Regular examination and testing of the main circulating oil is important. Samples should be taken from a pipeline in which the oil is flowing and not from a tank or container in which the oil is stationary where it could possibly have stagnated and accumulated contaminants that are not from the engine. If this happens a representative sample of the oil lubrication of the engine will not have been taken and if an adverse analysis is subsequently returned then the wrong corrective action could be taken Smelling the oil sample may give an indication of fuel oil contamination, or if an acrid smell is present this could be a sign of heavy oxidation. Dark colour gives an indication of oil deterioration possibly due to xidation and a black colour denotes the presence of carbon Dipping fingers into the oil and rubbing the tips together might detect reduction in oiliness generally due to fuel contamination and the presence of abrasive particles. The latter may occur if a filter has been incorrectly assembled, damaged or automatically by-passed. Water vapour can condense on the surfaces of sight glasses, thus giving an indication of water contamination. But various tests are available to detect water in oil, for example, immersing a piece of glass in the oil, water finding paper or paste- copper sulphate crystals change colour from white to blue in the presence of water- plunging a piece of heated metal such as a soldering iron into the oil causes spluttering if water is present