Not Hanna. Standard titration tests. I observed a similar pattern to the one you're stuck in. And I did some experimenting with the test methods I had available to me.
I made reference standards per guidance I found from Randy (using calibrated autopipettes and scales), and tested Hanna, Salifert, and Trident with them. Only Salifert was spot-on. Hanna was off by just over 0.5 dKh and Trident (calibrated and recalibrated with Neptune reference solution) was "off" by more than 1 dKh.
Overall, I believe test kits are less error-prone than electronic measurement methods (don't have to worry about equal glass thickness, fingerprints/smudges, microbubbles, water tint, battery SoC, etc). With test kits, it’s basic chemistry. That’s it.
Now, is this the most scientifically rigorous method? Of course not. It's far less than ideal. But we're using hobby grade equipment here, so we need to work within the bounds of what we have available. More than accuracy, what we should focus on is precision and trust. Ultimately, it doesn't matter a whole lot if we keep alkalinity at 8.5 or 9 dKh. What matters is that it's stable. Which is why we need precision.
The accuracy portion is more based on trust. And since the most accurate measurements of the reference solution I created were obtained by using Salifert titration kits, I decided to place my trust in them and use those as my golden standard upon which everything else is referenced to.
I now rely on the trends provided by Trident, and verify monthly with Salifert and it's been working well for me.
