Page 322 - Introduction to Marine Engineering

296 Instrumentation and control

compensating resistor is incorporated in the measuring circuit. The
insulating plunger varies the water flow in order to correct values to
3
20®C for a convenient measuring unit, the microsiemens/cm or dionic
unit. A de-gassifier should be fitted upstream of this unit to remove
dissolved carbon dioxide which will cause errors in measurement.
Oxygen analyser

The measuring of oxygen content in an atmosphere is important,
particularly when entering enclosed spaces. Also inert gas systems use
exhaust gases which must be monitored to ensure that their oxygen
content is below 5%. One type of instrument used to measure oxygen
content utilises the fact that oxygen is attracted by a magnetic field, that
is, it is paramagnetic.
A measuring cell uses a dumb-bell shaped wire which rotates in a
magnetic field. The presence of oxygen will affect the magnetic field
and cause rotation of the dumb-bell. The current required to align the
dumb-bell is a measure of the oxygen concentration in the cell.
The sampling system for an inert gas main is shown in Figure 15.22.
The probe at the tap-off point has an integral filter to remove dust. The

PRESSURE REGULATING
VALVE

VACUUM
BREAK

SEPARATOR

XVALVE
C A LIBRA!
OXYGEN
GAS
ANALYSER

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DRAIN
Figure 15.22 Oxygen analyser