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Oil Analysis And Testing

Flashpoint Testing

Engine Oil Analysis – Detecting diesel engine fuel through Flash-point analysis?

When you see a flame, what you are seeing is a burning mixture of fuel & Oxygen. Flammable liquids are volatile and have a high vapour pressure, which is related to how much of that fuel has evaporated and exists in the air as vapour. This vapour pressure is closely proportional to the liquid’s temperature; So as the temperature of the liquid increases, so does evaporation and therefore the concentration of fuel vapour that exists in the air. The flash-point of a liquid is the lowest temperature at which if enough flammable vapour is present, it ignites.


All flammable liquids such as diesel fuel and petrol have a flashpoint. As stated above; It is defined as the lowest temperature at which the liquid can form an ignitable mixture with air. The flammable liquid we are referring to in the case of oil analysis is usually diesel or petrol – fuel which has contaminated the oil.

Engine Oil Contaminated By Diesel Fuel

The oil in your diesel engine can be badly affected by fuel contamination. Diesel & Petrol fuels act as a thinner to your engine oil, causing the viscosity to drop dramatically. As we point out in the viscosity section, an oil’s viscosity is one of the single most important defences against abnormal wear and/or equipment failure.

Should the flashpoint indicate the presence of diesel or petrol fuel, then it may suggest that fuel is entering the crankcase by way of the combustion chamber. This is called blow-by. Another cause of fuel dilution is by raw fuel entering the crankcase due to dripping faulty injectors.

The flashpoint test works hand-in-hand with the viscosity test and together they can help us tell the difference between an oil thinning due to oil transfer and an oil thinning due to the presence of diesel fuel/petrol.


OCLS uses three Setaflash Series 3 closed cup flash testers, renown for their accuracy and durability.

Setaflash Series 3 Flash Point Tester

A new model Setaflash Series 3 Flash Point Tester