Hi there,

Since the KHD is providing consistent values and you've already checked the pH probe, that means everything on the unit itself is working properly which is great! Having also checked and verified the sample tube volume further shows the device is providing reasonable values. That leaves us with looking at the Hanna checker itself...

These instruments have been found to provide different values with each test being heavily dependent on the age of the reagent powder. We've written an article which discusses the cause of these differences. You can find that here, if you're interested:
https://www.aquariumcomputer.com/kno...oubleshooting/

The best way to fine-tune your KHD would be to run a few tests using the 7.5dkH Reference Solution. This will be far more accurate than traditional kits as it rules out the common variables of dust, fingerprints, sample size interpretation, etc. You also have an adjustment tool in the KHD settings which lets you adjust the KHD's end result to match that of the reference solution if need be.