Taylor K-2006 FAS-DPD question

[Mimipell521]

Hey everyone, I have been using the Taylor K-2006 test kit for a week or so and I have been pretty confident in the results I get. I am pretty particular and detail oriented in my everyday life, so I never really had reason to question my results. That is until this morning.....

I am finishing up a SLAM (I have a different thread on that) and trying to keep my chlorine level at 12 for the next 24 hours so I can complete an OCLT tonight (I had been unable to do it the last couple nights due to storms). I got up at 4:30 for a fitness class and tested before I left...I was at 11FC. I was running late so decided I would add the small amt of chlorine I needed to bring it up to 12 after my class. I retested my chlorine at 7:30am (sun was now on the pool) with the plan to add chlorine to the pool. This time it tested at 13FC. ????? I'm stumped. The chlorine should not have increased at all, ESPECIALLY by that much in such a short period.

I have gone back and reviewed my test kit instructions as well as the sticky on here with extended directions. The only thing I can see that might be off is if the miniscus wasn't exactly on the line, if maybe it was the top of the water....would that make this much of a difference? I also probably don't hold the dropper at a straight vertical angle. I will be sure to do those two things next time I test to rule that out, (but would even those two things throw the test off this much?) but I'm just wondering if anyone else has any ideas about technique or lighting or ANYTHING that might be throwing off my results?? Both tests were done at my kitchen counter with good lighting and I never question when the test is over because the line between slightly pink and perfectly clear is very evident to me.....

thanks for for any ideas you have!!

 

[JVTrain]

Did you test for FC after you did the addition or just use dose with the calculated amount to reach 12 ppm?

 

[Mimipell521]

Just retested again making the changes as stated above (made SURE the line was at the bottom of miniscus and that my dropper was straight up, letting the drop "hang" at the tip before going in), and tested two different samples on opposite ends of the pool, both elbow-deep. I got 11 and 11.5. I wish the results were more consistent so I could be more confident in this process. I know this time they aren't "that" far off, but shouldn't it be exact?

Thanks!

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Did you test for FC after you did the addition or just use dose with the calculated amount to reach 12 ppm?

Last night I did not test after adding chlorine, I calculated how much I needed to get to 12 overnight and ran my pump for about 3 hours after. This morning I haven't added any yet because my test results seemed off....

 

[duraleigh]

but shouldn't it be exact?

Absolutely not. The tolerance of the test is plus or minus .5 ppm even when the results (drop count) are identical.

You are measuring quantities in the parts per million.....think about how small that really is.

Perhaps a scientific laboratory could yield better precision but then none of us could afford it.

The FAS/DPD test is the most accurate test for FC that is available (at any reasonable cost). It is a centerpiece of the methods we teach at TFP and is the single most expensive test we use. But like everything, there is a margin of precision.....it's .5ppm

I would add that most folks get more accurate with time. I can pretty much get drop precision (same drop count) each time but I have been performing that test for over ten years.

 

[JVTrain]

For the OCLT, you must retest for FC 30 minutes after your last addition of chlorine to verify your starting FC for the OCLT. Otherwise, you're guessing at the starting FC. Even if you calculate the proper addition and the desired FC outcome, the variables of not knowing exact pool volume, bleach/chlorine strength... you need to test FC after your last addition.

 

[Mimipell521]

Ah, ok, so my 11 and 11.5 are acceptable, then. I'd say it was my 13 that was off....I will be sure to focus on technique each time and not get "lazy" as that was probably why my 13 was off, then.

Thanks for for the prompt replies!! I shall keep on keeping on! :-D

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For the OCLT, you must retest for FC 30 minutes after your last addition of chlorine to verify your starting FC for the OCLT. Otherwise, you're guessing at the starting FC. Even if you calculate the proper addition and the desired FC outcome, the variables of not knowing exact pool volume, bleach/chlorine strength... you need to test FC after your last addition.

I knew I wasn't doing the OCLT last night because of the rain and storms. Had I been doing it, I would have certainly retested last night. I just wanted to raise the chlorine level to keep it at shock value for one more day so I could do the OCLT tonight. Thanks for checking, though!