FAS DPD Testing - Experience needed

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Pool_Fool

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Apr 24, 2012
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Lynchburg, VA
I have been using the DPD testing powder 0870 for only a short while.
I am thoroughly enjoying the precise numbers it will give.
But I have recently come across a head scratcher.

I just opened a pool not 7 days ago.
It was a bit green but nothing I couldn't handle.... Or was it?
I initially shocked the pool with Cal-Hypo, that did most of the green in.
I removed the dead algae by vacuuming to waste. The FC levels dropped, as expected.
So I hit again with Cal-Hypo. And again wasted the dead algae. The water tests
As of this morning are as follows.
PH 7.4
TA 95
CH 110
CYA 35ish
But then I went to the reliable DPD test for the FC, and here comes the curveball. I put in the
two scoops of 870 then I ran the next part of the test with the drops of 871. It went
From a very light to pink to a very dark pink on the second drop. The 871 is suppose to change the pink to clear to give you the FC ppm. But it does the opposite it goes pink then pinker then clearer.
Of course once it goes clear I test the CC with the 003 drops and get a reading of .5 or less

When I test again using the tests strips I get a very low FC reading.
Versus the test using the DPD and 871 drops.
Anybody have any idea what might be going on here?
Why is the 871 test drops turning my sample pink when it is suppose to be turning it clear?

And to top it all off the pool is very cloudy at this point... Clearing up ...but slowly.
Does anyone suppose the cloudiness may have something to do with this whole testing kanundrum?

Thanks for any help here,
PF
 
Does anyone suppose the cloudiness may have something to do with this whole testing kanundrum?
Click to expand...
No, I doun't think they are related.


1. Is your clear cylinder rinsed thoroughly before you perform the test?

2. Do you have an OTO chlorine test? (the one that takes 5 drops of a yellowish reagent?) If so, do that test and report the results.

3. Regardless that it turns it turn pinker, it still goes clear at a point that makes sense to you, right?

4. The strips are worthless

5. The cloudiness may be caused from using Cal Hypo. You should be using liquid chlorine to shock.

6. What is your current FC? (using FAS/DPD and using OTO)
 
Thanks for the quick responses, but I did forget to mention that this is a vinyl pool.

The vile seems clear to the eye. I do give it a rinse before hand as well.
I do not have the OTO but I do have the taylor test kit.
The FC is as of this am is .5-1 ppm
But here is a new twist, with my results from yesterday being all in the normal range, I started
to put some pucks in the chlorinator(tr-Chlor) and now I have a TC reading of
6 - (this of course using the taylor test kit)
That is a CC of 5-5.5!

I have found that I use the strips only for a quick snapshot of what is going on
In the water. I only rely on them as a maintenance measuring stick in between true testing with my Taylor test kit.

I have been doing some poking around on the liquid bleach, ie Clorox. Am I not mistaken that using
That contributes more heavily to the TDS, by adding so much inert material?

Jason, The TA is 95

Thanks for all your input.
 
Am I not mistaken that using That contributes more heavily to the TDS, by adding so much inert material?
Click to expand...
It adds salt (which is calculated in TDS) but TDS is irrelevant in the first place.

You either have some testing issues or you really need to shock the pool. CC's of 5 or so would indicate a DESPERATE need to shock. Has your pool water ever been clear since you added the Cal Hypo?

.5 - 1ppm FC is not nearly enough FC.

Please read the article up in Pool School on "How to Shock your Pool" . It is beginning to sound like you have never completed the process.
 
Thank you David.

I promise you I have shocked quite a few pools quite a few times, with great success
Using Cal Hypo

I think maybe I have given far too much info about my pool situation, and lost focus on
What question I am trying to get help on.
I do make mistakes when it comes to pool care, that is for sure. And this most likely falls into
That category.
But please if you can help with the DPD question that is where my main concern is.
I have never seen this particular test act this way and I was hoping to have some light shed on it.
My guess at this point is because the CC was so high, and I am sure I got it there while trying kill the algae, ...that may be what is affecting the testing in this way.

Do you think that the DPD powder or the 871 drops can go bad?

Thanks again.

And thanks for the freshing up on the "how to shock your pool" it was a good read.
 
If the FC is high, the indicator may have been partially bleached out. The titrating reagent (R-0871) reduces the FC and the indicator is no longer being bleached out.

If the indicator completely, or partially, bleaches out, add an extra scoop of R-0870 powder and do the test as normal.

Alternatively, you could dilute the test sample water with 50 % distilled water and the multiply the result by 2.

Have you used a "Yellow-out" type product in this pool?

Also, since you routinely shock pools, get some OTO to use as a verifier. OTO won't bleach out and can help identify the existence of high chlorine levels.
 
To all thanks for your help with the DPD mystery.

But I think it may be solved.

I believe the high CC was messing with the the Powder and the 871 drops.
After a thorough shocking last night, I tested again this morning and the DPD powder and
871 drops worked perfectly. An FC reading of 2, Then I tested the CC with the 003 drops and I am happy to report A big fat zero for CC.
All of these numbers were confirmed with the drops from the other Taylor test kit.
So, for all those that may run into this DPD issue in the future, check those CC numbers.
.... Of course try not to get yourself into this mess in the first place.

Thanks again to all.
Looking forward to further discussion.

PF
 
When the CC level is much higher than the FC level, CC can "leak" through into the FC portion of the test if you don't move the test along quite quickly. That could result in exactly what you describe, the pink getting more intense as CC contaminated the FC portion of the test.
 
Pool_Fool said:
After a thorough shocking last night, I tested again this morning and the DPD powder and
871 drops worked perfectly. An FC reading of 2, Then I tested the CC with the 003 drops and I am happy to report A big fat zero for CC.
Click to expand...
You shocked it last night, and the FC this morning is only 2 ppm?

How much chlorine did you add to how many gallons?
 

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There was a CC level of 4.5 that had to be burned out.
Once that was Burned out all that was left was the FC and that was only 2.
The pool is about 25,000 gallons. I added 17lbs of Cal-hypo.
Isn't chemistry fun!?!
 
17 pounds of 65 % calcium hypochlorite will provide 53 ppm of chlorine to a 25,000 gallon pool. The 4.5 ppm of CC would only require 2.25 additional ppm of chlorine to oxidize the CC. Therefore, there has to be something else going on.
 
That was the number I was shooting for. I was increasing the FC to ten times the CC(with a little extra, to be safe and get it) to release the chlorine lock.

That is how I have learned to remedy that issue. It has worked for years, and if it ain't broke
Don't fix it!.

Do you know some other method?

Thanks
PF
 
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