Measuring Oil Content

Accepting that the term "oil" lacks specificity, we should accept that oil content measured can only be a reference and not a scientific value.

Scientific measurement of hydrocarbons can not be done on line, as it generally requires some kind of extraction method. A scientific instrument would be too complex for marine installation and could not analyze the effluent within the time constraint imposed by MARPOL for bilge water processing.

The solution MARPOL Annex I provides is both practical as well as reasonably accurate,and it prevents pollution from oil.

Since "oil" as used on ships covers quite a range of hydro-carbon chains, as well as oils from various origins, there is currently not one single accurate way to determine oil content expeditiously enough to use for continuous in-situ operation as is required for bilge water. Therefore Annex I does two things, it accepts that oil is a reference term and allows for reference measurement of oil content.

Today there are basically two measuring principles used. One is light scattering, the other fluorescence. None of the meters in use today, old or new, measure oil; they measure the effect of oil in the water on the light used in the analysis.

The "accuracy" of the meters has increased over the years, because IMO changed the reference mixtures used in the approval process, to better mirror real-life-bilge-water.

In the approval process the oil content meter is "calibrated" with simulated bilge water that has a known oil content. The meter is calibrated so that the meter reading corresponds to the ppm level of the sample water. The meter is not calibrated to read the oil, rather the meter is calibrated to the optical effect of the sample water.

That this is understood by the administrations is reflected by the fact that in a port state control situation, the accuracy of the meter is verified by the approved manufacturer's method rather than verification of the hydrocarbon content of the effluent in a lab analysis.