Since the results are consistent with itself, that means the KHD is working normally and providing results within factory tolerances. If you have 7.5dkH Reference solution, you can also test with that and fine tune the results using the adjustment tool in the KHD settings page.

The widely fluctuating results suggest that there may be an event or scheduled dose happening around the time of a scheduled KH test. If you'd like to test this, you take a cup of tank water and test out of that cup. As long as there are no air bubbles in the sample or reagent tube, you will likely see more consistent results.