I’ve got a Hanna salinity pen and a refractometor and so far the pen has been hold calibration well. But I do check it periodically with the frac to verify it’s still good. If my eyes were better then I’d old skool it with the colors but it’s just healed to differet the colorsI use all Red Sea test kits. My Hanna pen for PH was so off even with calibration before use… point of sale in my opinion. Red Sea test kits are awesome across the board