Accuracy and repeatability come up in nearly every introductory discussion to clamp on ultrasonic. This is best answered initially with the general published specifications of +/- 1% accuracy “out of the box” with between 0.15% and 0.25% repeatability depending on the manufacturer, with at least ten pipe diameters of straight run pipe in liquid measurement applications, and a fully turbulent flow profile (read more about straight pipe requirements here). Out of the box means programming the meter for the application, mounting the transducers and letting it run. No massaging the numbers, no calibration factors programmed or the like. That can be done later.
Accuracy is how close the device performs to the true measurement value. If a flowmeter is measuring 990 to 1010 gallons per minute consistently, with a reference value of 1000 gallons per minute, it’s performing to within +/-1 %.
Repeatability is how reproducible the flow meter publishes its measurements under the same conditions. If your measurements are 998, 996, and 997, the repeatability of that meter is 0.20% (i.e., the values are twiddling about+/- 2 out of 1000).
With this background established, we can now introduce the effect of single and double traverse configurations on measurement accuracy for clamp-on ultrasonic flowmeters.
With single traverse configurations, the instantaneous velocity being measured is only in one plane and in one direction. As a result, the measured transit times tend to vary significantly more than multi-traverse configurations. This is mainly due to the variations in the flow profile at any given moment in time of measurement. The variations are typically referred to as cross flow. For single traverse flowmeters, the beam is directed in one direction only, and thus cross flow may alter the measurement depending upon its direction relative to the ultrasound beam (i.e., when cross flow is directed closer to the direction of the beam then the flow measurement may increased, and when directed away form the beam the flow measurement may be reduced). This happens because the fluid motion physically changes the relative wave speed depending upon whether the cross flow is directed with or against the ultrasound (similar to a person rowing a canoe with or against a river current; in which case the river can dramatically increase or decrease the canoer’s speed relative to the shore.
To assist in correcting for the cross flow fluid variations, two traverse configurations take two slices of the profile and each in a different direction relative to the cross flow. The two different directions reduce the cross flow influence of the published flow rate.
It is quite typical to read significantly more stable flow rates in two traverse meter configurations, particularly with pipes six inches in diameter and larger. And in general, this should be the default configuration for best measurement. But there is a trade off to consider with ultrasound power.
If you have a measurement that is consistently producing broad repeatability in single traverse and has good signal diagnostics, switching to a two traverse configuration will nearly always “line out” the measurement (Old school for more stable measurement). The main reason single traverse configurations are employed is when a two traverse measurement has poor signal diagnostics and cannot maintain consistent and reliable flow measurements. Single traverse has the strongest signal capability, at the expense of repeatability. Each application varies and sometimes you must get a measurement regardless of anything else.
With this knowledge in your hands now, you should be able to navigate which configurations work best for various applications. The best way to understand and learn this concept is to go and make measurements with the Orcas portable meters to see for yourself. You may be very surprised at what you find.
Have questions? Give us a call or shoot us a note at sales@soundwatertech.com.
