DP transmitter 7MF4433 fill fluid expands on either side

DP transmitters SITRANS P, DS III model 7MF4433 were commissioned in 2015 for RO plant.  After 3 to 4 years I have gone through  issue where DP value start drifting towards positive or negative side. Even  after calibration of DP transmitter, value again starts drifting. Till now I have replaced more than 7 transmitters. After a lot of struggle I found one common issue in most of them and that is fill fluid (silicon oil) expands within the measuring chamber on any one side of DP transmitters.   Although the process fluid is sea water with temperature of 22-34 C maximum. and no chance of high temperature. Picture is attached for reference. When i opened measuring cell sealed alien screw ,I found gas or air exhausted and diaphragm comes to its normal position. 

Anyone knows about this phenomenon, because ambient temperature is in between 22-34 degree during whole year.

Dear Danw,

Thanks for information. This hydrogen permeation is something new & interesting for me.