or intance when i test for kh i can use 4 drops and it turns light yellow then upto seven to get it to look like the guide, now do you take 4 or7 or go in the middle and get a 5.5 lol all this can greatly change your results.
These are not colour matching tests per se: You should see a pretty much instant change from blue to (some kind of) yellow. This is the point at which I stop counting drops.
Measuring of KH can be (& therefore is) horribly inaccurate anyway using this method because they measure the carbonate buffering capacity and relate this directly to a measurement of KH but ignore any phosphate buffering. If any KH measurement is thrown off by the presence of phospahtes in the water, then automatically the measurement of CO2 is off.
Note that in-tank permenant CO2 testers I don't think are intended to
measure levels of CO2, rather you mix your accurate solutions to target a
desired level of CO2 in the tank (usually 30ppm, but it does not have to be this). You then use them to see if you have too little CO2 (blue indication), just right (green indication), too much (yellow indiication).
would using this manufactured KH4 solution be similar to you technical guys making your own
When us 'technical' guys make our own, it is also recommended that the solution is replaced every other week thus ensuring continual maximum accuracy.
I cannot speak for commercially available alternatives as I have never used one, but if part of its continual use of such devices involves no maintenance (i.e. replacement of test solution), then go figure.
LOL btw, if having access to some weighing scales, distilled water, bicarb and an oven makes us 'technical', then 7 years at college was an utter waste of time for me! I think it makes us more chef than teccie!
Andy