The causes of these deviations are usually particle counters of different technical quality, external disruptive factors when using portable instruments, and differences in sampling and sample handling.
Particle counters, such as those used in the OELCHECK laboratory, usually work according to the principle of light attenuation or light blocking. A laser diode serves as the light source. A photocell with integrated optics receives the incoming light. The larger the particle, the greater the voltage drop generated by the shadow area on the photocell.
Particle counters are calibrated with a test liquid and precisely specified test contamination according to ISO 11171:1999 and 11943:1999. Devices that output results according to ISO 4406 or SAE AS4059D should therefore show almost identical cleanliness classes.
Portable particle counters are available in quite a wide range, but their technology sometimes shows severe differences. This is also reflected in the prices, which vary between around 1,000 and 15,000 euros. The most advanced devices ensure extremely high precision. They achieve this, for example, by means of full illumination of the oil cross-section in the measuring cell and the digital technology used within the evaluation electronics. Usually the particles are categorized in eight channels according to their size. Technically simpler products are available at low cost due to less complex optics, fewer counting channels and a measuring cell cross-section that is only partially illuminated.
For the user, the differences are not obvious at first glance. Nevertheless, they can be the cause of deviations in the results of comparison measurements, although the requirements of the calibration standards are equally fulfilled. Particularly when counting particles in fresh oils, counting devices calibrated in the same way often yield significantly different cleanliness classes than laboratory devices equipped with a strong light source.
Gear oils usually cannot be counted on-site undiluted because too little high-viscosity oil flows past the light source, which is designed for higher flow velocities, or the cold oil is not conveyed through the 150 to 200 μm diameter bore of the counting cell. Particularly in the case of portable devices, other disruptive factors can occur which in turn influence the result of the particle count.