Page 31 - Instrumentation and Measurement

2.5.3 Ultrasonic Time of Flight Flow Meter
Figure 2.28 shows one-way ultrasonic waves can be used to determine the flow rate of a fluid. There
are either a pair of ultrasonic receivers and transmitters with one pair on each side of the pipe through
which the fluid flows and the other pair on the other side or a transmitter and receiver on one side of
the pipe and a reflector on other. Either way, there is one beam of ultrasonic waves travelling in the
direction of the flow and one beam in the direction against the flow. The flow meter can either
measure the average fluid velocity by averaging the difference in measured transit times between the
pulses of ultrasound going across the tube into and against the direction of flow or by measuring the
frequency shift using the Doppler effect, this depending on the fluid containing discontinuities that
reflect the ultrasonic wave, e.g. slurries and liquids with bubbles. Ultrasonic methods can be used for
pipes from 75 mm to 1500 mm diameter, with fluid velocities from about 0.2 m/s to 12 m/s with an
accuracy of about ± 1% or better.

FIGURE 2.28 Ultrasonic flow meter.

2.6 Other Measurements

2.6.1 Viscometer
Viscosity control of fuels is essential if correct atomisation and combustion is to take place.
Increasing the temperature of a fuel will reduce its viscosity, and vice-versa. As a result of the varying
properties of marine fuels, often within one tank, actual viscosity must be continuously measured and
then corrected by temperature adjustment.
Newton investigated the viscosity of fluids and postulated, for most fluids under prescribed
conditions, that flow rate is proportional to applied stress, more exactly that applied shear stress is
proportional to velocity gradient.

Where η is a constant called the coefficient of viscosity. Applying this equation to the case of flow
through a small-bore tube of radius (r) and length I gives:

Page | 31