In the measuring tube of the swirlmeter a helical-shaped vortex appears, rotating with a frequency proportional to the flow rate. In a conventional design this frequency is detected by a pressure sensor which senses the tiny pressure change appearing in the rotating vortex area.
However, the sensitivity of this sensor is limited by the high line pressure which is superposed on the flow-rate-proportional pressure fluctuation. Obviously, easing the sensor from this high system pressure would allow a significant increase of sensitivity and hence of measuring range.
Thus we introduce a measuring tube with two pressure ports placed opposite to each other, which results in two signals that are equal in amplitude but with a 180" phase difference, i.e., a push-pull signal. By connecting these two ports via a differential pressure sensor one thus gets rid of the line pressure, leaving the pure alternating signal that is wanted. Besides this suppression of the 'common mode pressure', a further idvantage of this design lies in the fact that all parasitic signals stemming from periodic pulsations of the original flow as well as vibrations of the tubing are rendered harmless.
The suitability of various designs of differential pressure sensors (capacitive, piezoelectric) and the influence of the coupling between the measuring tube and the sensor are discussed.