visatron OMD

Most text books only cover the operating principle of the older type of Graviner oil mist detector. A lot of engines are fitted with the Visatron oil mist detector manufactured by Schaller automation. A venturi driven by low pressure compressed air draws the samples from the crankcase compartments. The samples from the crankcase pass through an oil droplet separator which removes any droplets of oil by centrifugal force before passing across the measuring track of the oil mist detector. A receiving photodiode converts the intensity of an infra red beam of light from an infra red light emitting diode into an electrical signal which is fed to the electronic evaluation unit. As the opacity of the sample increases so the intensity of the infra red beam being measured by the receiving photodiode will decrease. This will produce a lower electrical signal. To prevent the infra red filters becoming soiled by the crankcase samples and affecting the reading, samples of air are drawn in via sintered bronze filters into scavenging air chambers. A flow monitor fitted in one of the chambers verifies that the vacuum in the measuring chamber is sufficient to draw the sample through. A temperature sensor allows a temperature compensation to be made. If the opacity of the sample rises above a set level, an alarm sounds and a search run is initiated. This opens and closes the valves in the control box in sequence to indicate the location of the high oil mist. During the search run, groups of compartments are measured in relation to each other. Upon completion of the search run, the fault location is indicated. Maintenance consists of changing of the sintered bronze filters if they become blocked and testing the operational circuits and LED indicators using the test button. it should be noted that despite the scavenging air chambers, soiling of the infra red filters is still possible, especially with age. The equipment will interpret this as an increase in opacity. This can be compensated for by adjusting the opacity reference point to a maximum of 70% opacity. Physical testing of the equipment can be carried out by introducing a test vapour (from an aerosol can) into one of the sample tubes. The air leaving the venturi, together with oil drained from the separator is led to the fuel oil drain tank.