I have seen a similar problem with a pressure transducer calibrated to read tank level (feet of liquid). These were Rosemont level transmitters, measuring pressure but calibrated at the factory to read a certain range from 0 to X feet. I had 3 tanks of different heights, with 3 different transmitters. The problem was, these were bought used and the maximum height on each was different and none were built for the exact tank height needed.I then put another 2000lbs in and now, there still reading the same, bout now 2500lbs. I'M BAFFLED! it's 4000lbs of water in the tank.
I had one heck of a job finding a scaling equation for each transmitter that would read the correct level. I finally had to do it the hard way, by emptying the tanks, reading the signal at 0', then filling with water and reading the signal at max height. With the min and max, I could then set up the proper scaling for each tank.