I have been through three Hanna Salinity checkers. I have one sitting in a cabinet collecting dust. I use a Vee Gee refractometer which is one of the most expensive on the market. Super reliable and easy to use.My problem with Hanna was the hand held salinity checker. They want you to calibrate it I think once a month. I noticed that it drifted with result after about a week or so. My acro's began to lose color and some even began to have tissue loss. I began using the calibration packets about every week and a half to keep the salinity right on because all other parameter were great. Got sick of buying those calibration packets so I decided to go to a Milwaukee Digital Refractometer, that's when the horror story was revealed. Hanna showed 35ppt, first test on the Milwaukee showed 38ppt! Tested three times and the same result. About dropped my shorts. Adjusted my salinity to 35ppt via the Milwaukee and within week the acro's started coming back. Within a month completely back. Threw the Hanna in the garbage, would never trust their hand held salinity checker again!