Become a TFP Supporter Pool Math Forum Rules Pool School
Results 1 to 4 of 4

Thread: Different pH readings w/ different tests during stain trtmt.

  1. Back To Top    #1

    ChuckDavis's Avatar
    Join Date
    Aug 2010
    Location
    Durham, NC
    Posts
    73

    Different pH readings w/ different tests during stain trtmt.

    I'm in the middle of an ascorbic acid (AA) stain treatment. I dropped the CL to zero and the pH to 7.2 and then added the AA.

    After adding the AA and letting it circulate overnight my LaMotte ColorQ and Taylor test kits still registered 7.2, but my LaMotte test strips and the local pool store read a pH of 6.8-7.0.

    I spoke with LaMotte, and the tech I spoke with said that it was possible that the ascorbic acid might "complicate" the ColorQ and Taylor test kit readings.

    I assume that the pH will drop, and the two sets of readings will converge, as I "burn off" the AA with chlorine.

    My question is: Which testing tool and pH reading should I use as I try to hold the pH near 7.2 while bringing up the chlorine level?

    (If pH readings during an AA treatment do depend on the testing tool being used, TFP might want to add this information to the stain treatment "cookbook.")
    15,000 gallon IGP, epoxy surface, waterfall, borates (!), not closed in winter
    SuperPump with 2.4 THP EcoTech variable speed motor, Tagelus TA-60 filter with zeolite media, Liquidator, AquaComfort heat pump, Blue-White flowmeter, Smartpool Wall Climber
    TF-100, LaMotte metal sequestrant test kits
    Sundance Capri hot tub

  2. Back To Top    #2

    In the Industry

    duraleigh's Avatar
    Join Date
    Apr 2007
    Location
    Sebring, Florida
    Posts
    30,084

    Re: Different pH readings w/ different tests during stain tr

    Years of reports here at TFP basically suggest there is little credibility with test strips and not much more in pool stores. Trust your own testing.......it works.

    Your pH should remain pretty stable as you add back in FC. Again, trust your own testing....not test strips or pool stores.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

  3. Back To Top    #3

    ChuckDavis's Avatar
    Join Date
    Aug 2010
    Location
    Durham, NC
    Posts
    73

    Re: Different pH readings w/ different tests during stain tr

    I was surprised that adding the ascorbic acid didn't lower the pH as measured by the ColorQ kit. The TPH stain treatment "cookbook" says that it will. (I didn't start using the Taylor kit until the discrepancy in readings surfaced and I was looking for another "vote.")

    I'm not a fan of strips (hence the ColorQ and Taylor kits), but two different bottles of strips that I have (both LaMotte, so probably same tech), plus a test strip at the pool store (unknown brand), all gave readings similar to the (fairly new) test system at the store.
    15,000 gallon IGP, epoxy surface, waterfall, borates (!), not closed in winter
    SuperPump with 2.4 THP EcoTech variable speed motor, Tagelus TA-60 filter with zeolite media, Liquidator, AquaComfort heat pump, Blue-White flowmeter, Smartpool Wall Climber
    TF-100, LaMotte metal sequestrant test kits
    Sundance Capri hot tub

  4. Back To Top    #4

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: Different pH readings w/ different tests during stain tr

    How much ascorbic acid did you add in what volume of pool water? What was your TA level? Do you have borates in the pool?

    The recommended amount of ascorbic acid is 1/2 to 1 pound per 10,000 gallons. With 80 ppm TA and no borates, then starting with 7.2 pH one would expect the pH to drop to 7.08 (with one pound) or 7.14 (with 1/2 pound). Also note that at the lower pH the rate of carbon dioxide increases so the pH will be rising over time faster so measurements need to be taken soon after the ascorbic acid is mixed well in the pool water (i.e. not 8 hours later or the next day).

    It's when starting from a higher pH that the ascorbic acid addition has a more noticeable pH drop where one pound would go from 7.5 to 7.31. This is because the carbonate buffer system gets stronger as the pH gets lower.

    I don't see how the ascorbic acid would interfere with the pH test. Your results are much more consistent with test strips not doing a great job, though usually for pH they are often OK. Obviously that's not true in your situation.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •