Particle counting & purity classes in the OELCHECK laboratory
Clean is not pure! Even oil that appears pure at first glance might be contaminated by the smallest particles. Contaminants in the oil are one of the biggest risk factors for hydraulic systems in particular. In all methods, the number and size of pollution particles is determined. The number of particles in different size classes is determined from an oil sample and then converted to the number of particles per 100 ml. The following standard methods are commonly used to count particles and assign them to cleanliness classes:
Counting using a microscope
The oil is diluted with solvent and drawn under vacuum through a rastered 0.8 µm or 0.45 µm filter membrane. The particles lying on the filter are "counted" under the microscope. Whereby this counting is mostly a comparison of images. Reference photos of filters with a known purity class are used. It is estimated which purity class best matches the image under the microscope. The evaluation can also be performed using image analysis software. However, with this method only an allocation into 2 classes, > 5µm and >15 µm, is possible according to ISO. Dark oils, such as gear oils, cannot be counted with the filter method, since it is not possible to distinguish the particles using a dark filter paper.
Counting with laser sensors
With this counting method, the oil flows past a light source and, depending on the size of the individual particles, reduces the intensity of the light ray detected by a photodiode. The change in intensity of the light ray triggers voltage changes on the photodiode, which are a direct measure of the particle size. However, a prerequisite for correct determination is that the particles pass the light ray one after the other. With this measuring method, air bubbles and water droplets can falsify the result.
Since laser sensors from different manufacturers emit light of different brightness, the "bright" sensors may have relatively high numbers, especially in fresh oils, while older sensors do not see such subtle differences in oil-soluble "particles". Therefore, differences in counting fresh oils in different laboratories using laser sensors and automatic particle counters (APC) will often occur even despite the best calibration of the equipment.
Counting via differential pressure method
Many of the online counters and the relatively inexpensive portable counters operate according to the screen analysis principle. In this process, the differential pressure increases when particles are deposited on a screen, thereby increasing the flow resistance, e.g. in the area where the particles > 4 µm, >6µm or >14 µm "wash up" and clog the filter pores. The pressure increase compared to the unobstructed flowing reference oil or the electrical contact that rises when the increasingly clogged filter bulges allow an assignment to cleanliness classes without actually counting particles. No distinction is made by type (soft or hard), origin (water, dust, air bubbles) and by shape of the particles (long or round) in any of the methods presented so far.
Counting with imaging methods
The ideal method for counting particles in oils is to photographically image and then measure the particles. The oil flows evenly through a cell formed between two glass plates. The particles are spread over a large area in the cell, and laser light illuminates them. A high-speed camera "shoots" images of the particles. The size of the particles is calculated using the number of pixels. With this evaluation based on a defined pixel size, no calibration of the counter is necessary, since the size of the particles is not calculated via a representation of an equal-area circle. Using the imaging method, it is possible to distinguish between individual particles and their cause of formation in the form of optical particle analysis (OPA).
