Page 64 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

Releasing nitrogen or carbon dioxide in a confined or unventilated
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area can lower the concentration of oxygen to a level that is
immediately dangerous to life or health. Calibration should
therefore only be carried out in well ventilated areas.
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The analyser readings are directly proportional to the pressure in the
measuring cell. The unit is calibrated to a specific atmospheric pressure
and the small error due to atmospheric pressure variations can be
corrected if required. Continuous samples should be supplied to the
instrument by positive pressure. They should not be drawn through the
analyser by negative pressure as the measuring pressure then becomes
uncertain.

The filter should be cleared or replaced when an increase in sample
pressure is required to maintain a reasonable gas flow through the
analyser. The same effect is produced if the filter becomes wet due to
insufficient gas drying. The need for filter cleaning or replacement should
be checked regularly.

2.4.10.2 Electrochemical Sensors
Analysers of this type determine the oxygen content of a gas mixture by
measuring the output of an electrochemical cell. In one commonly used
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analyser, oxygen diffuses through a membrane into the cell, causing
current to flow between two special electrodes separated by a liquid or
gel electrolyte.

The current flow is related to the oxygen concentration in the sample and
the scale is arranged to give a direct indication of oxygen content. The
cell may be housed in a separate sensor head connected by cable to the
read out unit.

The analyser readings are directly proportional to the pressure in the
measuring cell, but only small errors are caused by normal variations in
atmospheric pressure.

Certain gases may affect the sensor and give rise to false readings.
Sulphur dioxide and oxides of nitrogen interfere if they are present in
concentrations of more than 0.25% by volume. Mercaptans and hydrogen
sulphide can poison the sensor if their levels are greater than 1% by
volume. This poisoning does not occur immediately but over a period of
time; a poisoned sensor drifts and cannot be calibrated in air. In such
cases, reference should be made to the manufacturer’s instructions.

2.4.10.3 Maintenance, Calibration and Test Procedures

As these oxygen analysers are of vital importance, they should have a
valid calibration certificate and should be tested strictly in accordance
with the manufacturer’s instructions before use.

It is essential that, each time an instrument is to be used, a check is
made of batteries (if fitted) and zero point (21% oxygen) setting. During
use, frequent checks should be made to ensure accurate readings are
obtained at all times.

Testing is simple on all analysers using atmospheric air to test the
reference point (21% oxygen) and an inert gas to test the 0% oxygen

30 © ICS/OCIMF/IAPH 2006