This Appendix provides the basic theory of a light-scattering
OCM to assist the troubleshooter in diagnosing problems with an OCM.
There are three types of (OCM). The older 60(33) OCM and most of the newer 107(49) OCM are based on light scattering. They are essentially turbidity
meters. These meters measure the cloudiness of the water. The newer 107(49) light scattering meters are better
calibrated to detect oily emulsions than the older 60(33) meters. This increased sensitivity
of the 107(49) light-scattering
OCM to oily emulsions does sometimes lead to situations where other
light-scattering particles will be detected. These meters are, however,
designed to discriminate between oil and oily emulsion, and iron oxide
particles. The third type of OCM uses a different type of detection
technology, fluorescence detection technology. These OCM also meet
the 107(49) standard. Fluorescence
occurs when a molecule absorbs light energy of one specific wavelength
and emits light energy of a longer wavelength. Fluorescent compounds
(such as oil) each have a unique wavelength signature. These compounds
can be detected and correlated to the concentration of oil in water.
Silt/algae/iron oxide and other particles do not fluoresce at oil’s
wavelength. This allows an OCM based on fluorescence technology better
to discriminate oil and oil emulsions from other contaminants. Until
recently, this technology was limited to use in the offshore oil industry.
There are now models for shipboard use. However, this promising technology
has a limited track record at this writing.
The following
discussion focuses on light-scattering OCM models (from http://oilinwater.org/theory.html).
All light scattering OCMs use similar components but may be arranged
differently. There is insufficient experience available to discuss
fluorescence detection OCM technology in shipboard use in this Guide.
A cross-section of the probe body of a light-scattering OCM is depicted
in Figure 3.
In a light-scattering OCM, a light shines from the Lamp
(light source) to the other side of the chamber. The light is detected
at a Transmit Photocell and a Scatter Photocell. When clean water
is introduced, the light received at the Transmit Photocell will be
of the same quality as that which left the light source. Water is
a good transmitter of light so it does not attenuate the light.
As the amount of oil in the effluent is increased, the light
received at the transmit photocell will be reduced due to the oil’s
opacity. The curve in Figure 4 shows this decrease in light transmission.
This curve can also be distorted by the presence of solids and emulsions
as well as ageing of the light source(s).
Also in the detection chamber is a rod which blocks or “occludes”
the light to the Scatter Photocell. The light from the light source
cannot reach this Scatter Cell directly. Oil droplets act as prisms,
refracting or scattering the light around the occluding rod so light
will be picked up at the Scatter Cell, and less light will be received
at the Transmit Photocell. This physical phenomenon is used to measure
the oil content. The Scatter Photocell light curve as a function of
oil concentration is depicted in Figure 5. The amount of light received
at the Scatter Photocell is zero with no oil present [due to the occluding
rod] and increases with effluent oil content.
In summary, a light-scattering OCM is essentially a meter
that detects cloudiness or turbidity in bilge water. The newer 107(49) OCM use a variety of light wavelengths
from several light sources, including near infra-red and white light,
and usually have multiple diodes to detect both direct and scattered
light. In this way, the newer OCM are tuned to detect oily emulsions
and reject, within limits and type, most but not all
solid particulates. Due to the construction of the meters, some non-oily
emulsions and soot can also be detected as oil.
Detection of non-oily substances: Although it is not supposed
to occur, materials which are not oil are sometimes detected by the
OCM. The most common are:
-
Fine particulate matter – Soot as a result
of contamination from cleaning operations; iron and iron compounds
as a result of biological contamination; and biological detritus or
particles (usually accompanied by a foul odour in the bilge).
-
Non-oil organic compound – Soaps and solvents
together or alone will form droplets in water (i.e. emulsified droplets
approximately 0.1-1.0 millimetre in diameter). These droplets will
scatter light just as emulsified oils do and will be detected by the
OCM.
Detection of non-oily substances can result in occasional false
positives (high readings) or an inability to get reproducible readings.
This is not to say that the 60(33) or 107(49) meters are somehow defective. A 60(33) is unable to detect clear emulsions
of any type and therefore may yield false negatives (low readings).
In that sense, a 60(33) is imperfect by design in that clear emulsified
oily wastes pass through the OCM undetected. All instruments are able
to detect certain materials and are unable to detect and/or will falsely
detect others. Technology commonly used in shipboard oil content meters
is the best current solution when one takes into account cost and
potential problems with other detection instruments. The 107(49) light-scattering OCM is more sensitive
by design than the older 60(33) units. It detects both turbid and
clear oily emulsions while the 60(33) units
do not detect clear emulsions, allowing these emulsions to pass. Newer 107(49) light-scattering unit OCM also
distinguish some types of particulates (e.g., iron oxide compounds).
If one understands how the OCM operates and performs regular maintenance
and calibration, it can be a reliable instrument. The instrument alone
cannot diagnose all the additions to the bilge which may have occurred
on a ship. It is important for proper OCM operation to prevent exposure
of the OCM to these confounding factors. Refer to annexes 3 to 6 to
diagnose and troubleshoot these problems.
Understanding
the theory of operation of an instrument allows one to be aware of
potential factors which may interfere with the OCM’s accurate
functioning. All analytical instruments have Achilles heels and the
new 107(49) OCM is no exception.
Understanding these susceptibilities will allow the marine engineer
properly to operate the instrument and interpret data. The only sure-fire
way to accurately and reproducibly detect organic oils in aqueous
solutions is through voltometry using a method called cyclic voltometric
stripping (CVS). These instruments cost approximately US$100K and
are not able to operate continuously in line under shipboard conditions.
Short of this, any instrument which detects the presence of oil through
indirect means will experience similar pros and cons just as the current
generation of the newer 107(49) light-scattering OCM units.