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RE: CO2 tests vs. looking it up in a table
Roger has been advising Sylvia on methods to determine the CO2 levels in
aquarium water.
He gives the following advice regarding the use of an algorithm given on the
KRIB:
> If you do have the case where there is a small difference between the two
> extremes (implying a large buffer capacity), then you will need to use a
> pH meter to get numbers that are accurate enough to use.
< snip>
> The biggest problem with using this equation is that your pH numbers need
> to be precise; if any pH measurement is off by 0.1 unit up or down, the
> resulting CO2 estimate is off by 20% to 25%.
Roger, doesn't this same caution apply to the use of the CO2 - pH - KH
table, which a lot of people here seem to depend upon? In looking at the
table (posted by George Booth and archived at
http://www.thekrib.com/Plants/CO2/co2-booth-faq.html), the pH measurement is
incremented in units of 0.2 and the KH in units of 0.5.
I don't argue with the accuracy of the chart, or the advice you gave to
Sylvia, but it seems to me that the same advice you gave to Sylvia needs to
be given to everybody using the chart as well. There are many pH test kits
being used which are not capable of giving results accurate to 0.2 units
(most cheaper kits read to 1.0 or 0.5 pH units. If someone wants to rely on
this chart to determine the CO2 content of their water, they should make
sure that they are using pH and KH test kits that give results with the
required level of precision.
As George stated in his post, "Pay close attention to the accuracy - if the
KH kit is accurate to +/- 0.5 dKH and the pH kit is +/- 0.2 units, the range
of CO2 values is quite large."
James Purchase
Toronto