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Thread: At what levels do high chlorine interfere with PH readings?

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    At what levels do high chlorine interfere with PH readings?

    Hello, I recently SLAMmed by pool, due to a particularly bad algae infestation. To be honest, I left it at SLAM level a long time - I can't remember how long, but it was more than a week, maybe more than 2 weeks. This was because the water remained cloudy a long time.

    Finally the water cleared up, though I can't say it was sparking "mountain water" clear, but it was taking so long I was starting to thing something was wrong, as it seems like it shouldn't take that long. I decided to stop SLAMMing, and let the chlorine drop so that I could do a full battery of measurements, to see if something was out of whack that might explain the difficulty I was having getting the water "sparkling" clear.

    I think the water actually since then has gotten even clearer, but maybe not 100% sparkling clear yet.

    As of yesterday, there will still 1.0 ppm of chloramines, which means it doesn't pass the test for stopping SLAMming.

    However as of this morning, there is only 0.5 ppm of chloramines, which does pass the test, which is ironic, since the free clorine is now down to 7.5 ppm, which is way below SLAM level, but still above target level.

    I think my CYA acid, if I remember clearly, is around 80, but to be honest, I'm never sure, because I find the CYA test the most difficult one. I'm supposed to keep adding drops until the black dot disappears, but that black dot disappears very gradually, and it's very hard for me to say exactly when it has disappeared. Sometimes, it's like I can see it in my peripheral vision, but not when I look straight at it.

    Anyway, now to the meat of my question, now that the background is there. I assumed, maybe incorrectly, that with the free chlorine down to 7.5, it was low enough to start doing my other tests, including PH. But I was alarmed at the PH reading: it's off the scale at the low end... 6.8 or possibly lower, but I think it's pretty close to 6.8. Historically, it's always high, not low (by the way, I have a salt system, which I think is why it's always high).

    Admittedly, we've had a fair amount of rain since before SLAMMing, so maybe there was acid in the rain. Still, 6.8 seems awfully low, lower than I've ever seen in that pool. So my question is: is 7.5 free chlorine still too high for me to trust my PH reading? Maybe I should wait some more to let the free chlorine drop even lower, and do another PH reading.

    Oh, one more thing... I didn't do a salt reading before I started SLAMming, thinking that I didn't care, because I'm using chlorine bleach to control the chlorine levels during the SLAM, but now that I think of it, the recommended CYA levels supposedly varies depending on whether you have a salt system or not, so I wonder if my combination of salt and CYA is off. (Since for all I know, my salt is low.)
    18 foot diameter, 52" tall above ground Cornelius pool, with vinyl liner, and resin wall caps.
    Roughly 7600 gallons
    Blue Wave "Sandman" Sand filter/pump, model NE6150. Pump is 1 HP
    Intex SWG, model # 54601E

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    JasonLion's Avatar
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    Re: At what levels do high chlorine interfere with PH readings?

    If you are using a Taylor brand PH test, the test results are valid up to an FC of 10, and will be a bit higher than actual from FC of 10 to 18 or so. Not too far above 18 they become completely invalid. All of the other brands of PH test tend to be worse, i.e. more sensitive to FC levels.
    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: At what levels do high chlorine interfere with PH readings?

    I think the PH test says Taylor K-1000, so that sounds like then I have genuinely low PH. Before I started SLAMming, I had high PH, and I had to add muriatic acid a couple of times until I got it down to 7.5 Sudden it's at 6.8; how weird.

    I guess I need to add something to bring it up. It's been a long time since I did that; I remember I bought a box of white powder last year from Walmart, though I can't remember what it was called, but I'm sure if I search my old threads here at this forum, I'll find it.

    Meanwhile, maybe I should do the rest of my tests, including CYA to make sure those levels are all adequate.
    18 foot diameter, 52" tall above ground Cornelius pool, with vinyl liner, and resin wall caps.
    Roughly 7600 gallons
    Blue Wave "Sandman" Sand filter/pump, model NE6150. Pump is 1 HP
    Intex SWG, model # 54601E

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    JasonLion's Avatar
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    Re: At what levels do high chlorine interfere with PH readings?

    Borax is the best thing for raising PH. Soda ash, sold in grocery stores as Washing Soda is also OK.

    You shouldn't adjust the PH while SLAMing, that tends to lead to this kind of problem.
    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: At what levels do high chlorine interfere with PH readings?

    In my case, I adjusted the PH before SLAMming, and not during, which is as recommended in this site, as they say to ensure that PH is close to 7.5 before SLAMming.

    I think washing soda is what I used to use, which I bought at Walmart, because I couldn't find it in my grocery store. Out of curiosity, who would sell borax?
    18 foot diameter, 52" tall above ground Cornelius pool, with vinyl liner, and resin wall caps.
    Roughly 7600 gallons
    Blue Wave "Sandman" Sand filter/pump, model NE6150. Pump is 1 HP
    Intex SWG, model # 54601E

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    JasonLion's Avatar
    Join Date
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    Silver Spring, MD
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    Re: At what levels do high chlorine interfere with PH readings?

    Borax is sold in the laundry isle of many larger grocery stores.
    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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