Water Content in Used Oil Analysis

Water contamination in oil can be extremely detrimental as it can cause any of the following:

  • Rust/corrosion
  • Sludge formation
  • Increase in oil oxidation
  • Decrease in fatigue life (bearings, gears)
  • Increased wear
  • Reduced additive protection
  • Filter plugging


Water contamination can come from various sources such as:

  • Condensation
  • Low temperature engine operation
  • Inter/after cooler of air compressors
  • Leakage (oil coolers)
  • Ingress
  • Water in the new oil
  • Combustion gas

Water Test

Water content is measured by various means depending on the oil application, system criticality and level of detection required.

Crackle Test

This is used as a screening test when only an indication of water contamination is required. A few drops of oil are placed on a preheated 200oC hot plate and the analyst listens for crackling sounds, similar to when you are cooking and you place anything wet into hot oil. This test detects free and emulsified water and can be sensitive to 0.05% (500ppm). This test may not detect dissolved water and does not determine the quantity. It is also a very subjective test.

Fourier Transform Infrared (FTIR) Analysis

This is primarily used for engine, transmission and gear oils where the water content alert level can be between 0.1 – 0.5% (2000-5000ppm). Sometimes hydraulic and compressor oils are also tested by FTIR. However this is not the best method for critical systems as the warning alert levels are usually between 0.5 -0.1% (500-1000ppm), which is close to the detection limit of this method. 

The water content can be automatically determined by a computer from the size of the water peak on the resulting Infrared (IR) oil scan as seen below.

Karl Fischer Coulometric Water Test

This test is used when the water alert levels are very low, e.g. in the range 5-5000ppm. This is generally used for testing critical hydraulic, turbine (gas, steam & wind) and refrigerant systems.

In this test the sample is often heated, the water evaporates from the oil sample and then automatically transferred to the solution in the test cell. Here it is titrated electrochemically and the water quantity determined. Alternatively the oil sample can be directly injected into the test cell if an evaporator is not employed.

Karl Fischer Volumetric Water Test

This is normally used for refrigerant and some gear oils where the water content can be as high as 1-100%. In this test the sample is directly injected into the test cell.

So when should you change the oil?

As in the case of all tests you should consult your oil supplier and/or testing laboratory to ascertain the appropriate alert levels, as these would have been determined from their technical experience and historical data.

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