I'd like to create a standard and/or control solution to check the accuracy of my test kits measuring FC. I don't know how to do the math but I figure a teaspoon of 6% naocl in ?? amount of distiled water that should test at ?? FC.

Thanks

0.01 ounces (0.296 ml.)of 6 % bleach in 1 gallon should provide a reference sample of 4.8 ppm.

1 teaspoon (4.93 ml.) in 10 gallons will provide a reference sample of 8 ppm FC. Although, I think that you would have a hard time getting the amounts exact enough to be accurate.

You also can't count on the strength of the bleach. Bleach loses strength as it ages, starting out stronger than what it is labeled and going down over time. There can be as much as a 20% variation in strength from bottle to bottle.

My results from Aquacheck sticks, orthotolidine and DPD are consistently reading together 2.0 ppm less than my fas-dpd test from my TF-50 kit. I want to figure out where the error is.

Trust the FAS-DPD. You can use the 25 ml sample size to get more accuracy.

Keep in mind that the OTO test is completely unreliable at distinguishing anything that is 3 or higher from anything else 3 or higher, and is already questionable at 2. Test strips are dramatically imprecise. The error on the DPD test varies significantly over the available range and is much higher at higher FC levels. And none of the tests are ever better than +-10% of the measured value, and are usually worse.

Since the error varies so much with the level, what levels are you measuring with the various tests?

Yesterdays FC check:
AquaCheck, OTO and DPD=2.0
Fas-DPD=4.0

and these results have been typical.

That is a dramatic difference. I don't believe the OTO test or the test strips, but neither the DPD test or FAS-DPD test should off by nearly that much in those ranges.

I like JamesW's idea of doing a 25 ml sample by 0.2s with the FAS-DPD test. That reduces a couple of the possible sources of error, though not all of them.

Exactly. I'm being very careful to follow instructions as well. I'm using the extended version that you posted Jason and I'm double checking my technique. I don't know.