This device is fitted to monitor the presence of an oil mist in the crankcase. In this monitoring process samples of the air and vapours mixture taken continually from the crankcase of a diesel engine. This device will detect the level of oil mist concentration which must remain below level. At which explosion may occur. In case of coming to this level a warning is given to allow avoiding action to slow the engineer and prevent serious damages and explosion. 

The presence of an oil mist in the crankcase is the result of oil vaporization caused by a "Hot spot" this hotspot is produced by other heating of any engine bearing or overheating of any other moving parts of the engine time chain or gear drives. Explosive condition can result if build up of oil mist is allowed. Hence continuous monitoring of vapours intensity is required. A diagrammatic view of an oil mist detector is shown in the sketch.

Measure any increase in oil mist density. The detector consists basically of two parallel tubes of equalize, each having an electric current directly proportional to the intensity of light falling on the surface. Lenses are fitted to seal the tube ends of each tube but allow light to pass. One tube is sealed to contain clean air and is termed the reference tube. The other tube is called Measuring tube, has connections through which samples of the crankcase vapours are drawn by extractor fan. If the concentration of explosive mixture is reached in the oil mist sample, light will be obscured(not clear) before reaching the cell of measuring tube, electrical balance between two cells will be disturbed and an alarm will be operated indicating excess concentrations in oil mist. Sampling points should be fitted to each cylinders crankcase and their connections are brought to a rotating selector valve which is driven from the fan motor. This repeatedly connects each sampling point  to the measuring tube in sequence.

In the event of oil mist being detected the rotator stops to indicate which sampling will commence again its sequence.
The detector should be checked daily and the sensitivity tested. Lenses and mirrors should be cleaned periodically. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tube on to the cells which are then in electrical balance and if light will be obscured before reaching the cells, electrical balance will be distured and an alarm will be operated.


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