Ultrasonic flowmeters - multichannel

Although the first ultrasonic differential flowmeters were not very accurate, due to better transducers and DSP (Digital Signal Processing) techniques a lot of catching up has been done. Ultrasonic flowmeters are now suitable for calibration measurements. For a calibration-worthy flow measurement, the ultrasonic flow meter is always equipped with a number of sound paths. This way more information is gathered about the present flow profile so that deviating flow profiles can be compensated. The more paths, the more information the instrument can gather about the (laminar or turbulent) flow profile, a possible asymmetrical flow profile, cross flow and swirl.

The ultrasonic flow measurement of gases

Gases have a low viscosity. Therefore, the flow profile almost always has a high turbulent character. The viscosity of liquids on the other hand can vary more, and can also be high. The Re number can be so low that laminar flow behaviour arises. Modern multichannel ultrasonic flowmeters can measure and correct for this.

At this moment there are a number of suppliers offering 2-, 3-, 4-, 5- and 6-channel ultrasonic flowmeters for gasses. With a 2-channel instrument flow conditioning must be applied. Without this the necessary accuracy can certainly not be achieved. 3-channel flowmeters are possibly applicable in somewhat smaller pipelines, but usually instruments with 4 or more channels are used in calibration applications and certainly in larger pipelines. Although not always necessary, the use of a flow conditioner is common for gases.

Tests performed by the South-West Research Institute in San Antonio, Texas, have shown that the various flow conditioners have different effects on the various ultrasonic flow meters. Their recommendation is therefore, that if a flow conditioner is used in the installation, it should be calibrated together with the ultrasonic flow meter and the up- & downstream piping.