Convinced FAS-DPD is giving false high

Oct 9, 2021
7
New Orleans, LA
Surface
Plaster
Chlorine
Liquid Chlorine
Hi,
First time pool owner. After a month with a pool made the mistake of using test strips for CYA and added too much. Then we lost power during the hurricane and hence green pool. Found this helpful website and SLAMMED to get it blue, swam once, and green again. That was my fault bc I didn’t check chlorine after swimming. Meanwhile I ordered the Taylor K-2006 kit to get more accurate readings.

So on Tuesday I started the SLAM process again, this time using the FAS-DPD test 1-2x a day and the PoolMath calculator to take into account CYA. I’ll post that info below. But after 3 days at SLAM levels there was absolutely no improvement in the green color and surprisingly my chlorine levels were relatively stable overnight, which meant the algae wasn’t eating it up. Very confusing.
So my husband took the same sample to the pool store and they registered a chlorine at 5 ppm instead of 20 ppm by my FAS-DPD. Which I guess is why my SLAM wasn’t working. But this really bothers me bc I’m a scientist and work in a lab! So I tested the water again 3x, making sure I’m adding the powder correctly, holding the dropper exactly vertical, trying both the 25ml and 10ml tests. No matter what I did I was still testing near 20ppm which can’t be correct. What gives???


------------------------------------------
Test Results 10-08-2021 @ 07:21 AM
------------------------------------------
Free Chlorine: 20.0
Combined Chlorine: 0.5
CYA: 55

------------------------------------------
Test Results 10-07-2021 @ 05:43 PM
------------------------------------------
Free Chlorine: 17.0
Combined Chlorine: 0.5

------------------------------------------
Chemical Addition 10-06-2021 @ 04:54 PM
------------------------------------------
+ 16 Ounces of CalHypo53

------------------------------------------
Test Results 10-06-2021 @ 04:49 PM
------------------------------------------
Free Chlorine: 13.5
pH: 7.6
CYA: 42

------------------------------------------
Test Results 10-06-2021 @ 07:23 AM
------------------------------------------
Free Chlorine: 15.0
Combined Chlorine: 1.0
pH: 7.8

------------------------------------------
Test Results 10-05-2021 @ 07:31 PM
------------------------------------------
Free Chlorine: 15.0
Combined Chlorine: 0.5
pH: 8.0

------------------------------------------
Chemical Addition 10-05-2021 @ 06:29 PM
------------------------------------------
+ 24 Ounces of DryAcid

------------------------------------------
Chemical Addition 10-05-2021 @ 06:11 PM
------------------------------------------
+ 40 Ounces of CalHypo53

------------------------------------------
Test Results 10-05-2021 @ 06:05 PM
------------------------------------------
Free Chlorine: 0.5
Combined Chlorine: 0.5
pH: 8.0
Total Alkalinity: 140
CYA: 48

------------------------------------------
Test Results 10-02-2021 @ 10:55 AM
------------------------------------------
Free Chlorine: 7.5
Combined Chlorine: 0.5
pH: 7.6
Total Alkalinity: 120
Calcium Hardness: 350
CYA: 60
 

Mike1162

Silver Supporter
Jun 13, 2015
1,103
N Tonawanda, NY
Forget the 25 ml test & just use 10 ml then divide the number of drops by 2. Forty drops = 20 ppm. Please confirm that's what you're doing.
 
Oct 9, 2021
7
New Orleans, LA
Surface
Plaster
Chlorine
Liquid Chlorine
Go by your testing not the pool stores. How long was it between getting the sample from the pool & get it tested at the pool store?
Maybe 30min max for travel time to pool store. I tested a sample both before and after the pool store and it read 20ppm both times while their reading was 5ppm. Normally I would agree to trust my test not theirs except that I was SLAMming for 3 days according to my test and no improvement. Followed the pool store recommendation and water turned blue overnight. So in my mind that means my test wasn’t correct.
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
5,150
NW Ohio
You've already made the mistake of trusting bad testing and got your CYA messed up. It appears that's a mistake you felt needed to be made twice.

The FAS-DPD test was not and is not incorrect. I'm very afraid to ask... what "pool store recommendation" did you follow?
 

Mdragger88

Bronze Supporter
Jun 1, 2018
3,140
Hernando, Ms
Pool Size
26000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
You can double check your chlorine level w/ the yellow oto test - if u have 20 ppm it should be a much darker color than the highest one on the comparator block. It’s not very accurate but it will give u an idea if one of your fas/dpd components is bad. This can happen due to improper storage before it got to you or expired regeants/ compromised powder.
Do u have a plaster pool? Why the cal hypo?
Please fill out your signature 👇
Also a SLAM Process entails testing & adding fc multiple times per day (read more than twice/day) otherwise it can take alot longer. Be sure to check/scrub all your nooks & crannies as well - light niches, ladder rails, skimmer throats & weirs, drain covers, water features etc.
 
Last edited:
Oct 9, 2021
7
New Orleans, LA
Surface
Plaster
Chlorine
Liquid Chlorine
You've already made the mistake of trusting bad testing and got your CYA messed up. It appears that's a mistake you felt needed to be made twice.

The FAS-DPD test was not and is not incorrect. I'm very afraid to ask... what "pool store recommendation" did you follow?
I’m a little offended by your tone but I will respond. Obviously I did not feel that this was a “mistake that needed to be made twice” since I followed SLAM instructions to a T with the FAS-DPD kit. But when there was no improvement in 3 days I knew something wasn’t right. Pool store said chlorine was low and we added calcium hypochlorite and overnight we are blue. So obviously something was either wrong with the test that gave high FC levels or something was wrong about the SLAM poolmath calculator that made me underestimate the amount of chlorine to add, and I’m guessing the former. The reagent is not expired and also was clear not yellow when testing the color on a paper towel.
 
Oct 9, 2021
7
New Orleans, LA
Surface
Plaster
Chlorine
Liquid Chlorine
You can double check your chlorine level w/ the yellow oto test - if u have 20 ppm it should be a much darker color than the highest one on the comparator block. It’s not very accurate but it will give u an idea if one of your fas/dpd components is bad. This can happen due to improper storage before it got to you or expired regeants/ compromised powder.
Do u have a plaster pool? Why the cal hypo?
Please fill out your signature 👇
Also a SLAM Process entails testing & adding fc multiple times per day (read more than twice/day) otherwise it can take alot longer. Be sure to check/scrub all your nooks & crannies as well - light niches, ladder rails, skimmer throats & weirs, drain covers, water features etc.
Yes plaster pool and I think I made my signature. :) Truthfully wasn’t scrubbing all day as I had to work so this could have contributed to the lack of improvement but still the fact that we got overnight results by adding more should mean that I wasn’t adding enough, right? QUESTION: How long after shocking can you expect to see at least some improvement before thinking something is wrong?


I didn’t compare with the Oto test but can do that going forward to see if one of the FAS-DPD reagents is bad. Truthfully I feel like that has to be the issue - I just want to prove it! So now I have four tests - two test strips, Oto, and FAS-DPD, 🤣 I’m going to figure this out!
 
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Oly

Gold Supporter
Jun 28, 2017
2,339
Fresno, CA
Pool Size
27000
Your CYA tests of 42, 48 and 55 are not valid as the turbidity test cannot be read between the lines. Round up and use 60 as your CYA ppm and adjust and maintain your SLAM FC for best results.
Don't base a SLAMs success on observing results in three days.
Second guessing yourself with pool store test results will drive you nuts. :crazy:
 

Mdragger88

Bronze Supporter
Jun 1, 2018
3,140
Hernando, Ms
Pool Size
26000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
Yes plaster pool and I think I made my signature. :) Truthfully wasn’t scrubbing all day as I had to work so this could have contributed to the lack of improvement but still the fact that we got overnight results by adding more should mean that I wasn’t adding enough, right? QUESTION: How long after shocking can you expect to see at least some improvement before thinking something is wrong?


I didn’t compare with the Oto test but can do that going forward to see if one of the FAS-DPD reagents is bad. Truthfully I feel like that has to be the issue - I just want to prove it! So now I have four tests - two test strips, Oto, and FAS-DPD, 🤣 I’m going to figure this out!
Comparing w/ the oto may provide some clarity as to which test is inaccurate - it doesn’t need to be done every time. It’s just to somewhat validate high/low chlorine level. As stated previously it isn’t a numerically accurate test but more of a low, high, really high type gauge.
In PoolMath be sure to toggle on csi tracking- you don’t want scaling to occur from your use of cal hypo. Calcium Hardness can only be lowered by water exchange. Switch to liquid chlorine when possible to prevent this. Liquid chlorine is recommended for the slam process.

As far as SLAM Process goes it looks like most of your cal hypo additions fell somewhat short of achieving slam level for your cya. As mentioned above always round up aka- 55= 60 cya - there are no inbetweens.
Just so we’re all on the same page - just being blue doesn’t mean the slam process is complete. You must meet all 3 end of slam criteria to ensure algae is eradicated. It is possible you ended your previous slam prematurely.
Based on the fact u have only been testing & replenishing fc 2x’s/day, & those times may have not actually achieved slam level I think the progress you have seen is to be expected. Hopefully if you make adjustments (testing/adding more frequently, adjusting fc to correct slam level) Going forward you will see an uptick in improvement. Also Remember to backwash when pressure rises 25% over clean pressure.
 
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BowserB

Silver Supporter
Jul 29, 2018
647
"Old" Katy, TX
Pool Size
15000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Hi Sarah. I'm Bill. I'm not an alcoholic. I grew up in Baton Rouge, and I miss it. A couple of points. It seems unlikely (but not impossible?) that you have bad 870 powder and/or bad 871 reagent, but the cost to find out is pretty minimal. If you have Amazon Prime, you can order those two products separately and have them in a day or two. If they prove your existing 870 and 871 are OK, you'll just have plenty on hand for frequent testing. BTW, if you don't have one, when you order from Amazon, you might add a Taylor SpeedStir. It makes drop and swirl about 100 times easier, faster, and probably more accurate.

SLAM generally calls for liquid chlorine, which is usually a lot less expensive than cal hypo--which is also driving up your TA and CH. Walmart recently here had 10% liquid chlorine at about $4 a gallon. I don't know the situation in "Nawlins" these days, though. My brother and sister in Prairieville and Denham Springs have complained of shortages locally. Acid. There are various chemicals in dry acid that are not in plain 31.45% muriatic acid (which my local Ace Hardware sells at a better price than the pool store or Home Depot.) I don't recall what those chemicals are in dry acid, but they can't be good, as Pentair warns not to use dry acid with their salt water chlorine generators.

Finally, if you are continuing to get .5 to 1.0 ppm of combined chloramines, I think you still have some kind of bad stuff in the water. I get concerned at .5 CC and bump the chlorine level until it drops (happens very rarely since I keep the FC at the 6-6.5ppm range.)

Best of luck to you.
 
Oct 9, 2021
7
New Orleans, LA
Surface
Plaster
Chlorine
Liquid Chlorine
Thank you everyone! I will be sure to round up for CYA from now on. I can see in poolmath how rounding up ie from 42 to 50 greatly increases the recommended chlorine to add. I still don’t understand why (even if I wasn’t reaching appropriate shock levels) the FC readings were so stable from one day to the next considering how green the pool was.
 

Bperry

Gold Supporter
Aug 20, 2020
1,001
Knoxville, TN
Pool Size
27000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-60
I’d be curious to take a water sample to the pool store that I KNEW had 20ppm of FC and see what their test comes up with. Or even take some distilled water and add some chlorine to it and see if your test measures it accurately (there’s a dilution method to do that floating around here somewhere)
 
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Oct 9, 2021
7
New Orleans, LA
Surface
Plaster
Chlorine
Liquid Chlorine
I’d be curious to take a water sample to the pool store that I KNEW had 20ppm of FC and see what their test comes up with. Or even take some distilled water and add some chlorine to it and see if your test measures it accurately (there’s a dilution method to do that floating around here somewhere)
Good idea, I’ll try that
 

kimkats

Mod Squad
LifeTime Supporter
Jul 10, 2012
49,194
Tallahassee, FL
One of the best things you can do it take pics of your water each day in the same spot about the same time of day if at all possible. It might be your eyes cannot see the change but the pictures will show it to you.

Do not bother doing anything with the guess strips except throwing them in the trash or see if the store will take them back as they REALLY don't work.
 

Charles Bezzina

Well-known member
Jul 10, 2021
82
Sydney Australia
Pool Size
38000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-9)
I try to keep my fc around 7 but often it's up around 9.

My local pool store tests have never indicated anything higher than 5 for me. I suspect their equipment can't measure any higher than 5.
 
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