Back in the day, I was lucky enough to get one of the first machines put out and I’ve loved it ever since. I calibrate it using Hanna for alkalinity and Salifert for CA and Mag. Probably do this once a month… I have used their exchange program to send the machine back and get a rebuilt unit twice over the years. Costs a couple hundred bucks but it’s worth it to me. You can get your own rebuilt kit for cheaper but things are very small in there so I just do the replacement. Like others have said, it’s a little off accuracy wise but it’s close. That’s why calibrating it helps. The unit gives me a good baseline on the tank. Now the NP is another story. Many, many people have issues with it. I’ve been waiting to see if another version comes out before I spend serious money on that. I’m on the sidelines.