Two different FC readings?

pitmanr2003

Well-known member
May 21, 2010
143
I was testing this morning how much my chlorine dropped over night and got two completely different results back to back. I was at 10Fc before i went to bed and when I tested at sun up I was at seven. My first reaction was Im going to have to shock for sure. I decided to check it again because I wasnt sure If i had put enough dpd in it. the next time I got a 9. Does the ammount of dpd make a difference in the outcome. bothe were full scoops, but the last one was over a full scoop.
 

duraleigh

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Does the ammount of dpd make a difference in the outcome.
no. As long as you use enough powder to turn the water sample to pink/red, the outcome should be the same. Undissolved portions of powder in the water do not affect the test, either.
 

frustratedpoolmom

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Were they from the same sample of water, or did you pull out two different samples?
Were the sample(s) from the same location in the pool?
Was the pump running the whole time, or had it recently been turned on?
Is it possible (being early am) there was testing error...(drop miscount?) ( I know I'm not fully awake yet LOL :))
 

Richard320

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Jan 6, 2010
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That's happened to me before, too.

I was trying to conserve the powder. "Pink" is not good enough, I don't care what Taylor says. It has to be dark pink- saturated. If I don't see two or three undissolved granules after mixing, I add a bit more.

I don't understand the chemistry involved, but those undissolved granules dissolve when I start adding R-0871.

I get very consistent results when I add enough powder.
 

pitmanr2003

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May 21, 2010
143
I take a 8 oz cup and get sample and pour from the cup. So yes they were from same place same time and pump was on. Could of been half. Asleep. Lol
 

Beez

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May 19, 2009
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Richard320 said:
That's happened to me before, too.

I was trying to conserve the powder. "Pink" is not good enough, I don't care what Taylor says. It has to be dark pink- saturated. If I don't see two or three undissolved granules after mixing, I add a bit more.

I don't understand the chemistry involved, but those undissolved granules dissolve when I start adding R-0871.

I get very consistent results when I add enough powder.
Finally someone else can back me up on this! I've been saying the same thing ever since I started using the FAS-DPD test, but every time I bring it up it's like talking to a brick wall. No one, not even Taylor, will acknowledge that it is possible under certain circumstances to get varying shades of pink depending on how much powder you use. And you can't convince me that a lighter starting shade of pink will give the same result as a darker starting shade. It just doesn't make sense. It may make sense theoretically, but definitely not practically.

Now here is an interesting thing I have learned about the powder: The original powder that I noticed the variability with was grayish and inconsistent with regard to granule size. Meaning that there were larger chunks of varying size in addition to a much finer ground powder. The powder I have now is snow white and is very finely ground throughout. I have tried to replicate my original results with this newer powder, but I have to really try hard to get the varying shades of pink with it, and honestly I don't think it is possible to do accidentally.

YMWDV, (your mileage will definitely vary)
Dave
 

dmanb2b

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I think the key is to follow the test instructions...one (Heaping) scoop. I've had the same variability, but what I noticed is that it was me causing it by being to skimpy on the DPD powder (resulting in light pink). Add enough powder to turn the water bright pink, otherwise you may get a test variance of 0.5-1ppm. Even then, the test is much more accurate than any other FC testing method readily available to the residential pool owner. Another reason to ensure you follow FC targets (not minimum) on the FC/CYA chart to allow for a 0-2ppm FC testing margin of error.

Theoretically the R-0871 is a chlorine neutralizer so having the color change to clear is is what you are looking for, but I have noticed that it can take a drop or two from the point of the color change Bright pink to clear...(Bright pink, one more drop = light pink, one more drop = very light pink, then another to clear...either way, you could have a variance of 1ppm or so)
 

Beez

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May 19, 2009
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dmanb2b said:
I think the key is to follow the test instructions...one (Heaping) scoop. I've had the same variability, but what I noticed is that it was me causing it by being to skimpy on the DPD powder (resulting in light pink).
Point taken, but you're skirting the issue. The company line is that as long as the solution turns pink, the amount of powder added is irrelevant. Light pink, darker pink, all pink is good pink according to Taylor. This advice runs contrary to my experience with the test.
 

pitmanr2003

Well-known member
May 21, 2010
143
dmanb2b said:
I think the key is to follow the test instructions...one (Heaping) scoop. I've had the same variability, but what I noticed is that it was me causing it by being to skimpy on the DPD powder (resulting in light pink). Add enough powder to turn the water bright pink, otherwise you may get a test variance of 0.5-1ppm. Even then, the test is much more accurate than any other FC testing method readily available to the residential pool owner. Another reason to ensure you follow FC targets (not minimum) on the FC/CYA chart to allow for a 0-2ppm FC testing margin of error.

Theoretically the R-0871 is a chlorine neutralizer so having the color change to clear is is what you are looking for, but I have noticed that it can take a drop or two from the point of the color change Bright pink to clear...(Bright pink, one more drop = light pink, one more drop = very light pink, then another to clear...either way, you could have a variance of 1ppm or so)
here is my next question. Is it amount of drops till it looks clear or till when It changes. Like you said it may change colors and still take more drops to go completely clear. Also while we are on this subject I was shocking pool all morning due to the drop at night or not drop at night. lol. i have never got higher than .5 cc but my last test before going to work yielded a 1.
 

dmanb2b

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amount of drops til it is clear. If you just barely yeilded a 1cc try bumping up your FC by 2-3ppm and perform an overnight FC loss test.

Beez--I was not trying to skirt the issue on what Taylor may recommend...I hear your point, but just wanted to share what I have found works more consistently for me.
 

Beez

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May 19, 2009
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Dallas, TX
dmanb2b said:
amount of drops til it is clear. If you just barely yeilded a 1cc try bumping up your FC by 2-3ppm and perform an overnight FC loss test.

Beez--I was not trying to skirt the issue on what Taylor may recommend...I hear your point, but just wanted to share what I have found works more consistently for me.
OK, sorry if I sounded argumentative. It sounds like you and I agree that the amount of powder used can indeed affect the accuracy/repeatability of the test, especially in the context of the overnight FC loss test where .5-1ppm determines whether you are through shocking or not. I believe this is the situation pitmanr2003 finds himself in.
 

dmanb2b

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My opinion, I would agree...whenever I try to do the overnight test, I aim to get a nice bright pink. Fortunately, even with my dark crumbly DPD powder :lol: it turns bright pink when I truly use a "heaping" scoop. Then I just count the # of drops until clear. The other thing I look out for on the overnight test is if I do lose 1ppm and my CC is zero, I would repeat the FC measurement, just to make sure :goodjob:
 

JasonLion

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Beez, if you use too little powder you will certainly see the effect you describe. If you use enough powder that should not happen.
 

Beez

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May 19, 2009
785
Dallas, TX
JasonLion said:
Beez, if you use too little powder you will certainly see the effect you describe. If you use enough powder that should not happen.
Thanks for clearing that up Jason, but why is that bit of advice so hard to come by? The standard response when the question arises is always the same: "The amount of powder added is not important. As long as the sample turns pink the test will perform correctly." This recent thread is a perfect illustration, but I have seen the same question/response come up periodically.

I know I must be missing something... :scratch:
 

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