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Operating Instruction Visatron VN 93
06. 04
Function description
All VISATRON® devices continuously draw the atmosphere from every compartment
in the crankcase area through the suction system. The suction vacuum required is
generated through a wear- free air jet pump in the device, fed with compressed air
(drive air), and is consequently independent of the power supply.
The sample flow, consisting of the sucked-in atmosphere of the crankcase
compartment, is guided through an optical channel for measuring turbidity. The
turbidity of the sample flow is measured by the absorption of infrared light.
% Opacity is used as the dimensional unit of the turbidity; 100% Opacity means total
absorption, and 0% Opacity means no absorption. Oil mist becomes explosive from
a concentration of approx. 50 mg of atomised oil in 1 litre of air and up, which
corresponds to an opacity of approx. 40%.
The sensitivity, i.e. the opacity that is required to trigger an alarm, can be set at 10
different levels in all devices.
A slow increase in the operating opacity or the contamination of the optics is
com-pensated in the microprocessor system up to a maximum safety alarm
threshold.
The contamination is strongly limited through scavenging air chambers that are
supplied with fresh air and are located in front of the optics.
The VN 93 series is distinguished from the VN 87 series through the following
features:
- Scavenging air chambers are not supplied with fresh air from the engine room, but
instead with forced air, so fumes from a fire in the machine room cannot lead to an
alarm.
- The measuring head heating unit for the avoidance of precipitating condensation
water in the device, which can be delivered as an option to the VN 87, is integrated
in the measuring head of the VN 93.
- An optional RS 485 interface with a history storage device permits the connection of
the VN 93 to the ship's computer or PC.
- A pre-alarm relay, programmable in the sensitivity, is activated before an engine
stop will be triggered by a high concentration of oil mist.
- The possibility of a calibration with fresh air exists for the types VN 116 / 93 and
VN 215 / 93. It permits the measurement of the absolute opacity.
