Science of Water : Concepts and Applications

Water Hydraulics 87
Meter
backpressure is provided to keep the magnetic fl owmeter full of liquid during fl ow measurement.
Two diameters downstream should be acceptable (Mills, 1991).
√ Note: Magnetic fl owmeters are designed to measure conductive liquids only. If air or gas is
mixed with the liquid, the output becomes unpredictable.
ULTRASONIC FLOWMETERS
VAC
FIGURE 3.29 Magnetic fl owmeter system.
Flow
FIGURE 3.30 Time-of-fl ight ultrasonic fl owmeter.
Signal
conditioner
Transceiver
Transceiver
mA DC
Ultrasonic fl owmeters use an electronic transducer to send a beam of ultrasonic sound waves
through the water to another transducer on the opposite side of the unit. The velocity of the sound
beam varies with the liquid fl ow rate, so the beam can be electronically translated to indicate fl ow
volume. The accuracy is ±1% for a fl ow velocity ranging from 1 to 25 ft/s, but the meter reading is
greatly affected by a change in the fl uid composition.
Two types of ultrasonic fl owmeters are in general use for closed pipe fl ow measurements. The
fi rst (time of fl ight or transit time) usually uses pulse transmission and is for clean liquids, while the
second (Doppler) usually uses continuous wave transmission and is for dirty liquids.
Time-of-Flight Ultrasonic Flowmeters
Time-of-fl ight fl owmeters make use of the difference in the time for a sonic pulse to travel a fi xed
distance, fi rst in the direction of fl ow and then against the fl ow. This is accomplished by opposing
transceivers positioned on diagonal path across meter spool as shown in Figure 3.30. Each transmits
and receives ultrasonic pulses with fl ow and against fl ow. The fl uid velocity is directly proportional
to time difference of pulse travel.