Koepel:
I see two pumps, the means pump ripple interference :o
When the pumps are indeed in series, they electrically influence each other an cause even more air ripple :o :o
The air ripple of those pumps is not very bad. Perhaps there is something else going on.
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.