Use a refractometer that measures seawater and not a brine solution like most. DD H2Ocean is my go to refractometer and I also have the Tropic Marin glass hydrometer. Neither of which have failed me. The Milwaukee digital tester isn't very reliable, as you have to calibrate each time with RO, which is a big no no. You should always test with a known solution and if it says to shake the bottle before use, then throw that garbage away. Aqua Craft is the only solution I'll use for my refractometer. Don't buy a cheap refractometer, they're just not reliable. I've known a couple people who like the Hanna, but I've never used one.