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Thread: Chlorine test mismatch + larvae

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    Chlorine test mismatch + larvae

    I am concerned about a mismatch in my chlorine test results.

    I use a salt chlorinator system and CYA is about 25 ppm (need to increase). I usually test for chlorine with Taylor's FAS/DPD test, and chlorine levels over the last couple of weeks have been (unusually) steady at 4.5 to 5 ppm, regardless of when I test the pool.

    Tonight I noticed a lot of larvae swimming up to the lights in the pool, became concerned as to how they could be surviving, and therefore tested my chlorine yet again. Sure enough, about 4.5 ppm (9/10 drops to clear the pink in 10 ml). Suspicious, I tested again using Taylor's R-0001 and R-0002 (bottles new from the store last week). The color suggests about 0.5 ppm and is nowhere near dark enough for 5 ppm. I have redone both tests with new chemicals with the same results. Anyone have any idea what is happening?

    I had a black algae outbreak last year and I am now really anxious that the FAS/DPD test has been giving me false readings. Is that possible? I am using Taylor's Speedstir so chems are well mixed...

    If consensus is that FAS/DPD is reliable, then can anyone advise re how to kill the gazillion larvae zapping around the pool? Pump is running about 18 hours a day but I guess they swim well enough not to get drawn into the circulation and filtered out, and clearly the chlorine is not killing them.
    21,500-gallon tiled gunite with 65' infinity edge. Two Intelliflo VS pumps, one for trough one on pool. Pentair 520 cartridge filter on main circuit ,Pentair 200 cartridge filter on edge circuit. Easytouch, Intellichlor 60, Intellichem with external acid pump, Intellibrite 5g lights.

    Testing: Taylor K2006 (with FAS/DPD). Taylor K1766 salt.

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    dmanb2b's Avatar
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    Re: Chlorine test mismatch + larvae

    Turn off the SWG, add one gallon of 8.25% bleach and let circulate for 30 mins. Your FC should jump up ny about 4-5ppm in a 20K gal pool. As for the larvae, you may need to go through the shock proces.
    24'x52" AGP (13,500 Gallons), Intex SWG, (2)Solar Bear 4x20 panels, Hayward S220T Filter, 1/2hp Pentair Superflo

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    Re: Chlorine test mismatch + larvae

    Thanks for your response - I will try that tomorrow. Can larvae appear and survive at 5 ppm? I don't understand the difference in the tests....Is it simply that the R-0001/ R-0002 test sucks? I am really hoping that I can have faith in the FAS/DPD results...
    21,500-gallon tiled gunite with 65' infinity edge. Two Intelliflo VS pumps, one for trough one on pool. Pentair 520 cartridge filter on main circuit ,Pentair 200 cartridge filter on edge circuit. Easytouch, Intellichlor 60, Intellichem with external acid pump, Intellibrite 5g lights.

    Testing: Taylor K2006 (with FAS/DPD). Taylor K1766 salt.

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    Re: Chlorine test mismatch + larvae

    I would trust the fas-dpd test over the color match test..they are far more accurate
    18x52 intex ultra frame pool 6981 gallons, 1 HP LL pump with 19 in sand filter,BBB method,borax 50ppm
    leslies DPD test kit + fas-dpd chlorine test kit + borates test strips
    thru wall skimmer,hard plumbed with 2 returns 1 1/2 sch 40 pvc
    1 large fountain 1 inch sch 40 pvc,shut off ball valves on all

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    JasonLion's Avatar
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    Re: Chlorine test mismatch + larvae

    The FAS-DPD titrant, R-0871 can go bad, which will result in incorrect test results. This usually only happens when it is a couple of years old, has been left in the sun or otherwise gotten hot for an extended period of time (or really hot for a shorter period), or frozen. R-0871 should be a clear colorless liquid. If it has turned yellow it has gone bad.

    The most common problem with the DPD test (R-0001 & R-0002) is false low readings caused by fairly high FC levels. FC around 5 would not cause this problem, FC needs to be above 10 before there would be an issue, so it seems unlikely that it applies in your situation.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: Chlorine test mismatch + larvae

    Are you testing for combined chlorine? If not, check that. If yes, then what are the results?

    How many gallons is the trough, and are you keeping it circulated and chlorinated?

    What flow rate are you using on the infinity edge?

    Also, using an OTO chlorine test can help confirm other readings.

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    duraleigh's Avatar
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    Re: Chlorine test mismatch + larvae

    Curious where you live that you are getting larvae in the pool?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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