I can see we have two issues here:-
1) Fiber optics intermittent signal. This will affect the sensor reading. What we can do is to test the fiber optic using fiber optic tester (if you have the tester). Otherwise the termination of fiber optic need to be done again so that we have good communication on fiber optic. Another thing we can try to swap the cable. What I mean is we have one pair for normal use (pair 1) and one pair for spare (pair 2). Now we used the spare cable (pair 2) which consist (Tx and Rx). We can try use Tx (pair 2) and Rx (pair 1) or Tx (pair 1) and Rx (pair2) to see which one is giving consistent signal without alarm. We need to remove this alarm first before we go to next troubleshooting.
2) Permittivity reading at 3.0 in air. This is cause of layer on sensor. It can be oil layer or conductive layer. If soaked with diesel doesn’t work, normally we need to clean the sensor manually. We have to rig down MPFM and use wood stick with cotton rag to clean sensor. But safety precaution need to apply. Never put hand inside the sensor since we have gamma radiation inside the sensor. The picture as below:-