Page 321 - Introduction to Marine Engineering

Instrumentation and control 295

bore) tube. The liquid flow in the capillary is such that the difference in
pressure readings taken before the capillary and after it is related to the
oil viscosity. A differential pressure gauge is calibrated to read viscosity
and the pressure values are used to operate the heater control to
maintain some set viscosity value.

Salinometer
Water purity, in terms of the absence of salts, is essential where it is to be
used as boiler feed. Pure water has a high resistance to the flow of
electricity whereas salt water has a high electrical conductivity. A
measure of conductivity, in Siemens, is a measure of purity.
The salinity measuring unit shown in Figure 15.21 uses two small cells
each containing a platinum and a gunmetal electrode. The liquid sample
passes through the two cells and any current flow as a result of
conductance is measured. Since conductivity rises with temperature a

Bimetallic strip

Gunmetal ring—HE532C
Meter
Insulating tube—

Platinum ring
O
Supply
Insulating tu

\7

Gunmetal ring

Flow
Figure 15.21 Salinometer