I calibrate mine before every use using the pinpoint calibration fluid then I put RO
water on it to make sure it's zeroed out. By doing this I found out the first refractometer I bought had somehow broke and wouldn't you know it, this thing decided to break when I was putting my fish into hypo treatment (Thank god I was only 1/2 way there). I know a bunch of you have had bad experiences with Drs but when I explained the problem they overnighted me a new one (I went with the more expensive one Vital sine)on their dime and I just picked up the difference in the cost between the two. Like Larry said it only takes 30 seconds if not less. I dry it off with paper towel then wipe it down with warm fresh water, dry it off and store it in it's case with one of those moisture obsorbant packets.