Refractometer...although to be honest, I wish I'd skipped the LED lighted one as that feature is pretty useless. And it's a rare day when I don't check the calibration with cal fluid before use. I would probably do that even if I had a digital one. Plus I like the mad scientist-y feel of finding the best light and peering into some dark eyepiece to unravel the mysteries of the universe. More likely it's to find out how much I gacked up the salinity by guessing on salt amounts by filling a half empty storage tank, which is the same thing right?