Servo-operated Gas Flow Meter

Abstract

This paper describes a new gas flow meter which is sensitive and accurate enough to be adapted for measuring small rate of flow without pressure drop. The flow is obstructed by an inserted plate in the measuring pipe, and the produced difference of pressures at the both sides of the plate is detected by means of a thin flexible horizontal diaphragm which moves an electrode between fixed upper and lower electrodes in a water cell. The output a. c. voltage from the movable electrode, which is proportional to the pressure difference, is amplified, and drives a servomotor. Thus a positive displacement type flow meter which is connected to the motor and works as a pump, transports the gas from the upstream side to the downstream, so as to nullify the difference of pressure. The servo performs an integral action as the movable electrode with the weight balanced to the buoyancy is floating in the cell. For the stability required, a tachometer feedback loop is also provided. As results of tests, the errors of the instrument were found to be within ±0.3% in the range up to 5 litres per minute, while the pressure drop was actually at zero in this range.