Make sure you are rinsing with RODI after use, make sure before zeroing and also before the actual measurement that you cleaning the vial with a microfiber cloth to make sure there are no dried water drops or fingerprints. If you do back to back measurements and it off by more than 5ppb or 5% then I would contact Hanna.The unit I have will bounce all over the place. Maybe user error. I’ll look into it more and see if I’m just a dunce.
I’ve used 736 and have 774 and both have always been accurate and trustworthy.