This is how precisely OELCHECK detects water

Water is one of the most common contaminants of lubricants and operating fluids and often causes serious operational problems. It can affect not only the lubricating film formation of lubricants, but also hydraulic properties or dielectric strength. Through evaporation processes in the friction point, water can accelerate their aging process and cause corrosion or deposits. For good reason, we therefore devote a considerable amount of attention in the OELCHECK laboratory to any contamination by water.

Table of contents

  1. Visual inspection and crackle test
  2. Infrared spectroscopy
  3. The Karl Fischer method

Visual inspection and crackle test

Before a sample is tested in the high-quality analytical equipment in the OELCHECK laboratory, it is visually inspected and subjected to a crackle test. After the sample has been stored upside down for at least 30 minutes, water droplets may appear on the white lid seal. Streaks or a milky clouding of the oil also indicate contamination with water. However, even if water is not conspicuously visible, it may be present in unacceptably high concentrations.

Therefore, the visual inspection is always followed by a crackle test. Using a pipette, a drop of oil (0.2 ml) is sprayed onto a 150 °C hot plate. If the drop contains more than 5% water, this escapes from the oil with a crackling sound, often in conjunction with steam bubbles. The laboratory technician rates the progress of the test with a numerical value from 0 (no reaction) to 3 (strong reaction). This assessment is based on his subjective observation and does not provide a quantifiable numerical value. Therefore, no "crackle value" is given in the laboratory report.

For the OELCHECK tribologist, however, a "crackle value" of e.g. 3 is just as much a warning sign as visible streaks or turbidity of the oil. They cause him to take a particularly critical look at the water values determined in the further process with IR spectroscopy and, if necessary, with the Karl Fischer method.

Infrared spectroscopy

Infrared spectroscopy is one of the most important tools in lubricant analytics. IR spectroscopy can be used to detect oil aging, detect mixing with other oils, and calculate contamination such as with water. By comparing a spectrum of the current sample with that of the previous trend sample or fresh oil, conclusions can be drawn about changes in the oil due to oxidation and nitration, additive degradation can be assessed, or any percentages of carbon black and water can be indicated. However, the detection of water in oil is relatively inaccurate. Values below 0.1% (1,000 ppm) cannot be detected by IR spectroscopy; higher concentrations can be reported in 100 ppm increments (0.11%, 0.12%, etc.). It is the ideal instrument for detecting water in mineral oil-based engine and transmission oils, or lubricants from applications where there are no problems from relatively low levels of water contamination, such as from condensation of moisture in the air, and/or where moisture that has penetrated the oil can evaporate during operation. In many cases, if more than 0.1% water is detected by IR spectroscopy, it is leakage, water that has entered the system from the outside, or a sample taken from the bottom of a reservoir after extended machine downtime.

The Karl Fischer method

However, for some types of oil, such as glycol- or ester-based synthetic oils, water determination using the IR method reaches its limits, just as it does for lubricating greases. In addition, the water content in hydraulic and transformer oils, for example, usually has to be determined much more precisely.

In these cases, the water content is determined with a titrator using the Karl Fischer method. Titration is a method of quantitative analysis in chemistry. A known substance, such as water, whose concentration in the oil is unknown, is reacted in a specific chemical reaction with a titrating solution whose concentration is precisely specified. Depending on the type of lubricant, a coulometric or volumetric Karl Fischer titration method is used. The coulometric method can be used to detect extremely low water concentrations in the trace range from 10 ppm (mg/kg) to values of about 10,000 ppm (mg/kg), or 1%.

It is the method of choice for:

  • All turbine, gear and hydraulic oils tested with an OELCHECK all-inclusive analysis kit 3 and higher.
  • Oils from refrigeration compressors where the limit value for water (according to DIN 51503-2) is max. 60 ppm, depending on the refrigerant and lubricant.
  • Mineral oil-based insulating oils for use in electrical equipment (according to DIN IEC 60422). In some cases, they may only contain a maximum of 15 ppm water at high stresses.
  • Synthetic lubricants based on polyglycols, such as worm gear oils.
  • Ester-based oils, such as rapidly biodegradable products. Water can decompose esters into their components, alcohol and organic acid. This "hydrolysis" can accelerate corrosion, among other things. OELCHECK tribologists therefore advise additional drying or a change already at water contents above 450 ppm.
  • Lubricating greases, because their water content is difficult to determine via infrared spectroscopy.
  • Diesel fuels according to DIN EN 590 (contains up to 7% biodiesel) as well as heating oils, which may have a maximum water content of 200 ppm, and biodiesel (maximum 500ppm).
  • In addition, this method is recommended for:

- Large oil volumes, such as in oil circulation systems of paper machines.
- Industrial plants with connected cooling circuits from which water can penetrate into the lubricant.
- Oils with relatively complex friction- or wear-reducing additive packages.

The volumetric variant of Karl Fischer titration is used to determine extremely high water concentrations of more than 1 % up to 100 %.
It is used for lubricants that inherently have a high water content, such as:

  • Fire-retardant HFC hydraulic fluids whose water content is between 20 and 50 %.
  • Water-miscible metalworking fluids.