Crankcase of each cylinder is connected to the OMD, which continuously checks the air sample from each cylinder. If the amount of mist increases, OMD raises an alarm. OMD doesn’t reduce or prevent the formation of mist, but only gives warning in case the concentration rises above the level at which an explosion can take place.
Construction
The assembly consists of;
> Extraction fan-draws the sample from the sample points through the reference and measuring tubes via non-return valves.
> Rotary valve-This valve is externally accessible and is so marked so as to indicate which sample point is on line. In the event of exceeding the set point , the valve automatically locks onto that point so giving a clear indication of the locality of the fault condition.
> Reference tube-measures the average density of the mist within the crankcase, as there will always be some mechanically generated mist.
> Measuring tube- measures the opacity of the sample by means of a photoelectric cell as with the measuring cell. To exclude variables in lamps a single unit is used with beams directed down the tube by mirrors.
Working
The arrangement of OMD consists of two tubes of the same size, one is called reference tube and the other one is called measuring tube. Both these tubes are placed parallel to each other. At one end of each tube, a photo-electric cell is fixed. Photo-electric cells generate an electric current when light falls on their surface. The amount of electric current generated is directly proportional to the intensity of light falling on it. The other ends of both the tubes are sealed by fitting lenses that allow light to pass through them.
Equal intensity of light is reflected on the photo-electric cells using a lamp. Light passes through the lenses after being reflected by mirrors. One of the tubes has an inlet and outlet connection for introducing oil mist.
Measuring tube has a connection for oil mist, which is extracted from the crankcase with the help of an electric extractor fan. Samples from each cylinder are monitored by using a rotating selector valve, which connects each cylinder in sequence to the OMD. If the concentration of Oil mist in the measuring tube rises, the intensity of light reaching the photo-electric cell reduces. Now as both the tubes are electrically connected, reduction in the generation of electric current will induce an electrical imbalance between the two cells, which will lead to ringing of the alarm.