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Re: [APD] CO2 measurement

True points. The margin of error with color comparison pH tests is rather wide for some of us. Some do rather well with them -- "spot on" reliable -- and some of us might as well throw dice as pick a color on the strip.
But I wouldn't want anyone to take this for shooting at a rule of thumb the way some shoot at the wpg rule of thumbs ;-)  . After all, while no, the CO2/KH/pH table is not as good as actually having a direct measure of the CO2, it's the best practical measure at hand that folks can be reasonably expected to use with a generally reliable, repeatable performance.
Of course, rules of thumbs and measurements be darned, ultimately, one must follow the sage guidance to watch the plants. And, as a rule, I'd not recemmend otherwise. *However*, I would not want a newbie, dissatisfied with the rate of growth with a new tank set up to crankup up the CO2, assuming the measurements must be wrong. The rule of thumb I'd advise is use the table unless you have very good reason to suspect that your test measurements are systematically incorrect or some tank compound is fouling the CO2 -- carbonate system. Basically, jsut like the wpg rules of thumb, I'd recommend use of the table and if in strong doubt based on persistent results, make small incremetnal changes to suit.

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----- Original Message ----
From: Thomas Barr <tcbiii at yahoo_com>
To: aquatic-plants at actwin_com
Sent: Thursday, March 2, 2006 4:51:15 PM
Subject: Re: [APD] CO2 measurement

There's little issue with the CO2/pH/Kh table.

The problem is in the measurement of KH and/or pH.
Ph is fine if using a pH calibrated pen/probe etc and all the
electrical devices are off or well grounded.

Alkalinity test, the KH, measure only total alkalinity, not
specifically carbonate hardness. So hydroxide, borate alkalinity
and phosphate acids can play with CO2 determination as well as
user errors.

While seeking accuracy is useful in some cases, the reality of
the tools to determine the method are quite another matter.

It is not uncommon for careful aquarist to have 50-200ppm of CO2
and healthy fish, while some might have 30ppm reading and
gasping fish(generally due to low O2, but not always).

Accuracy to 5ppm +, - is about the best folks can realistically
hope for given the error of the best alklanity test kits sold,
is about 4ppm.

But.....even that assumes all the alk is bicarb, which is not
always the case.

Every method has a trade off and a set of assumptions.
Some better than others.

Tom Barr

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