How often should I need to calibrate my refractometer? I calibrate it every time before use and it is always off. One time (At Band camp) I had a hard time getting my fresh SW to the right SG. Added a little salt, tested, added a little more tested, added more, tested and so on and so forth. Got to a point that I checked the calibration and it was off again. Luckily it was only at 36ppm so just added RO/DI. But is it a problem with my Refractometer of is this just how it works, what would make it go out of calibration so easily?