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Thread: FAS-DPD test results + adding Salt question

  1. Back To Top    #1
    Lana537's Avatar
    Join Date
    May 2009
    The Triangle, NC

    FAS-DPD test results + adding Salt question

    Hello All~~

    Our pool has been happy and balanced so far this summer. This evening I got an unexpected, surprising, and significant drop in FC test. The question is: Can the addition of salt throw off a FAS-DPD chlorine test?

    Here's the story:

    Our Aquacheck salt strips had been saying we have 3720 of salt; the Pool Pilot was saying 2700. Concerned that our strips had become old (bought one year ago, expire in August of 2010), we went to the pool store today and bought a fresh container of Aquacheck test strips (expire in 2011 sometime). We noticed that Aquacheck has actually amended their numbers on their test since last year, but that's a whole 'nuther thread).

    The new batch of salt test strips confirmed what the Pool Pilot was reading--obviously age is a factor. We needed and added 3 40-pound bags of salt. Pool Pilot later registered 3300 of salt--roughly our goal. Test strips confirmed. All was happy in the back yard. Joy all around.

    This evening, I did the second general testing of the day. I saw a very unusual drop in FC. My question is--can the addition of 120 pounds of coarse solar salt throw off a FAS-DPD chlorine test (for a while)?

    Before I post numbers, please let me tell you that in addition to the specs listed in my sig below, we have Chemtrol equipment in line (even though we have an outdoor pool). DH wishes to be able to control/monitor our pool during the times that we must be away for several weeks at a time, so we have extra gadgetry with online control.

    Our goal for FC is 2.5 with a range of 2.0 to 3.0
    Because of the Chemtrol equipment, we keep our CYA at ~ 25 (I know that this sounds low, but the Chemtrol cannot "see" if CYA is high!)

    So for the past four weeks:

    FC is usually 2.2 to 2.4 (25ml Taylor testing)
    CC usually 0, sometimes .2
    pH stable as a rock with CO2 injection at 7.5, lowered only if TA walks up, and I add acid to bring it back to....
    TA 80
    CH 410
    CYA 23 ish
    Phos 1000-2500
    Borates 50
    Salt now 3300
    TDS off of Chemtrol: ~8090
    LSI ~ 0.0
    CSI ~ - 0.4
    Pool temp ave. 85 degrees

    This evening--I do my usual FC test. Holy moly! It reads a low 1.0! Combined Chloramines are dead zero. I surely did not muck this up. I draw another water sample, and I test again. Darn! 1.0 FC and 0 CC again! No change in any of the variables--sun, weather, bathers .....all is the same since we've been swimming along at these levels since mid-June. A whole month I've been getting 2.0 to 2.4 on the FC, never more than .2 of CC on a 25mL test. Pump runs 24/7; no unusual weather, testing twice a day.

    Has the addition of this salt altered the abilities of the FAS-DPD chlorine test? I have backup containers of FAS-DPD powder and the other reagents--all of which are new from Taylor this season.

    I have jugs of chlorine to dump in if the test, which I will do with different containers of reagents, comes out low tomorrow--don't wish to have to throw the SWCG into boost/wait for it to be effective.

    Thanks for reading this, and thanks for any ideas.

    Happy Swimming~~

    23,600 gallon, refurbished 1960's inground concrete, Jandy sand filter w Jandy 2 hp variable speed pump, Pool Pilot Digital SWCG+Chemtrol 2100+CO2 tank; PebbleTec White Pearl with 10% Cobalt Blue Dark Beadcrete by Olympic Pool Plastering, Georgia. Taylor K-2006 Test Kit; Aqua Check Salt test; LaMotte borates test, and Jack's Magic Sequest test kit; SparklyPoolitis level: extremely high.

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: FAS-DPD test results + adding Salt question

    Salt has no effect on the FC level or the FC test.

    I would add a little extra chlorine tonight and the do something about what dmanb2b is about to mention.
    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

  3. Back To Top    #3
    dmanb2b's Avatar
    Join Date
    Apr 2009

    Re: FAS-DPD test results + adding Salt question

    your CYA @23 is way too low for a need it in the 60-80 range.
    24'x52" AGP (13,500 Gallons), Intex SWG, (2)Solar Bear 4x20 panels, Hayward S220T Filter, 1/2hp Pentair Superflo

    Pool School, TFTestKits, Pool Calculator

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