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2. Transducer Construction (cont.)
BNC Connector
Transducer
Head
Cover
Figure 1: General C-PT Transducer Assembly
3. Measurement Methods
C-PT transducers can be installed using a number of configurations and clamping fixtures; however, how this is done
largely depends on the flowmeter's measurement mode. Although many of GE flowmeters have one mode called
Transit-time, some GE flowmeters have an additional measurement mode called TransFlection. Although both
methods use time measurement, the method in which time is measured is different.
3.1 Transit-Time Method
When in Transit-time mode, the flowmeter transmits ultrasonic pulses through a moving liquid. The pulses that travel
in the same direction as the fluid flow (downstream), travel slightly faster than the pulses that travel against the flow
(upstream). The flowmeter uses various digital signal-processing techniques, including cross-correlation, to determine
transit times, and then uses these times to calculate flow velocity.
3.2 TransFlection Method
To measure flow, one transducer transmits a group of pulses (typically 16 pulses) at regular intervals (approximately
5,000 to 10,000 transmissions/sec). The ultrasonic pulses travel through the liquid, reflect off scatterers (i.e. bubbles,
particulates) and the signal is then received by the second transducer.
In essence, these ultrasonic signals are "pictures" taken continuously at the same location in the pipe. The flowmeter
compares these pictures to one another as each picture is received. By comparing (averaging) these pictures, the
flowmeter is able to eliminate stationary objects by subtracting signals that do not appear to move in all or most of the
pictures. The flowmeter measures the time difference between the remaining "moving" objects on each successive
picture. The time difference is called T
and is used to calculate flow.
m
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C-PT Ultrasonic Flow Transducer Installation Guide
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