Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/120548
OIL ANALYSIS Once the sample cools, an approximately 25-microliter aliquot is dropped onto chromatography paper (or filter paper) and allowed to spread or wick for 1 hour in an oven set at 80 degrees C (176 degrees F). The filter paper is then placed under a light source in order to locate the various rings. The calculation of a dispersancy index is provided by the measurement of the different sizes of the diffusion rings Figure 2. Lubricants at different of the oil and the levels of wear pollutants. The current blotter test method by visual quotation remains subjective and not based on mathematical models. Although the blotter test offers limited value measuring soot concentration, it provides an excellent assessment of the lubricant's dispersancy performance. An oil that is properly dispersing soot and other insolubles produces an evenly graduated blotter. A blotter indicating a high soot load but even graduation suggests that the oil is still fit for service but should be watched closely for degradation. When dispersancy begins to fail, the insolubles start to form a dense ring on the exterior of the absorbing oil drop, as seen in spot 7 of Figure 2. Spot 9 indicates the characteristic dense black dot and shape periphery that forms when the oil completely loses dispersancy performance. From a maintenance perspective, when the ring begins to form around the exterior of the oil blotter, it is time to look at scheduling an oil change. Certain laboratories have established their own quotation method of the spot by a measurement of the diameter of certain halos (or rings) and calculations of diameter ratios. Because these methods are not published, it is almost impossible to compare the results obtained between various laboratories. Moreover, with the introduction of new oils and fuels (e.g., Figure 3. Oil spot preparation biodiesel, ethanol, etc.), the appearance of multiple rings caused by various pollutants (carbon particles, etc.) is noted. For these reasons, the visual/manual interpretation of the various rings is very complicated and not easily exploitable. 30 March - April 2013 | www.machinerylubrication.com There exists an automated apparatus that facilitates the interpretation of the spot and eliminates the subjective aspect from the manual method. This instrument is equipped with a monochromic charge coupled device (CCD) camera and does not use the information color of the spot or differentiate each ring of the spot. The apparatus compares the diameter of the spot with a theoretical diameter and analyzes the opacity homogeneity of the spot. Of these two parameters, the device calculates a dispersancy index that varies from 0 to 100 (with 100 being the ideal dispersancy). A New Approach The goal of this new approach was not to reinvent the blotter test. A computer is used with dedicated software that was specifically developed for recognition and analysis of color images. With the digital imaging analysis of the spot, in particular its opacity and its spreading out by means of a dedicated algorithm and the choice of perfectly adapted filter paper, it becomes possible to evaluate in an objective and quantified way the residual power of a lubricant to disperse insoluble matter. The general principle of the method for the preparation of the spot remains virtually unchanged. It consists of depositing a volume of 15 microliters of oil on a specific filter paper and analyzing the rings of the spot, which are representative of the dispersion of the pollutants. The sample volume was decreased from 20 microliters to 15 microliters to limit the size of the spot and to make it compatible with the image-analysis system, as well as to be able to analyze all types of lubricants. The deposit of the oil on the filter paper is carried out at room temperature or in certain cases at 200 degrees C in order to free itself from the viscosity of the sample. The filter paper is then placed in a drying oven at 100 degrees C for 24 hours. The instrument used for this new approach features a light source positioned above the measurement table (direct light) and a color CCD camera equipped with a high resolution. It also has dedicated software that is capable of monitoring both the light source and the camera. The software memorizes the numeric color picture of the spot. This instrument makes it possible to take a digital color picture of the spot as the human eye sees it but with higher resolution. The image is memorized for the treatment and added to the test report, which enables you to visually check the results reported by the software. The use of a color camera allows you to identify the various rings by obtaining chromatic information on the spot. In order to analyze the spot under the same conditions of lighting for optimized reproducibility, the calibration of the device is carried out on a white sheet of filter paper. The software reports the following data: • The color image of the spot as the human eye perceives it. • The digital model in levels of gray associated with an opacity of the delimited ring by its real form. • The number of rings present in the spot, with ring 1 being the last external ring. • The diameter of each ring.