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Thread: Why is it so darn hard to get an accurate pH reading?

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    Why is it so darn hard to get an accurate pH reading?

    According to my new test kit, my pH has been holding around 7.6-7.8 & I have been steadily adding acid every few days trying to bring it down to around 7.2, but have noticed very little color change on my tester.

    Yesterday, I decided to pull 2 water samples & carry them to two different pool stores to get my water tested. All numbers came up about the same as what I come up with using my big test kit...except pH. Yesterday morning I still showed a pH a little higher than 7.6, the first pool store came up with 7.8 (test strips)...the second pool store came up with 7.2 (little cannisters they inject water into then stick the cannister into a reader thing)

    Why can't I come up with a definitive pH? Am I using some diluted acid? It is from last year & has been stored in my shed all winter.

    Is there a better way of checking pH? Anyone else besides a pool store that can check it & maybe have a more accurate way of doing so?
    27ft Above Ground Pool
    Sand filter

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    JasonLion's Avatar
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    Re: Why is it so darn hard to get an accurate pH reading?

    Don't trust the test strips.

    The standard phenol red PH test included with most test kits is really quite good. The test reagent will eventually go bad, but that normally takes years. The only, at all common, thing that throws the reading off is high FC levels. When FC is above 10 there can be problems with the PH reading higher than it actually is.

    If your TA level is high it can take an enormous amount of acid to lower the PH. High TA can also cause the PH to rise fairly quickly, which might give the impression that you were not able to change the PH when in reality it went down from the acid and then up again later. Plaster that is less than one year old will also cause the PH to constantly rise.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: Why is it so darn hard to get an accurate pH reading?

    Alk is only at 120...

    PH has been checked at several different FC levels...anywhere from 12 down to .5 (oops ) just before shocking the pool. Regardless, it reads about the same! I may try buying another one of the pH drop test kits & trying that.

    Pool is vinyl liner AG pool so plaster isn' the problem...

    I would get another set of numbers this evening but chlorine levels are being held at 15 right now after my little oops (see above) & letting the chlorine level drop on me...luckily, I think I bumped the Cl levels way up to shock level before the water got TOO bad.

    Not sure if it makes a difference, but I have been using both bleach & Cal Hypo shock to quickly raise the levels of chlorine. Does either of these affect pH much?
    27ft Above Ground Pool
    Sand filter

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    JasonLion's Avatar
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    Re: Why is it so darn hard to get an accurate pH reading?

    How much acid have you been adding? I calculate that it would take about 5 cups of 31% muriatic acid to bring the PH down from 7.6 to 7.2. Not everyone would be able to see the difference one cup of acid made in the PH test.
    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: Why is it so darn hard to get an accurate pH reading?

    By now, probably half a jug!
    27ft Above Ground Pool
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    Re: Why is it so darn hard to get an accurate pH reading?

    I have found that on my pool, that the acid calculators way underestimate the amount of acid need to change my pH. For example in order to lower the pH from 7.6 to 7.3 usually requires at least 32 oz of 31% acid.
    7,500 gal, IG pool, L shape 22' x 15', 1.5 hp pump, cartridge filter, AquaPlus SWG/Controller, Pebble-Tec liner.

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    Mod Squad JohnT's Avatar
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    Re: Why is it so darn hard to get an accurate pH reading?

    I've found that adding 7 drops of the reagent for the pH test makes my test far easier to read. I only use 5 myself, but my interpretation doesn't change between 5 and 7 drops. At 7 drops, the colors are almost identical to those on the test vial. I have the kids put 7 drops in while I'm traveling so I can trust their email reports more.
    TFP Moderator
    20K Gallon 20X36 Vinyl Inground
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    Re: Why is it so darn hard to get an accurate pH reading?

    How much acid you need to move the pH will depend on the TA. All the calculators and charts make some assumptions as to this so the BEST plan of attack is to add some acid (good starting point is 1 pt per 10000 gallons) wait about 30 minutes or so and retest pH and repeat this until you get the pH you are looking for. If you have a test kit with an acid demand test you can use that but be aware that this is still an estimate and you still might need to make adjustments to the pH anyway. The best use of an acid demand test is when you need to make a HUGH change in pH. It will get you in the ballpark faster but it won't always get you to the right endpoint.

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    AnnaK's Avatar
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    Re: Why is it so darn hard to get an accurate pH reading?

    Quote Originally Posted by JohnT
    I've found that adding 7 drops of the reagent for the pH test makes my test far easier to read. I only use 5 myself, but my interpretation doesn't change between 5 and 7 drops. At 7 drops, the colors are almost identical to those on the test vial. I have the kids put 7 drops in while I'm traveling so I can trust their email reports more.

    This is GREAT advice!

    I have a hard time with the 7.5 reading. I know it's not 7.2 or 7.8 but to my eyes it could be anywhere in between. Using john's 7 drop method of phenol red lets me know very definitely that it's 7.5. The hue is the same but the saturation is much richer.

    Thank you very much for posting this tip, John.
    — AnnaK —

    12,000 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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