If you're looking to buy another Potassium test kit, I've used the Elos recently
I don't like the lack of range between 3-400ppm
It's hard to know what your results are
Look at how small the range is
If you can get your hands on one, the Salifert is supposed to be easier to use, and has a more defined range as it uses the titration technique
Count the drops and there's your reading