FAS-DPD and Speedstir -- strange results

[tpspeier]

I switched our pool over to the BBB method about two weeks ago. I purchased a Basic OTO kit and a Taylor K-2006 kit for testing. I have been very regular about testing and the chroline level has been very consistent (around 11 every night). Over the weekend I picked up a SpeedStir kit. I used the SpeedStir for the first time last night, and the chlorine level indicated was 8.5 (17 drops using 0.5 ppm per drop). I repeated the test without the SpeedStir (by manually swirling) and the level indicated was 11 (22 drops). I tried again and saw the same results. I then used the OTO test and the yellow was darker than the yellow for the 5-10 range (indicating the chlorine was above 10).

The delta between these tests seems too high to be accounted for in the margin of error. Does the SpeedStir somehow skew the measurement of the FAS-DPD test? I found another thread where chem geek indicated that the amount of swirling can lower the result slightly (http://www.poolforum.com/pf2/showthr...?6427-Fas-dpd&), so I'm not sure if the SpeedStir "over-swirls" the test.

Thanks!

[PoolDoc]

I'd recommend calling Taylor, and asking - 800-837-8548. If you are not getting clear answers, ask to speak

Ivusich, Wayne J.
Manager, Pool & Spa Sales
410-472-4340 Ext. 121
or
O'Grady, Jody
V.P. Sales Operations and Support
410-472-4340 Ext. 122

However, I think if you want to run this down, you're going to need to do some experimenting on your own. In fact, you'll be likely to get a more thoughtful response from Taylor (and especially, from Wayne) if you've done the process below, first.

Do this:

1. Get 2 gallons of distilled water @ Walmart (NOT spring, bottle, artesian, etc: "distilled"!)
2. Get a fresh jug of bleach.
3. Soak your test tube and your speed-stir bar overnight in 2-3 cups of distilled water + 1 cup of bleach
4. TRIPLE rinse your gear in distilled water.

5. Using your cleaned test gear, run the pool water test, side by side again.
(This eliminates stir bar or other 'carry-over' as the source of the difference.)

6. Re-immerse all gear in distilled water. Use the Speed stir in the distilled water, to clean the stir-bar

7. Empty your 1st jug of distilled water, and fill it with pool water. Take it into the house, and cover it with a towel so that it's NOT exposed to any light. Wait 24 hours.
8. Using your cleaned test gear, run the pool water test, side by side again.
(This eliminates oxidizables in your pool as the source of the difference -- the improved Speedstir mixing could result in the chlorine oxidizing goo that's already in your pool. If THIS proves to be the case, it would be a VERY interesting result, with significant implications for pool care!)

9. Re-immerse all gear in distilled water. Use the Speed stir in the distilled water, to clean the stir-bar.

10.Using your cleaned test gear, run the pool water test, side by side again. BUT, use the Speed stir to pre-mix BOTH samples before testing. WHILE testing, use the Speed stir on only ONE sample.
(This distinguishes oxidizable POOL goo, from oxidizable DPD reagent materials as the source of the difference)


11. Re-immerse all gear in distilled water. Use the Speed stir in the distilled water, to clean the stir-bar.

12. Using your cleaned test gear, run the pool water test, side by side again. BUT, add 1/2 of your FAS dose, swirl, and then wait before stirring. Let the non-Speed stir sample sit for 5 minutes, plus however long you use the Speed Stir. Let the Speed-stir sit 5 minutes before stirring.
(This checks to see if different timing due to stirring is the cause of the difference.)


(I wrote this quickly, and haven't proofed it carefully, so if something doesn't make sense -- I probably screwed up.)

[PoolDoc]

(membership updated . . . AND test process edited, since you rec'd notification email)

[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.