Quote Originally Posted by singingpond View Post
I've been following this thread with interest. Glad you figured out a reason for the discrepancy!

However, the reason you discovered (a difference on how hard you swirl the sample) makes me wonder which result is actually the more 'correct' one. If the aggressive stirring result gives the truer report on the actual chlorine levels, that would suggest that a lot of us doing the manual swirl are getting results that are too high. I certainly don't 'almost shake' between drops.

I suppose as long as one uses the same technique every time, the differences between readings from day to day are still meaningful. However, one really wants correct readings, not just repeatable readings.

I'm going to try your experiment (doing the FAS-DPD manual measurement on two identical samples, just varying the amount of swirling between drops), to see how large a difference I can get.
I was wondering the exact same thing. I also don't swirl/shake agressively. I would think if my results were inaccurate that I would have noticed an algae issue (I have not) since I tend to keep my CL level at the lower end of the Best Guess recommended level.

I guess the way to find out is to add a known level of CL to water known to be CL free, then test using both methods.