FAS-DPD and Speedstir -- strange results

[singingpond]

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.

edited to add:

It does seem worthwhile to look at the other thread you linked earlier:
http://www.poolforum.com/pf2/showthr...-Fas-dpd/page2

A comment from chem geek in that earlier thread:
> It's not important to be precisely accurate when it comes to chlorine. You want to be in the rough ballpark, but even if you are off by 20-30% it's not terrible assuming you maintain conservative chlorine levels (if you're off by 50% or more, then that could be a problem). <

It still leaves the question, though, as to whether the Speedstir result is closer to correct than the typical manual result, or whether it's the other way around.