Several contributors have written regarding experiences with test kits.
The only way that you can definitively establish the accuracy and precision
(repeatability) of test kits is through the use of standards. If you run
your standard through the test and you measure the value of your solution
standard, then you can be confident that the test is accuarate. If you run
your standard 10 times and each time you get the same value, +/- some
tolerance, then you can be confident that the test is repeatable.
Any other method will only tell you (if your lucky) if today's results are
relatively greater than or less than yesterday's results. Nothing else.
Hach test kits can be purchased with solution standards, have documented
tolerances, and are easy to use. Their catalogue can be obtained for free
by calling 1-800-227-4224. The cost per kit is approx. $50, not cheap, but
you get what you pay for.
A while ago, I ran some test comparisons between the aquarium store bought
test kits and the Hach test using lab standards prepared at UWEC's
Quantitative Analysis lab. The aquarium store kits did not even come
close. The Hach test kits were dead on.
Elk Mound, WI, USA