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.