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.