Oil mist detectors

There are two methods of oil mist detection, Obscuration and Light Scatter. The earlier forms of oil mist detector used an obscuration type detector, the most well known type being the early Graviners. Schaller use obscuration in their Visatron range of OMDs. Light scatter is a modern method of oil mist detection used by QMI and the Latest Graviner Mk6

OBSCURATION

The older type of obscuration type of detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end which generates an electric current directly proportional to the intensity of the light falling on its surface. Lenses are fitted to seal the ends of each tube but allow light to pass. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tubes onto the cells which are then in electrical balance.

The samples drawn from the crankcase are drawn in turn along the measuring tube by means of a selector valve. If a concentration of oil mist is present in the sample, the light will be obscured in the measuring tube: electrical balance between the two cells will be disturbed and an alarm will be operated.

Later types of obscuration detector use an infra red light source at one end of the measuring chamber and an infra red receiver at the other end.

The obscuration type of detector has the advantage of simplicity of operation. The disadvantages are that comparatively long runs of pipework from sampling points to detector are required, only one sample point is measured at a time, and regular maintenance is required if false alarms are to be avoided. The actual monitor is located close to the engine and therefore if a high oil mist alarm is activated, the natural instinct to check the detector should be resisted.

Another major disadvantage of the obscuration type of oil mist detector is the relationship between oil mist density and the read out. As can be seen from the graph, the relationship is non linear. When there is no oil mist, then the read out is 100%. As the oil mist increases then a large change in the density of the oil mist is needed to change the obscuration read out by a small amount. Because of this non linear relationship, the monitor can not be graduated in units such as mg/l. It looks for deviation to trigger an alarm. Having said that, manufacturers claim that it is set to alarm at an oil mist concentration of 2mg/l.

LIGHT SCATTER

Light scatter is the interaction of light and matter. All materials will scatter light. (Light scatter is what enables us humans to see objects.) It is composed largely of light that is reflected or refracted. An LED transmitter shines light through a sample. The light is scattered from the oil mist particles and is picked up by a measuring receiver placed at 90° to the light source.

An advantage of this system is that it is measuring from a true zero. If there is no contamination of the sample (ie no oil mist) then there is no read out. Another is that as the number of oil mist droplets increase in the sample the amount of scattered light increases in an almost linear relationship, thus making it easy to calibrate the read out in mg/l.

The system usually consists of individual detectors fitted to the crankthrow compartments of an engine crankcase. In some cases a separate fan is used to draw the sample through the detectors, in others each individual unit has its own fan. The density of the oil mist is measured and sent back to the central processing unit mounted in the control room at up to twice per second. The oil mist concentration for each crankthrow can be automatically logged and trends plotted.

A disadvantage of the system is that splashed oil can contaminate the detectors. To prevent this care has to be taken in the placing of filters and baffles correctly.

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