Transit Time Ultrasonic Flowmeter
About transit time ultrasonic flowmeter
The transit time ultrasonic flowmeter is
an instrument designed for measuring the volume flow rate in clean
liquids or gases. It consists of a pair of ultrasonic transducers
mounted along an axis aligned at an angle 0 with respect to the fluid
flow axis. Each transducer consists of a transmitter–receiver pair, with
the transmitter emitting ultrasonic energy which travels across to the
receiver on the opposite side of the pipe. These ultrasonic elements are
normally piezoelectric oscillators of the same type as used in Doppler
shift flowmeters. Fluid flowing in the pipe causes a time difference
between the transit times of the beams traveling upstream and
downstream, and measurement of this difference allows the flow velocity
to be calculated. The typical magnitude of this time difference is 100
ns in a total transit time of 100 µs, and high-precision electronics are
therefore needed to measure it.
There are three distinct ways of measuring
the time shift. These are direct measurement, conversion to a phase
change and conversion to a frequency change. The third of these options
is particularly attractive as it obviates the need to measure the speed
of sound in the measured fluid which the first two methods require. This
scheme also multiplexes the transmitting and receiving functions, so
that only one ultrasonic element is needed in each transducer.
Transit time flowmeters are of more
general use than Doppler shift flowmeters, particularly where the pipe
diameter involved is large and hence the transit time is consequently
sufficiently large to be measured with reasonable accuracy. It is
possible then to achieve an accuracy of ±0.5%. The instrument costs more
than a Doppler shift flowmeter, however, because of the greater
complexity of the electronics needed to make accurate transit time
measurements.

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