The basics of applying ultrasonic transducers for sensing objects or fluid flow
In a typical flow application, two sensors are placed a known distance apart. The flow rate can then be calculated given the distance and the transit time that it takes for sound to travel between the two transducers in both directions, as the moving fluid carries the ultrasonic energy at different speeds in each direction. This time difference is directly proportional to the velocity of the liquid or gas in the pipe. Determination of the flowing velocity (V f ) begins with the equation: V f = K × Δt/T L , where K is a calibration factor for the volume and time units used, Δt is the time differential between upstream and downstream transit times, and T L is the zero-flow transit time. Various compensation and correction factors are added to this basic equation to account for fluid temperature, and the angle between the transducers and the pipe, among other considerations. In practice, an ultrasonic flow meter requires real-world “hardware” and fittings (Figure 6). Transit-time flow meters work well with viscous liquids, provided that the Reynolds number at minimum flow is either less than 4,000 (laminar flow) or above 10,000 (turbulent flow), but has significant non-linearities in the transition region between the two. They are used to
Figure 6: An actual transit-time ultrasonic flowmeter requires various fittings and connections; note the dual ultrasonic transducers. (Image source: Circuit Digest)
measure the flow of crude oils in the petroleum industry, and are also widely used for measuring cryogenic liquids down to –300°C, as well as for molten metal flow metering – two temperature extremes. PUI offers ultrasonic transducers that are specifically designed for transit-time fluid flow applications. The UTR-18225K-TT operates at 225 ±15 kHz and has the narrow beam angle needed for this application of just ±15°. This transmit/receive transducer has a diameter of 18 mm and a height of 9 mm with 2200 pF of capacitance. It can be driven with a 12 V p-p train of square waves and up to 100 V p-p at a low duty cycle.
It also takes drive and signal conditioning circuitry An ultrasonic detection system comprises more than just the piezoelectric transducers. Appropriate and very different circuitry is needed to meet the drive requirements of the transducer in transmit mode and for low-level analog front-end (AFE) signal conditioning in receive mode. While some users build their own circuitry, ICs are available that can conveniently provide the basic drive and AFE functions along with additional features.
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