So here is an update for you, and a bit of a story.
So I have been using just a digital refractometer (Hanna) which I must say has been dead on accurate. Recently (within the last few months) I have calibrated it as the manual instructs. It has to be calibrated with RO/DI water at a 0 TDS.
I got a regular refractometer from a member on here recently and calibrated it with pin point salinity calibration fluid. I tested the tank water and noticed it low (33PPT) so I figured I would top off with saltwater instead of just RODI. I did my water changes today (like I do once a week) and I always test my salinity. My refractometer read 35PPT which was perfect, my digital refractometer read... 38PPT!
So I calibrated it again and it still read 38PPT even when testing the salinity calibration fluid. So I check the TDS of my RODI unit and it is at 0, then I test it in the storage tank where I draw all my RODI water, 8PPM TDS. Bingo! this is why my digital refractometer was off by so much.
A while back I kind of stopped testing salinity for a while and I tested it after I calibrated the digital unit and it read 33PPT. I am pretty confident now that my cause here has to do with salinity being too low. I will be correcting this slowly, RODI unit is getting new filters (precaution) water storage bin is being sanitized and cleaned out.
Lesson here is to have multiple devices that perform the same test differently, if they are available.
