FAS-DPD and Speedstir -- strange results

[tpspeier]

Ben, Thanks for the thorough response. I will work on these experiments over the next few days and report back before calling Taylor.

[PoolDoc]

I'll look forward to it: I'm hoping to learn something.

And, it's just curiosity, but what is your technical background? Not many people understand, much less use, the word "delta" in that sense.

[tpspeier]

My day job is a computer engineer, so I am quite particular about details and precision. This is one of the reasons I was so disturbed about the different results from the different tests.

I was re-reading Taylor's test kit instructions last night and realized I was doing one thing wrong. I was taking my samples from near the surface of the water (about 3-4" below the surface) instead of the recommended 18" deep. I took the samples at "elbow-depth" and observed the same results, but at least this rules out one possible source of variation.

[PoolDoc]

Hm-mh.

My own, very informal testing, has never shown much of a difference between surface and 6""+ down. But, if the surface is covered in goop, including suntan lotion slime, I always go deeper. If you want to go a step further, it would be interesting to know if there actually is a detectable difference between surface and immediate sub-surface.

One other note: on pools under full sun you would expect to see a difference in chlorine levels between the surface and 12" down, if the CYA level is zero. Half-life at the surface can be as little as 15 minutes.

[tpspeier]

Our pool is in full sun, but the CYA level is around 110-120 (I used the distilled water trick mentioned in a thread on this forum to measure since it was above 100) so I would suspect little difference in chlorine levels between the surface and below the surface. I may try the depth experiment once I resolve the Speedstir vs. manual-swirl variation.

[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.