FAS-DPD and Speedstir -- strange results

[tpspeier]

I finally did through step #5. The results were about the same: 24 drops for the manual swirl and 18 drops for the Speedstir. I now have the gallon of pool water covered to test tomorrow.

[PoolDoc]

Thanks.

You're going about it carefully enough, so that no matter what result you get, it will be helpful info for many.

[tpspeier]

Results of step #7 are about the same: 24 drops for manual swirl and 17 for the Speedstir.

For steps #10 and #12, does it matter if I use yesterday's pool water from the gallon jug or if I take a fresh sample?

[tpspeier]

I tried the first step #10 from Ben's list with the same results. I tried several other experiments involving varying the drop rate on the two samples, but all of those showed the same results too. I forgot to try the second step #10.

On the bright side, the pool is crystal clear and we've really been enjoying it.

[chem geek]

Have you tried slowing down adding the drops to the SpeedStir so that you take as long for the two tests? Also, if you let the SpeedStir water set still after the test, does it turn pink again after a minute or so? I'm just stumped.

[BigDave]

Two thoughts:
1) Have you compared the volume of the sample in the speed stir to the other test tube you are using?
2) Do you put the stirring magnet in after measuring the sample or before? I'm sorry for asking the obvious, but sometimes...

[tpspeier]

Thank you for asking the obvious. Often times things like this end up being something very simple and we spend a lot of time over-thinking the problem. Here are the steps I tried last night. Hopefully one of you can find something obvious I've done wrong.

1) Obtained 1 cup of water sample from the pool in a measuring cup. The sample came from the deep end at elbow-depth about 12 inches in from the wall near one of the returns.

2) Filled both test tubes exactly to the 10 mL mark with water from the measuring cup.

3) Dropped the magnetic bar into the Speedstir sample.

4) Added exactly two scoops of the DPD powder to each sample. I used a fresh container of powder since I had almost exhausted the container that came with my K-2206 kit.

5) Started the Speedstir and manually swirled the other sample at the same time.

6) Alternated adding drops of the titrating agent to the two samples (one drop to Speedstir, one drop to manual). I held the bottle vertically above the sample with the tip of the bottle at about the same height as the top of the test tube. I squeezed until the drop seemed to hold on the end of the tip and then allowed the drop to fall into the sample.

7) The Speedstir was stirring the whole time I was adding drops to both samples. I was only manually stirring after each drop added to the manual stir sample.

The results still showed a difference of 2 ppm less for the Speedstir than the manual swirl. As best I can tell, the fact that the Speedstir was constantly stirring was the only difference between the two tests. I guess I could try turning the Speedstir on and off between drops to see if that makes a difference.

As a sanity check, I also tried the TA test using both methods and got the same TA results from both methods.

This may be an obvious question to the forum, but has anyone else tried performing the FC test using the Speedstir and manual swirl side-by-side like this? Nobody has replied indicating they have seen the same behavior, but neither has anyone replied confirming that they get the same results both ways. I've assumed that it is generally proven that the two methods show the same results, but I wanted to ask to be sure.