As an alternative to laborious manual particle counting with the microscope, particle counting was automated. For this purpose, the first laboratory devices were developed during the 1970s. Most of the particle counting devices currently used in lubricant analysis function according to the principle of light attenuation or light blockade and use laser diodes as a light source. Two types of device are deployed here.
Liquid particle counters with optical sensors (APC)
A laser diode serves as a light source for particle counting. A photocell receives the impinging light. The larger the particle, the larger the voltage drop generated by the shaded area on the photocell. The devices are calibrated with a test liquid and pre-defined test pollution according to ISO 11171 and/or 11943. A precondition for a correct determination is that the particles pass through the light beam in sequence, in order to avoid overlap in the measurement cell. In this method air bubbles and water, or silicon droplets can adulterate the result.
Liquid particle counter with imaging techniques (OPA)
For counting particles in oils, OELCHECK uses a significantly improved method, optical particle analysis (OPA). During the investigation, the oil streams uniformly through a cell developed between two glass plates. The particles are scattered in the cell and illuminated by laser light. A high-speed high resolution camera „shoots“ images of the particles. The size of the particles can be computed by means of the pixel count. No calibration of the counter is necessary during this evaluation on the basis of a defined pixel size. The optical particle analysis procedure which we presented in detail in ÖlChecker Summer 2002, categorises individual particles and their formation mechanisms on the basis of particle form. On the basis of the image capture of the particle outlines, the particle is categorised by form of origin. With the aid of assignment to different wear mechanisms, significantly improved evidence can be obtained on the state of wear of the installation.
Test devices which function in accordance with the principle of light blockade or attenuation, nevertheless have the disadvantage that, e.g. air bubbles, water droplets or roughly dispersed components inherent to the oil can influence the count (for further details, see QUESTION TIME on page 8). As a rule, this technology cannot be used for liquids deployed as a diphasic mixture, such as HFC liquids or mineral oils, the application of which entails the permanent entry of water, depending on the process.
The differential pressure procedure: a compromise solution
As an alternative method, the differential pressure procedure was introduced at the end of the 1980s and early 1990s. The decisive advantage of this technology lies in the fact that neither air bubbles nor water droplets and/or where relevant, the clouding of fluid caused by them lead to the distortion of the results. The devices designated mostly as „particle monitors“ function according to the screening principle. The differential pressure increases or the volume flow decreases as a function of the particles accumulated on a filter. The pore size of the screen is 5, 10 or 15 μm depending on the manufacturer of the device. Pressure or volume flow behaviour is evaluated by comparison with variously contaminated reference oils. In this way, classification by purity class is possible. Particle monitors on the basis of the screen blockade are designed in a highly user friendly way, but do not fulfil most specifications which require information on the particle count for the respective size.