KevinRoach:
No it is the pumps causing the fluctuation and the code I am using is ok but not very fast on the reading and seems it has to settle in a bit for the reading to stablise.
Sounds like you need a Bessel digital filter, has the best settling properties (low phase distortion,
no overshoot or ringing).
Try the York uni app: https://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html - it generates C code for you