A liquid-independent volume flow measurement principle

Abstract

A novel flow measurement principle is presented enabling non-intrusive volume flow measurements of liquids in the ml/min range. It is based on an opto-acoustical time-of-flight principle, where the time interval is recorded in which a thermal label travels a known distance through a flow channel. Big advantages are the insensitivity to temperature drift and the fact that user calibration is unnecessary. The paper presents a set of physics-based criteria that define the working range of the measurement principle. A prototype of a flow meter was developed and evaluated in a test rig with flows of water, isopropyl alcohol (IPA) and mixtures of both liquids. Pure water and IPA flows of 0.1 to 2 ml min−1 were measured and found to coincide with the reference flows within 4%. The root-mean-square (RMS) value of the fluctuations did not exceed 3%. For flows of 2 ml min−1 the limited power of the laser source caused deviations of 7% with 5% fluctuations. Finally, flow measurements were done in water–IPA mixtures with concentrations between 10% and 90%–w/w at a flow rate of 15 g h−1 (≈0.3 ml min−1). The ratio of measured and reference flow appeared to be 4% to 5% below the theoretical value, but it was hardly a function of mixture composition. Hence, liquid independence of the measurement principle was proven.