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water chemistry

Here is a question for the water chemists.

I have a Milwaukee pH controller with a CO2 injection system on a 72 gallon
tank.  The pH is maintained at 6.9 (recently calibrated). I have a 4 mo old
Lamotte test kit for KH (BCG-MR indicator tablets and sulfuric acid
titration) and it measures 100 pm CaCO3 which equals a KH of 5.6 (conversion
factor =17.86).  According to the tables on the Krib, my CO2 is calculated
to be 22 ppm. I also have a 4 mo old Lamotte test kit for CO2, which uses
phenolphthalein and sodium hydroxide reagents.  Using this kit the CO2
measures 10 ppm.

Which is more reliable?  Which is "correct"?  What accounts or could account
for the difference? (Note that I do not add any salts to the water, nor any
pH adjusters. If there is excess "non-carbonate" alkalinity, what would it

Robert A Sirota