Page 314 - Introduction to Marine Engineering

288 Instrumentation and control

number of rotations will give a measure of the total quantity of liquid
that has passed. The rotation transfer may be by mechanical means, such
as gear wheels, or the use of a magnetic coupling.
Another method is the use of a rotating element which is set in motion
by the passing liquid (Figure 15.1 l(b)). A drive mechanism results in a
reading on a scale of total quantity. The drive mechanism may be
mechanical, using gear wheels or electrical where the rotating element
contains magnets which generate a current in a pick-up coil outside the
pipe,,

flow velocity measurement
The vmturi tube
This consists of a conical convergent entry tube, a cylindrical centre tube
and a conical divergent outlet. The arrangement is shown in Figure
15.12. Pressure tappings led to a manometer will give a difference in

Entry tube

•Head of liquid related to
fluid flow velocity
Manometer

Figure 15.12 Venturi tube

head related to the fluid flow velocity. The operating principle is one of
pressure conversion to velocity which occurs in the venturi tube and
results in a lower pressure in the cylindrical centre tube.

The orifice plate

This consists of a plate with an axial hole placed in the path of the liquid.
The hole edge is square facing the incoming liquid and bevelled on the