I have a pretty expensive ATC refractometer and I have been having problems calibrating it correctly. I have 35ppt pinpoint calibration solution and it seems to read differently between calibrations, sometimes 2-3 ppt within 10 minutes or so. Yesterday I calibrated it at 35 and today it was down to 33, I calibrated it back to 35 and cleaned my tank for a while, and about 15 minutes later I put the calibration solution again and it was reading back down at 33. The temperature did not change at all during this time. Is my refractometer just going bad? I have no idea how these things work so I don't know what might cause one to lose calibration faster over time. Thanks!