Water content in oils and fats

Water is a very common contaminant in oils and greases, along with contamination from dust. It impairs the formation of lubricant films and is a cause of corrosion of machinery and equipment. In order to exclude any water-related risk, OELCHECK recommends testing the water content of every oil and grease sample.

Table of contents

  1. Multiple causes
  2. Serious impact
  3. Secure verification

Multiple causes

A contamination with water can have many causes.

  • Oil expands when heated and contracts when cold. At a temperature difference of 50° C, the change in volume of a 100 liter oil filling is still approx. 3.5 liters.
  • Outdoor oil tanks become alternately hot and cold with the outside temperature. Incorrect barrel storage can cause rainwater to be drawn into the oil when barrels are stored with the bung-holes facing upwards.
  • Stop-and-go operation only heats up a motor or machine for a short time. Condensate forms during cooling due to the low operating temperatures. In engine oils, water also condenses from the combustion gases.
  • Gearbox housings or containers of circulation systems often "breathe" through the filler necks. A screen on these necks prevents the direct entry of dust and usually also water. But condensation-promoting moist air can enter via this screen.
  • Shaft ducts, hydraulic cylinders or oil filler necks are often treated particularly thoroughly with a high-pressure cleaner because the oil sweated out here acts like a dust binder. The seals often can no longer withstand modern steam jets with a pressure of over 80 bar, which is why water penetrates the lubricant or the system. If the steam jet water still contains a grease solvent, foaming of the oil filling can sometimes also occur due to this cleaning agent.
  • Water from the cooling circuit, which usually contains antifreeze glycol, can get into the lubricating oil or engine oil via leakage points such as faulty seals or cracks in the cylinder head or block. Defective heat exchangers can also be the cause. In addition to water, glycol, which is incompatible with mineral oil, causes massive damage.
  • If oil has become "acidic" as a result of oxidation due to a long period of use or high temperature, and this significantly increases the neutralization number or AN (Acid Number) compared with fresh oil, soldered connections in the cooling water circuit in particular will be attacked. The acidic oil is then the reason why leaks occur at these solder joints due to corrosion and inhibited water enters the oil. Aged coolant can also attack the solder joints from the other side.

Serious impact

Water, like any other contaminant, accelerates the aging of lubricants. Water, which consists of hydrogen and oxygen molecules, can split into these elements under certain conditions. Hydrogen can lead to hydrogen embrittlement of steel, oxygen can accelerate oil oxidation.

  • The load carrying and lubricating capacity of water is significantly lower than that of oil. If water droplets are in the oil as free water, a stable lubricating film will no longer form at these points. The asperity peaks of the paired surfaces touch each other. Localized welding and mechanical abrasive wear occur.
  • Water is the cause of rust and corrosion on ferrous and non-ferrous metals. The rust particles (non-magnetic iron oxides) and non-ferrous metals removed by corrosion, such as copper, cause mechanical abrasive wear and accelerate oil oxidation.
  • At operating temperatures above 80 ° C, the penetrated water begins to evaporate. The bearing surfaces of plain and rolling bearings can be partially washed free of grease or oil by the steam bubble effect. Corrosion, abrasive wear or galling occurs more easily. In hydraulic systems, the steam bubbles created during heating can cause cavitation, especially in the hydraulic pumps.
  • Water causes ice crystals to form at temperatures below freezing. The oil is thus less fluid and can no longer fulfill its task as a lubricant. Refrigeration oils in particular, but also hydraulic and gear oils that are used at temperatures below freezing, are at risk here.
  • Insulating or transformer oils must have an insulating effect and must not be electrically conductive. Therefore, they should contain hardly any water. With increasing water content, the breakdown voltage is considerably reduced.
  • Water, especially in connection with dust, leads to sludge formation in tanks and containers. Oil additives, which are supposed to neutralize the water and thus make it harmless, also settle together with the water droplets in filters or at the bottom of the tank. As a result, the additive content might decrease.

Secure verification

Traces of water can be found in every oil. Even fresh oils are not absolutely water-free but contain between 15 ppm (insulating oils) and 400 ppm (engine oils) of water.

The following water contents are generally considered typical and tolerable:

Fresh oils 250 – 400 ppm
Gear oils 1000 – 2 000 ppm
Hydraulic oils 500 – 800 ppm
Engine oils 800 – 1800 ppm
Fuels / heating oils 80 – 200 ppm
Greases 1500 – 3 000 ppm


At OELCHECK, each lubricant sample is checked in several steps with regard to an increased water content:


With a density of approx. 1.0, water is more than 10% heavier than oil with a specific weight of approx. 0.9. If the oil contains more water than can be kept in suspension by the oil itself or by the dispersing additives, it settles at the bottom of the sample bottle as clear water. In such cases, contamination by water is clearly visible visually as 2-phase formation. The proportion of free water can be easily estimated when using the transparent sample bottles.

In glycol-based synthetic oils, which have a density of approx. 1.0 like water, water does not settle out but causes streaking. A cloudiness of the oil is often a sure indication of an increased water content, which can be between 500 and 5 000 ppm depending on the oil type, without free water being separated out.

Infrared spectroscopy

Regardless of the external appearance, the water content of each sample is always determined in the OELCHECK laboratory using FT infrared spectroscopy by the intensity of the absorption band. Water values above 0.1% can be determined accurately with the IR especially if the fresh oil is known.

Crackle test

Since the fresh oil reference is often not known in detail, all oil samples are additionally subjected to a "crackle test" for water detection. For this purpose, a drop of oil (0.2 ml) is sprayed onto a hot plate. If the oil contains more than 0.1% water, it foams briefly and the water escapes with a splashing and crackling sound.

With a trained eye and the experience of thousands of samples examined, sufficient accuracy can be achieved with the crackle test, usually confirmed by IR.

Karl Fischer titration

For many lubricants, such as all synthetic or bio-oils, the water content should be known with particular precision. Every ppm of water also counts in transformer oils, refrigerator oils and fuels. An absolutely accurate and reproducible water content is determined with an additional method, the Karl-Fischer titration. Depending on the oil type and customer requirements, OELCHECK analysis kits 2 to 5 include a water determination according to the Karl Fischer method.

Measuring relative humidity

Since 2021, OELCHECK also offers the possibility of measuring the relative humidity at application temperature, as well as a saturation curve. In combination with the Karl Fischer test, the water content can thus be evaluated even more specifically for oil type and application.