A novel ultrasonic mass flowmeter for liquids

An alternative technique for determining mass flowrate is to use a volumetric meter in conjunction with a density cell since mass flow m is given by the product of density /spl rho/ and volumetric flowrate Q. This usually requires a sampling system to determine density. The meter presented in this paper uses a volumetric flow cell and a density cell, but makes all measurements ultrasonically on flowing liquid without the need for sampling. It was developed initially for use on aircraft but has a wider applicability. The meter is based on an ultrasonic time-of-flight cell (the Q cell). This can provide either volumetric flowrate Q or (more typically) the ratio of volumetric flowrate to speed of sound in the liquid squared (Q/c/sup 2/). in series with this is an ultrasonic cell (the Z cell) capable of determining specific acoustic impedance Z and speed of sound c. Since Z=/spl rho/c, equations can be used to determine mass flowrate. The flowmeter is controlled by a fixed point digital signal processor chip (DSP) which also computes volumetric and mass flowrates. Ultrasonic transit times and signal amplitudes are measured and converted to digital values which the DSP processes. To provide the required turndown ratio (about 50:1) and high accuracy at low flowrates, the DSP performs a rolling average on the data. However, it also computes non-averaged values of mass flowrate to provide a less accurate but faster responding output. This allows the flowmeter to respond rapidly to transients in the flow.