Taylor DPD Test Reports False FC Level

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uatuba

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Jun 21, 2014
12
Winfield, AL
I did the FAS-DPD Test twice this morning. According to it, my FC was 18.4.

This is not possible. My TC level was 0 according to an OTO Test, Test Strips, and the AquaChem TruTest Reader last night, and I added 3 gallons of 8.25% bleach which should have brought the chlorine level up to 16 ppm.

Additionally, my CYA is not measureable (I have been using Dichlor to supplement bleach additions so that I can raise it but it's not happening quickly enough, so I bought liquid stabilizer today), so I know that the chlorine SHOULD have dropped some based on the cute little piles of algae a the bottom of the pool a few hours after brushing.

Even after the 90+ drops of reagent, there still seemed to be a very slight pink tint to it but my wife says I was seeing things and I do have a difficult time seeing faint colors.

Any ideas?
 
Are you sure it was an OTO test? Yellow? What brand and age? I don't trust the strips or digital tester with a FC that high. If your color match test is pink, a DPD test, that too would bleach out at the high FC level.

The FAS-DPD is nearly fool proof, and if you added 3 gallons of bleach and expected 16, then the test could easily be 18.5 if your pool was a little smaller than you think and/or the bleach was strong.

Btw, you should use 10ml of water for the FAS-DPD test to save reagent. Each drop would be 0.5ppm.
 
The strips, OTO, and Reader were to confirm that I was indeed at 0. Yes, it was an OTO test (yellow). I did not use them when testing after I added chlorine. I just bought the Taylor Kit online. If bleaching out was an issue, the FAS-DPD test would report a false low rather than a false high. I did swirl after each drop, and I did make sure the powder was fully dissolved. I am certain that the chlorine was not 18+ ppm.

The Reader said 8.3, and the OTO wasn't deep orange like normal when at super high chlorine level...it was slightly darker than the max (5) on the comparator.

The amount of water in the pool is correct per my water meter. The bleach was the last Great Value concentrated of what Wal-Mart had and it hasn't been restocked in 3 weeks, so I doubt it's stronger than what is on the label.
 
You have a FAS-DPD test ... Not a DPD test. Just so we say the same thing.

The FAS-DPD was correct, not sure why you don't think so. The orange OTO puts the FC level up high. The strips and digital tester are worthless at levels that high.

Do it again using the 10ml of water next time.

Make sure you use a little extra powder when the FC level is expected to be high. You can not use too much and it does not all have to dissolve.
 
The OTO was not orange. I stated that, or rather that it was slightly darker than the max (which is still yellow). If I had a pool store around here I would have taken it to them to get it tested, but I'm sure that would be discounted, as well. Yes, it is an FAS-DPD test.

I will try with the 10ml test. My issue is that algae is still appearing, and adding 3 gallons to get to 16 ppm from 0 ppm should result in less than 16 ppm over night.
 
It's a bit hard to follow your post since you are mixing several subjects. Let's focus on your chlorine.

My TC level was 0 according to an OTO Test
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Not according to what you posted. You posted a dark yellow and that is not zero.....clear is zero. The OTO test is quite dependable in testing for the presence of chlorine but terrible at telling you HOW MUCH chlorine.

The FAS/DPD chlorine test is a precision test that will tell you, with a great degree of accuracy, HOW MUCH chlorine is in your water.

So, back to your post title before we work on the other issues.....
Taylor DPD Test Reports False FC Level
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Not really, The OTO test reported the presence of chlorine and the FAS/DPD reported exactly how much.....do you agree?
 
uatuba said:
I did the FAS-DPD Test twice this morning. According to it, my FC was 18.4.

This is not possible. My TC level was 0 according to an OTO Test, Test Strips, and the AquaChem TruTest Reader last night, and I added 3 gallons of 8.25% bleach which should have brought the chlorine level up to 16 ppm.
Click to expand...

I did not attempt to use the OTO (at first) as a test for the amount of chlorine AFTER adding bleach because I knew that I wouldn't be able to rely on it. When I got a result that indicated more chlorine than was possible in the pool, THEN I checked with OTO to see if kt was its usual dark orange or brown. It was not.
 
18 is not terribly different than 16. How sure are you about the accuracy of your pool volume? I've bought 8.25% bleach that measured closer to 9.5%, so sometimes it can be a bit stronger than you think.

Another thought also comes to mind. As you may know, the FAS-DPD sample can turn pink again after completing the test, but you don't keep adding drops. If you are using a speed stir, then maybe you got the 90 drops done in about a minute which is reasonable, but hand swirling may have prolonged the test by quite a bit, perhaps giving a false high.

Did you try the 10ml sample as suggested (giving a shorter time to complete the test).
 

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uatuba said:
I did the FAS-DPD Test twice this morning. According to it, my FC was 18.4.

This is not possible. My TC level was 0 according to an OTO Test, Test Strips, and the AquaChem TruTest Reader last night, and I added 3 gallons of 8.25% bleach which should have brought the chlorine level up to 16 ppm.
Click to expand...
This is confusing. If you added chlorine expecting 16ppm, then measured 18.4ppm, why would you not just think you might have overshot a bit?

What are you addressing with your water that would make you think your FC should have dropped to zero?

How old is the reagent for that OTO test?

Dom
 
domct203 said:
This is confusing. If you added chlorine expecting 16ppm, then measured 18.4ppm, why would you not just think you might have overshot a bit?

What are you addressing with your water that would make you think your FC should have dropped to zero?

How old is the reagent for that OTO test?

Dom
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I added 3 121 oz bottles (not really gallons, but that's what we call them). Poolcalculator calls for 354 oz of 8.25% bleach. The extra 9 oz of bleach wouldn't correspond to a 2.4 ppm overshoot, and I waited overnight. Since I have algae clumping on the bottom of the pool, I'm positive it should have dropped at least some.

My FC was zero when adding because I have hardly any CYA and the pool sits for hours in direct sunlight...I can't tell you how much CYA is in my pool because the dot never disappears.
 
If you want to verify the strength of your bleach and have access to a U-100 insulin syringe, testing is very easy. Measure out 1 liter of distilled or filtered water that has no chlorine and use the syringe to add 10 units of bleach. Mix and then test your solution. The PPM will equal the percentage (e.g., 10% = 10 ppm).
 
I still think this entire discussion is much ado about nothing.

Use 10ml of water and trust the FAS-DPD, I am going to assume testing error in trying to count 90 drops is an decent amount of time.
 
Just wanted to point out since you don't know your cya level that 16 is the shock level for a cya of 40. If your cya is 23 or less you are above mustard algae shock levels when you shoot for 16.
 
jblizzle said:
I still think this entire discussion is much ado about nothing.

Use 10ml of water and trust the FAS-DPD, I am going to assume testing error in trying to count 90 drops is an decent amount of time.
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epro05 said:
I agree with you.
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Well thank you both so much for your help. I used a stir plate the second test (I made my own for home brewing and used a stainless screw as the stir bar because it's the only thing that would fit in the tube). Clearly this is just a case of some idiot who can't count to 90 quickly.
 
I am not sure if you are upset with our responses or just poking fun at yourself.

We certainly did not mean to seem dismissive.

The point is that adding 90 drops of anything is prone to issues and can make the test take too long which will also introduce errors.

What did you get with the 2nd test?

I would not be too concerned about 1 time of testing having a discrepancy. If you continually notice a trend of being off (and not just with the chlorine), then maybe there is a volume difference.

Please add your pool details to your signature as described HERE as it will help us help you in the future.
 
uatuba said:
Well thank you both so much for your help. I used a stir plate the second test (I made my own for home brewing and used a stainless screw as the stir bar because it's the only thing that would fit in the tube). Clearly this is just a case of some idiot who can't count to 90 quickly.
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I'd like to point out that you are using a stainless steel screw as a stir bar. Most stir bars are Teflon coated as that reduces the possibility of interferences. If you look at the Taylor website, the FAS-DPD test lists "oxidized manganese" as a source of positive interference (makes your FC higher than it is). Some grades of SS are alloys that contain trace levels of manganese.

It's a long shot, but I think you need to ditch the stainless steel screw and get a real stir bar.


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