ASTM D7899 Standard Test Method for Measuring the Merit of Dispersancy of In-Service Engine Oils with Blotter Spot Method
4. Summary of Test Method
4.1 A drop of oil is deposited on a piece of specific filter paper held level and not resting on a surface. The filter paper is placed in an oven set at 80 °C for 1 h. The oil wicks across the paper. The oil spot is illuminated with a constant and homogenous LED (light emitting diodes) backlight. A CCD (charge-coupled device) camera positioned on the other side of the paper takes a picture of the spot in black and white mode. The software analyzes each pixel of the picture. Dispersancy characteristics of the oil are judged by how far the oil drop spreads, how large the central sooty area is, and how homogeneous the opacity of the spot is in comparison with a theoretical reference diameter of 32 mm.
5. Significance and Use
5.1 Dispersancy is the property that allows oil to suspend and carry away pollutants of diverse sources such as soot from combustion, metallic particles from wear, corrosion of mechanical parts, and insoluble products resulting from the aging of the oil.
5.2 When poured on a specific filter paper, oil that is properly dispersing soot and other insolubles produces an evenly graduated spot. The distribution of the different zones (Fig.1) will reflect the status of oil dispersancy.
5.3 While the oil spreads out on the filter paper, the oil carries contaminants, and due to the lamination phenomenon of the oil film, the particles of same size deposit on the paper on the same concentric zones.
5.4 This test method provides a simple technique for condition monitoring of the dispersancy property of in-service lubricants.
5.5 An oil that is properly dispersing soot and other insolubles produces an evenly graduated blotter (see Fig.2 - Spot 1). A ring of light debris on the outer circumference of the circular spot also indicates that the oil has retained its dispersancy properties.
5.6 A blotter indicating a high soot load, but even graduation, suggests the oil is still fit for service, but should be watched closely for degradation (see Fig.2 - Spot 2).
5.7 When dispersancy begins to fail, the insolubles begin to form a dense ring on the exterior of the absorbing oil drop as in Fig.2 - Spot 3. A brown or yellow stain on the blotter spot indicates oxidation.
5.8 Fig.2 - Spot 4 indicates the characteristic dense black dot and sharp periphery that indicates sludge and the loss of dispersancy as the particles have settled in the center and the oil has wicked outward.
5.9 From a maintenance perspective, when the ring begins to form around the exterior of the oil blotter, it is time to look at scheduling a drain. If the black dot is allowed to form, the situation is problematic because the undispersed portion of soot that has deposited upon surfaces will not be removed by the oil change. Often, several changes made at frequent intervals will be required to effectively scour the engine clean. Also, if dispersancy performance degrades at an unusually rapid pace, a more extensive review of combustion and ring performance should be undertaken.
