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Above - sketch to illustrate how an ultrasonic flow-meter works. An ultrasonic transducer (yellow) launches waves at an angle to the flow of gas/liquid/oil in a pipe, the waves are received at a second transducer (also yellow). Once received, the second transducer then launches a packet of waves that re-traces the path taken by the first ultrasonic pulse but in the opposite direction. One packet of waves travels with the flow but the waves travelling in the opposite direction travel against the flow. There is a difference in time for the waves to travel the same distance and this is measured by the flow meter and forms the basis of measuring the speed of the flow.

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Flow-meters

Transducer configuration

Ultrasonic flow-meters use at least two transducers aligned so that ultrasonic pulses travel across the flow of liquid or gas in a pipe at a known angle to the flow.  By changing which transducer is the transmitter and which the receiver, the path of the waves can be changed to be partially against the flow or partially with it. The difference in arrival times can be related to the flow speed. The method is commonly used for metering petrochemical oil and gas. Sophisticated flow-meters have several pairs of transducers - some pairs probe the centre of the flow in the pipe other pairs "swirl" pulses around the inside wall of the pipe to probe the boundary layer flow. A more accurate measurement of the flow profile can be made using more pairs to sample different parts of the flow-profile.

Cambridge Ultrasonics has helped clients in various ways, including:

  • Checking the performance of transducers using our visualization system.
  • Developing wave-guides for use with high temperature gases.
  • Designing electronic circuits to meet intrinsic safety standards.
  • Novel transducers for use under conditions of intrinsic safety.
  • Theory related to detection of arrival time.

When does a wave arrive?

In the case of flow-meters, an interesting and very pertinent question is, when does a pulse of waves arrive? It has close connections with quantum physics - Heisenberg's Uncertainty Principle (which in turn is derived from Fourier methods in mathematics). The Principle states that it is impossible to measure the energy and the time of arrival of a wave to better than a certain precision.

One way to measure when a wave arrives is to detect the first time that any energy arrives. When a wave first arrives its signal level is vanishingly small and consequently the signal-to-noise ratio is very poor so the result has relatively high random variability that result in high errors. It is also highly susceptible to dispersion (you can see dispersion in some of the FE videos on this web-site).

The industry uses a variety of criteria for arrival times, for example the time of the third zero-crossing of the electronic receiver signal. However, this is not all the criterion, in addition the criterion discards any measurements if they are different by more than say 10% from the previously accepted value (or possibly three values). This results in a system that is relatively insensitive to fluctuations in flow speed. It is widely accepted that the current technology of flow-meters fails to register fluctuations in flow. This method requires timing measurements accurate to about +/-5 ns or smaller. It's possible that the method breaks Heisenberg's Uncertainty Principle but the reason for this is subtle - this method cannot cope with randomly varying arrival times it requires arrival times to be consistently within a narrow range and there is a large degree of averaging of values in that narrow range. This method can only work well with more or less constant flow conditions.

One answer to the quesiton, "When does a wave arrive?" is to measure the time when the maximum energy in the ultrasonic pulse arrives. This approach requires simultaneously measuring energy and time and finding the time of maximum energy - it sounds simple but it most definitely falls into Heisenberg's Uncertainty Principle.

The best measure of the energy in a pulse of waves is found by match-filtering the received ultrasonic signal with the ultrasonic signal that is transmitted then calculating the envelope using the Hilbert transformation. The position of the peak in time after signal processing is the peak arrival time of energy. We at Cambridge Ultrasonics like this method and we have successfully used this technique in many projects not just flow-meters. It is unusual to use this method in ultrasonic technology but it is commonly used in radar.