A. The most common causes of OCM malfunction are
deposition of rouge or other deposits on cuvet (glass tube) surfaces
or detection of non-oily substances as oil. Air bubbles, very small
emulsified droplets of soaps and solvents, and particulate matter
such as soot and rouge, can cause the instrument to misread if introduced
to the sensor chamber. Uncertainty in OCM readings can be caused by
materials such as emulsified droplets of soaps or solvents which alter
the refractive index of the solution often without causing turbidity
or by particulate matter which may cause turbidity.
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▸ Coating of surfaces by rouge (iron oxide compounds).
For the purposes of this Guide, rouge is any iron-based compound of
small micron size. Although rouge is always present to some extent
when water and iron are in contact, certain conditions on a ship,
such as biological contamination of the bilge, can greatly increase
the amount of rouge that is present. In the initial stages of rouge
contamination, a black film, which is easily wiped away, can be seen.
As this condition progresses, a reddish deposit, which is resilient
and very difficult to remove by mechanical means, will develop on
surfaces. In the case of oil content meters, this deposited film reduces
the amount of transmitted light to the detector and causes a non-zero
reading on the OCM when conditioned with clean water. Most manufacturers
have recommended procedures using mild acids to dissolve rouge coatings
in their OCMs. Alternatively, a vinegar or 10% citric acid solution
is particularly effective for this purpose.
▸ Presence of air or other gaseous bubbles (e.g., water
vapour). The presence of air and other bubbles can cause OCMs to give
non-zero readings when zeroed. Bubbles interfere with the transmission
and detection of the light source and are perceived as turbidity by
the OCM. Readings are usually erratic when air bubbles are present
causing the OCM display to change rapidly from low to high ppm. The
presence of bubbles has several causes. The most common is cavitation,
caused by obstruction in the sampling tube, a leak on the suction
side of the separator, or pressure drop across a valve, any of which
can result in the formation of bubbles. This can be corrected by inspecting
and plunging out the obstruction in the sampling tube and/or by flushing
the valve and reinstalling the sampling tube. Air leaks in the system,
which can cause air bubbles bypass through the sample tube (and also
result in cavitation near the sampling tube) can be remedied by checking
for and repairing leaks. Other causes may exist depending on the unit
or physical installation.
▸ Sample outgassing: A less common cause is outgassing
of the effluent due to the presence of biological activity or other
physiochemical processes involving in situ generation
of gas. In this case, it is best to do the zeroing operation with
a source of water that is known to be clean.
Related problems include the inability to zero an OCM.
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▸ Attempt to zero the OCM with clean water flushing
using water that is known and confirmed to be free of turbidity and
air bubbles. If the meter still fails to zero, inspect the sampling
port tubes for obstructions. If obstructions are found, clearing of
these obstructions will usually correct the problem. Note that most
new OCMs have calibration modes that use the clean flushing water
piped to the cell.
▸ If the above measures fail to zero the OCM, one should
inspect the glass tube/cuvet surfaces for a black or reddish film
(rouge or iron oxide). If this film is present, it is most likely
the cause for the meter failing to zero. The OCM should be cleaned
per the manufacturer’s instructions. If there are no instructions,
vinegar or 10% citric acid solution can be used for this purpose.
Once this cleaning is done, the system should be thoroughly flushed.
Dissolution of iron deposits will result in outgassing and bubble
formation.
▸ If these measures fail, the OCM should be serviced or
replaced. The OCM does not need to be replaced if it is determined
to be detecting particulates or non-oily emulsions. Refer to annexes
2, 3 and 4 for recommended remedial actions.