What is Oil Mist Detector (OMD) ? Explain its construction and working. What maintenance are to be done on OMD?

The purpose of this detector is to detect the presence of oil mist at the concentration-levels well below the limit at which a crankcase explosion is liable to trigger.

FIG.: Oil Mist Detector(OMD)

Crankcase Oil Mist Detector Working

• It basically comprises two parallel tubes of similar size, each having a photo-electric cell (these are light sensitive and generate an electric current which is proportional to the intensity of the light falling on their surface) fitted at one end. The lenses seal the tubes but allow light to pass.
• Two identical beams of light from the centrally positioned same lamp, are reflected by mirrors to pass along the tubes on to the cells.
• One of the tubes contains clean air and is called the reference tube. The other tube has connections through which the vapours of the crankcase are drawn by means of an electrically-driven extractor fan.
• Sampling points are normally fitted to each cylinder crankcase and their connections are led to a rotating-selector-valve, which repeatedly connects each sampling point to the measuring tube, in proper sequence.
• If the concentration of the oil-mist is present in a certain sample, light shall be obscured before reaching the cell of the measuring tube, the electrical balance will be disturbed and an alarm will be operated.
• At the detection point, the rotator stops to indicate, which sampling point is to be investigated. After testing, the samples are led away to the atmosphere.

Note: The detector must be reset before the alarm ceases and then the sampling procedure will resume its cyclic monitoring. The detector should be tested periodically and its sensitivity and zero-setting checked. The lenses and mirror should be kept cleaned.