"will the KH kit have directions to follow to test for CO2 amounts?"
no. the test kit tests KH only.
" what is meant by "to cross reference KH"
this is how you get an estimate for CO2 ppm
on the left of that chart (going down) is KH, above, (left to right)
is PH.
so you test your water for KH (
me 3.5ish)
then PH (
me 7.5)
so I look at the KH side at 3.5, move left to right till I get to what is 33.2ppm on the chart. then move up and I see i'll need to inject the CO2 till I get around a 1.0 drop in PH (7.5 to 6.5) and that
puts me around
33 ppm of CO2

roud:
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"Measured your KH, then see how much you need to reduce the pH to get your target CO2 ppm.
Say you tap water is a KH of 5m say you want 35-40ppm of CO2, you should add enough to get the pH to 6.6 and be able to keep it there.
Warning, KH may not be entirely carbonate hardness. This means you will think and believe you have MORE than you actually do, thus you may be under dosing CO2.
This issues will never be the reverse, eg, you are adding more CO2 than you think.
So the error is always on the safe side usign this method.
As the KH in your tap drops, say your KH is 1-2 degrees, there's just not much room for other sources of KH other than carbonate, at 4-5 and above, there may be.
So assuming most of the KH is carbonate hardness for a KH or 1 degree is likely okay.
Say you want a CO2 of 50 ppm for a KH of 1 degree? the chart does not cover those ranges of pH's, but you can scale using a similar higher KH to see what the pH adjustment would be.
So about 5.9 pH would give about 39 ppm and a pH of 5.8 would give about 48 ppm of CO2."
-----tom barr------
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