The optical sensor on the D-D ATO has a small void in the middle of the sensor where the cable enters, shown as the shaded blue area in the below picture. This area is not watertight so it is possible for water to enter this section and is not a problem for its operation. To stop moisture getting to the working parts the front and back of the sensor are encased in resin this is where the electronics and the inner mounting magnet are housed.

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