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4. OIL END SENSOR
Oil end detection is based on the difference between the index of refraction
of air and the index of refraction of silicone oil.
The index of refraction of silicone oil [A] (1.40) is very near that of the sensor
casing (1.49); so, the oil-casing interface has a very large critical angle. As
result, when oil covers the sensor [B], light [C] from the LED [D] is only re-
fracted slightly at the boundary between the casing and the oil. The photo-
transistor [E] stays off.
However, the index of refraction of air is much lower (1.00). So, the critical
angle for the air-casing interface is much less than that of the oil-casing inter-
face. Light from the LED strikes the air-casing interface at an angle greater
than the critical angle. So
stead of refracting and activates the phototransistor.
when oil runs out, light from the LED reflects in-
j
1 Nov. '87
a
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