They should be calibrated from factory, but as with all things they can go a little out at times. You can get a solution for (at a guess £5-£10) but would just try it first. Try if on your tap water too so you can work out your rejection ratio through your RO if you wish. Also, if you test on water in a not so clean cup / container it'll give you a high reading. So fill container / cup, drain, fill, drain and go on for a few times then fill and test.