FAS-DPD and Speedstir -- strange results

[JimK]

chem_geek: I watched the Taylor video, and I would say that my original swirling (not the aggressive style) was about the same as in the video. One difference between my swirling and the video is that I am not yet able to swirl and drop at the same time (my coordination isn't there yet). I would add one drop, then swirl, then another drop, then swirl, etc. I noticed in the video that the presenter was manually swirling the entire time while adding drops (which is similar in behavior to the Speedstir). When I use the aggressive style swirling, the water keeps moving between swirls while adding the next drop.

Last night, I performed test #10 from PoolDoc's list. This involved using the Speedstir to mix the DPD powder in each of two samples, and using the Speedstir while dropping one sample and manually swirling the other sample (with my original style swirling). The Speedstir took 16 drops and the manual took 20 drops. I also tried chem_geek's suggestion of agitating the water before adding any reagents. I ran the Speedstir on the water sample for about two minutes before adding the DPD powder. I mixed the DPD manually and swirled manually after each drop. This took 20 drops. At least with this test, it appears as if the difference in results is due to the stirring method while adding drops. I'll perform tests #12 and #14 from the PoolDoc's list, but I need to order more reagent.

This is interesting.

I've always added drops while continuously swirling much like in the Taylor video.

[nefretrameses]

Doc or Chem geek: Just wondering if there was ever any resolution to this.

Santa brought me a speedstir and with pollen starting to fall in my pool, it's time to ramp up my testing program.

[chem geek]

No resolution as far as I know. It's very strange because your experiments would seem to indicate that it's the amount of stirring while adding drops that is making the difference. Agitation beforehand didn't affect the results. Did you ever contact Taylor about this? I know Ben mentioned you call them, but I didn't see any response about that.

[tpspeier]

Sorry I never replied back to this thread. Our pool developed a pretty serious leak around the time of my last post and we ended up losing (and replacing) a lot of water. As a result, the CYA level came way down (to about 40 ppm), and I brought the FC level down with it. After this drop in FC, I was no longer able to reproduce the differing results between the SpeedStir and the manual stir methods.

[nefretrameses]

Thanks for the update tpspeier. (I think Chem geek thought I was the OP). So maybe the differing results are related to the absolute CYA or FC levels or their ratios?

Your original CYA was 110 - 120 and now is 40. Original FC was 8.5 or 11 (without or with speedstir respectively). What's you current FC at CYA = 40?

[tpspeier]

Prior to shutting down the pool for the winter, I was keeping the FC around 5.