of the practical reasons why a constant pressure flowmeter cannot be used for generating of arbitrarily small gas throughput.