This type of meter is relatively new to flow measurement. Some meters permit measurements to be made external to the pipe, others require the sensor to be inside. The former are referred to as Clamp-On.

This type of meter is relatively new to flow measurement. Some meters permit measurements
to be made external to the pipe, others require the sensor to be inside. The former
are referred to as Clamp-On.

Principles of Operation

The meters employ acoustic vibrations to detect the flow either in ‘time
of flight’ or ‘Doppler Mode’.

1. Time of Flight Meter
This meter measures the difference in travel time between pulses transmitted
along and against the flow, beamed at an angle to the pipe axis. Clamp-on transducers
are capable of re-transmitting sooner and operating faster, whereas inserted
transducers make better sonic contact with the fluid. Variations in fluid properties
will cause variations in sonic velocities and hence will affect the accuracy
of the meter.

2. Doppler Meter
This meter employs the well-known Doppler Effect. A transducer transmits acoustic
energy into the flow to a receiver. With-no flow, the frequency received is
identical to that transmitted; however, with a flow the frequency reflected
from particles or bubbles in the fluid is altered linearly with the flow rate.
The frequency shift is linearly proportional to the flow rate. Some devices
have two transducers, for transmitting and receiving, whilst others use just
one to achieve both functions.

3. Differential Frequency Meter
This is a time of flight device in which the transducer is positioned so that
the ultrasonic energy is beamed at an angle to the pipe. One transducer is located
upstream of the other, and the frequencies of the beam in the upstream and downstream
directions are measured. The frequency shift is proportional to the flow velocity.

Operational Constraints and Performance

The meters are available for use in pipes larger than about 1 cm in diameter
and can measure flows in excess of about 0.5 L/min. The first two meters (i.e.,
the Doppler and differential time meters) will operate for Reynolds numbers
greater than about 4000 to 10000 respectively, whereas the differential frequency
meter can operate for Reynolds numbers less than 2000 and greater than 4000.

The Doppler meter usually requires reflecting particles to be present in the
fluid, the concentration of which varies with different designs. The minimum
velocities for the differential time and frequency meters, and the Doppler meter
are respectively 3 cm/sec and 15 cm/sec. Additionally the clamp-on meter needs
a pipe-wall thickness which is small compared with the transit distance of the
acoustic beam.

Accuracies of ± 0.5 to 10% FS, repeatabilities of 0.1% FS and linearities
of 0.15 to 0.5% FS are typical. In general, the Doppler meter is not as reliable
as the other two meters, since there is some uncertainty as to whether the latter
is measuring the average velocity or some other velocity since the penetration
depth of the acoustic beam is difficult to control. Slurries can sometimes be
totally opaque to the beam and hence no signal is measured.

In general, the Doppler meter can be used for fluids having some small amount
of entrained gas or solid particles, whereas the other two meters can be used
in clean liquids as well as in liquids having up to about 35% of solids.

The clamp-on type must have pipes which are sonically conductive.

Installation and Maintenance

The flow meters require upstream and downstream lengths of about 10 to 30 D
and 5 to 10 D, respectively. High intensity vibrations in connecting pipes should
be avoided as well as acoustic sources such as orifice plates, valves, bends,
etc.

Only routine calibration checks are required, no routine maintenance being
necessary.