Large Discrepancy Between FAS/DPD and Regular CL Test

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Palpatine

Palpatine

Bronze Supporter
Sep 3, 2019
292
West Hills, CA
I have both the Taylor K2005 kit and the FAS/DPD tests... but this is the first time I've used the FAS DPD test.

I've been consistently testing CL at about 3ppm in the AM (using the k2005 kit), adding a gallon of 12.5% liquid chlorine a day and some MA about 5 times a week to lower my PH... rinse and repeat and all has been great. Water looks and feels awesome.

Today, about 2 hrs after adding my gallon of liquid chlorine I did the FAS/DPD test twice and came up with FC 11 and FC 11.5. CC came to 0 both times, so that's the good news I guess... but why the huge discrepancy?

I did the k2005 test after, to double check my FAS/DPD results, and sure enough shows CL at about 6ppm, maybe 6.5.

Pretty sure I did the FAS/DPD test correctly... I used the 10ml mark so multiplied by .5 and I have the speedstir which was running the whole test (performed the whole thing rather quickly)... so what gives?

Thanks for the input.
 
The K-2005 uses the R-0001/R-0002 DPD reagent with color comparator. It is not designed to work above 10ppm FC as the indicator will start to get bleached out at high FC and give a false low reading...as you are seeing. When the FC is above 10ppm, you need to use the DPD-FAS test as it can handle FC values all the way up to 50ppm.
 
@JoyfulNoise and @duraleigh thanks for your replies. Should I never use the R-0001/R-0002 DPD reagent with color comparator? It's so quick and easy in the morning to test CL and PH with. Weird because it has never gotten to the darkest red color... that I can recall. It's been pretty consistent.

Also, and maybe a bit more concerning, I guess my FC has been closer to SLAM level (16) than the desired range (5-7) with a CYA level of 40. I know it's safe to swim in SLAM levels... but still. Maybe this is why my PH keeps shooting up. That and my plaster is only 7-8 months old.

Should I be using the FAS/DPD daily?
 
Why is your chlorine so high? You might want to do some more frequent testing using your FAS-DPD kit so that you can understand the impact of your chlorine additions...clearly you seem to be adding too much and constantly keeping your FC that high is simply a waste of chlorine as the farther you go above the recommended FC/CYA ratio, the faster you lose chlorine to sunlight. Overdosing is also not good for your new plaster.

Try to test more frequently then, when you feel like you really understand your pools needs, you can go back to using the simple DPD test. I use the R-0001/R-0002 test all the time but my FC is extremely stable and I have years of testing under my belt so my pool water is pretty much a known quantity for me.
 
JoyfulNoise said:
Why is your chlorine so high?
Click to expand...
Yeah, well that's the thing... I've been testing daily and I've been trying to keep it in range with the DPD test. Heck, as far as I knew I was pretty on the money. The test has been super consistent for me 2-3ppm every AM, so I add a gallon of CL. I guess now that I tried the FAS/DPD test I know that it's actually been consistently high.

Really curious to see what the FAS/DPD test shows in the AM tomorrow.
 
Not sure how much of a difference it makes, but can you add your chlorine in the evening instead? From what I understand you get a little more benefit as the sun wont degrade it like in the daytime.
 
BadDogPSD said:
Not sure how much of a difference it makes, but can you add your chlorine in the evening instead? From what I understand you get a little more benefit as the sun wont degrade it like in the daytime.
Click to expand...
I appreciate the suggestion but I kind of figure it's 6 to 1, half a dozen to the other. It's gonna burn off during the hot valley days no matter what... and my pool gets full sun all day. Just easier to knock out testing first thing in the AM. As long as I have at least 3ppm overnight I don't see it being a problem. Maybe I'm wrong though.
 
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Like Griswald, I always used that OTO (Yellow) chlorine test as a quick check to insure daily consistency. Very easy to see changes in the intensity of the yellow.....just difficult to tell exactly what the ppm is.
 
Hmmm, kind of interesting... FAS/DPD is showing 4.5 FC this morning and regular DPD is showing about 3... but I guess it could be 4ish. Hard to tell with those reds on the k2005 comparator.

Pool does get full sun all day, but seems like a big loss from yesterday's 11.5. CC is still 0
 
I suggest you careful measure your FC level after addition of chlorine and perform an overnight chlorine loss test. If your pool is losing more than 1ppm overnight (after sundown and before sunrise), you need to SLAM the pool.
 
JoyfulNoise said:
I suggest you careful measure your FC level after addition of chlorine and perform an overnight chlorine loss test. If your pool is losing more than 1ppm overnight (after sundown and before sunrise), you need to SLAM the pool.
Click to expand...
I'll wait to add CL until after sundown tonight... wait an hour, then test with FAS/DPD kit. I'll then measure first thing in the AM.

Even though CC is 0 you suspect I'll need to SLAM the pool?
 
Lack of CCs is not a positive indicator. You can have bacteria and algae in the water and no measurable CCs. CCs should only be considered important when consistent, positive and elevated values are measured. Then that tells you there is a problem. Transient CCs or CC of 0 doesn’t really indicate anything important.

The OCLT will tell you what you need to know.
 
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JoyfulNoise said:
Lack of CCs is not a positive indicator. You can have bacteria and algae in the water and no measurable CCs. CCs should only be considered important when consistent, positive and elevated values are measured. Then that tells you there is a problem. Transient CCs or CC of 0 doesn’t really indicate anything important.

The OCLT will tell you what you need to know.
Click to expand...
Thanks @JoyfulNoise, I appreciate your advice as always. I'll test and report back tomorrow.
 
I ran the OCLT and here are the results:

First, not sure if I mentioned that my pool is heated to 86 degrees... sometimes it gets up to 88-89 during the day. Not sure if that matters or not, but it's extra info if needed.

Last night I waited until sun went down and added about 3/4 of a gallon of 12.5 liquid CL. I waited an hour and tested, probably around 8pm, and CL was 8.

I just tested this morning, two tests at 6am, and one came back 7.5 and the second 7.

I guess I'm good, yes?
 
I just tested again with the FAS/DPD without adding any CL yesterday and FC was 4.5... seems about right, though we didn't have much sun yesterday til mid to late afternoon, a lot of June gloom... but I'll add about a 1/2 gallon of 12.5% CL now and see where it's at in an hour or so.

I guess this is the process until I get it dialed in.

Edit: Also interesting, I just checked with the DPD test before adding the CL and it was barely a 3.
 
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