A simple ionic flowmeter for measuring very small rates of flow of air.

A prototype direct-reading ionic flowmeter to measure rates of flow of air as low as 0·37 μg S−1 has been constructed. The flowmeter functions best at a pressure of 15 torr. A discharge is struck in the air and the discharge current is fixed at about 50 μA. Two ionization chambers are placed symmetrically in a Perspex tube, one on either side of the discharge. Currents through the ionization chambers are arranged in opposition so that there is a null reading on a Vibron electrometer when there is no air flow. When flow of air does occur some ions are carried out of the upstream ionization chamber while some ions are carried into the downstream chamber; the consequent differential current in the electrometer is about 2 pA when the flow is 0·37 μg s−1. The differential current increases linearly with the rate of flow of air at 6 μA (g s−1)−1 up to rates of flow of about 10 μg s−1; thereafter the relationship between differential current and rate of flow departs from linearity, reaching a maximum at a rate of flow of around 0·1 mg s−1. The behaviour of the flowmeter may best be described by regarding the ions as diffusing from the discharge (without recombination) to the ionization chambers.