Cause for deviating purity classes in comparative analyses

OELCHECK replies:

Even assuming that the samples are optimally distributed as a homogeneous mixture (samples withdrawn in sequence are already inherently distinguishable from each other) and that the calibration of the particle counters is carried out according to the same standards, the purity classes on deployment of automatic fluid particle counters (APC) may actually turn out to be divergent. The high viscosity ISO 220 or 320 gear oils are particularly affected by the differences, as they are deployed, for example, in wind power installations. These oils must normally be diluted before they can be examined with an automatic particle meter. Unfortunately, there is still no standard as to how and in which ratio this dilution should occur and how the particles present in the solvent are to be treated. The selected diluent and the respective mixing ratio can nevertheless significantly influence the determined purity classes.

For the dilution of the highly viscous gear oils, analytical laboratories use the following solvents in different mixing ratios:

• Heptane, xylene or kerosene, diluted with gear oil in a 1:1 ratio
• Pure toluene or a toluene mixture, in a dilution ratio of 1:3
• Specially cleaned aviation hydraulic oil, (basic oil for calibration) in a ratio of 1:1.

Used gear oil may contain the greatest variety of soft and hard particles. Hard particles, which get into the oil through pollution or abrasion, are particularly damaging to the rolling bearings. Particles which are relatively soft and which have a lubricant effect, such as additives and their reaction products, tribopolymers or silicon defoamer droplets normally do not harm the gears. For some types of oil, these soft components are desirable by-products for better reduction of wear. Small air bubbles and droplets can also interfere with the counting. During investigation, all of these particles, small droplets and bubbles pass through the optics of the automatic particle counter. The electronic counter can observe them as shadows. The counting procedure is not, however, able to distinguish soft particles or air bubbles from hard particles which may cause problems.

As a solvent, toluene can provide assistance here. It has the advantage of dispersing the majority of oil-based reaction products, tribopolymers and water and silicone droplets in oil so finely that the counter can no longer detect them. In this way, the particle counter only captures the hard particles. The deployment of toluene is highly practical for gear oils from a technical measurement perspective, but due to possible harmful effects on health, must be handled in the laboratory with particular care.

OELCHECK will be using toluene in future in connection with the optical particle analysis (OPA), diluting the oil in a ratio of 1:3 in such a way that it can pass the optical path at a steady speed. Before dilution, the particles are counted in a fresh solvent and taken into account as a blank value. Under these conditions, we can assume that the purity classes established in the OELCHECK laboratory reports reflect the actual condition of the oil with regard to harmful hard impurities as accurately as possible.

The values indicated for purity classes by on-line or on-site particle counters cannot be compared with the purity classes determined in the laboratory. These particle counters are highly suitable for tentative trend observation (e.g. the oil became cleaner). The displayed numerical value for purity cannot however be compared with calibrated particle counters.

If a sample should be found to have significant differences between the purity classes of OELCHECK and those of an external laboratory, the underlying conditions for the count should be ascertained and compared with our information.