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Thread: False high reading of CYA?

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    False high reading of CYA?

    I started with a fresh water fill this year. After filling the pool and getting everything balanced other than adding cya, my readings were:

    FC 5
    CC 0
    PH 7.2-7.6 (currently aerating and adding muriatic acid)
    TA 280
    Salt 3000ppm(calculated)
    CYA <30 but should be 0 because none was added to this fill with pool completely dry before filling and test showed no turbidity)

    Using poolcalculator.com, it calculated I needed 131oz dry stabilizer to get to 70ppm or 128oz for 68ppm. I bought 2-4lb containers of the Clorox brand stabilizer. I dissolved the contents of the containers(128oz) into a couple buckets of water and added it to the pool. It is said it could take up to a week to register on a cya test, but I was able to get all of the granuals to dissolve, so I tested the next day just to see. And to my surprise, my cya was a little above 100.

    So, an error came in one or a combination of these ways.
    1. I added more CYA than 128oz. I did not weigh the actual product and assumed each container had precisely 4lbs.
    2. I have CYA in my tap water
    3. I miscalculated my pool's water content by more than 20%.
    4. The test created a false high reading.

    Now to my actual question. Are there any conditions were one could get a false high reading using the black dot test? In looking around here, I've seen false low readings and false pool store high readings, but nothing really of falsely high readings. I am using a k2006 with TF replacement regents.
    Intex 26' x 52" (14.5k) Intex SWG. 14" sand filter. Taylor k-2006 kit

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    Mr Bruce's Avatar
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    Re: False high reading of CYA?

    1. Possible but unlikely IMO.
    2. Pretty sure that's nearly impossible
    3. Possible.
    4. Not likely IMO

    You forgot

    5. The margin of error in the test is +- 20

    So let's say it's actually 85. And you over estimated your pool by 1000. Then those numbers might make sense.

    This BTW is also why I do everything in halves, especially CYA.
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    Re: False high reading of CYA?

    That margin of error is like crazy! Could you review how to test the CYA "in half". Every early spring, my test results are way below 30 (black dot). I try to add a negligible amount - somehow CYA comes back way higher than I calculated. Maybe testing in halves would give me more control???
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    Re: False high reading of CYA?

    A couple things that come to my mind are:
    - Any chance that from the time you fresh-filled, you used another other chlorinating product, even for a short period, other than your SWG? Even if used for a short time we know tabs and many bags will add CYA.
    - When you did the CYA test, were you outside and was it a good, sunny day? We know inside is no good for a CYA test. But if you were outside and it was overcast, it's possible the dot would disappear sooner, hence giving you the appearance of a higher CYA.

    Add any of those to the +/- 15 degree of error for CYA testing, and we might be closer to an explanation.

    Oh, and I've never heard of any "high-octane" CYA as it were, but who knows? You would think the % on the bottle should be accurate. Most times if we ever sense a problem with strength, it's weaker than advertised. What if this time it was actually more concentrated? That's out of my area of expertise .... just some thinking outside the box. I've been wrong before.
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    Re: False high reading of CYA?

    It's VERY important to test in direct sun, with your back to it, holding the cya tube at waist level shaded by you.

    Pour in some solution till it just starts to make the black dot disappear, then look away then look at it. When you
    can't just barely make it out, thats about it.

    Also make sure the water you are testing is near 70 degrees if possible, colder also makes it more difficult
    to determine.

    Any other way of testing will give a false reading.
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    Re: False high reading of CYA?

    My experience -- and I've confirmed it using the CYA standard -- is that bright sun is vital. Even just wrapping all my fingers around the view tube messes with my readings, so now I hold it at the very top with just thumb and forefinger. The darker it is, the sooner the dot disappears, and the higher the CYA appears to be.

    Don't neglect to take multiple readings using the same solution. Just pour it back and forth. There's no shame in taking a dozen readings, especially the first season.

    Others report they have better success by filling to a certain line and then quickly glancing at the view tube, then add a little more and keep repeating until the dot appears. It avoids that optical illusion effect of seeing the dot when you really don't.
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    Re: False high reading of CYA?

    Thank you all for your quick responses. A few things to note and answer.
    1. Ambient light: It was pretty overcast this morning when I did the test. I didn't think that it could cause a big enough error. I was just expecting/hoping for 70 and wouldn't be too upset if I made 80 so got a bit nervous, having just filled the darn thing.
    2. Water temperature: my thermometer got hazy over winter, but my best skin to water test says right around or a little below 65.
    3. Additional chlorine and CYA: I haven't put any bags of shock or pucks whatsoever. I've been adding bleach to maintain FC until I have enough CYA for the SWG to keep up.

    I will definitely be re-testing and checking my figures so I can get a more definitive explanation, whether my error or not.

    A side note, I haven't seen anyone around the net complain(and I looked), but the stabilizer calculation in Pooltrac Chem Calculator Pro(Android app) gives a figure twice that of poolcalculator.com. If I would have gone by that program's calculations, I would have put in around 16 pounds of stabilizer instead of .

    But thanks again for all of your input.
    Intex 26' x 52" (14.5k) Intex SWG. 14" sand filter. Taylor k-2006 kit

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    Re: False high reading of CYA?

    Never mind, lol.
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    Re: False high reading of CYA?

    For your pool, to go from 0 to 70 ppm, PoolMath tells me that you should have only added about 5.5 pounds of stabilizer.

    ooops- never mind. The gallons was wrong at the top. Sorry!!
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    Re: False high reading of CYA?

    I have a 13,000 gallon pool.

    I use the chlorox granules.

    1 pound adds 10 ppm of CYA. (if that helps at all)
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    Re: False high reading of CYA?

    Quote Originally Posted by YippeeSkippy View Post
    For your pool, to go from 0 to 70 ppm, PoolMath tells me that you should have only added about 5.5 pounds of stabilizer.

    ooops- never mind. The gallons was wrong at the top. Sorry!!
    That's what I did, too, ha ha. I checked the gallons at the bottom but forgot to change it at the top.
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    Re: False high reading of CYA?

    Quote Originally Posted by quincy915 View Post
    That margin of error is like crazy! Could you review how to test the CYA "in half". Every early spring, my test results are way below 30 (black dot). I try to add a negligible amount - somehow CYA comes back way higher than I calculated. Maybe testing in halves would give me more control???
    I think what was meant by the comment about "doing [CYA] in halves" was to adjust your chemistries by adding half of the calculated amount of the chemical, let it circulate and mix for at least 30 minutes (except for CYA--it has to dissolve and circulate over at least a few days) I guenerally prefer to wait at least an hour or 2, for most chems to mix on completely... Retest at that point and see where that got you... Use those results to estimate how much of the remaining "half" of the chemical to add...
    • ex: pool calculator says you should add 4 lbs of CYA... Add 2 lbs, then once it is dissolved *thoroughly*, retest... let's say, the initial 2 lbs got you 75% of the way to your goal... then only add 1 lb more (50%) of the remaining half of the chem.... and retest again after that 2nd addition dissolves.


    The idea is that it's always best to undershoot the amount you add for any of the chemicals... it's a lot easier to add a bit more than to try to correct any problem caused by adding too much of something. We've all tried the route of adding the chem all at once, expecting that we will only need to retest to confirm that it's exactly where we expected it to be... more often than not you end up ...

    Unfortunately there are many places that errors may occur. Off the top of my head:
    • Often, pool volume ends up being underestimated, because it's rarely filled to its full depth, or because a feature in the pool isn't taken into account (a bench, steps, rounded corners at the bottom, etc).
    • Testing procedures that aren't precisely followed, and done exactly the same way everytime, will make test results be inconsistent.



    BTW... what do you mean when you say that you try to add a "negligible amount" of CYA? I didn't understand this statement.... What do you consider to be a negligible amount?

    Also, the scale on the CYA view tube is not linear... a little over 100 can really end up being 130 or even 150! That's because the scale on the test tube is logarithmic and even a "drop or two" added past the 100 mark represents a big jump in the actual value....

    To test CYA over 100... mix equal parts pool water and tap water, and use the resulting mixture to do the CYA test, just like you would with undiluted pool water.... just multiply the result by 2, to get the real value.

    If the result is still over 100 (before multiplying) then mix one part pool water to 2 parts tap water, and then multiply that result by 3....

    Everyone else has given good advice on how to do the test. I'll add that it's importsnt to do it the same way every time, to the extent that's possible. Richard's advice regarding pouring the test solution from the test tube back into the dropper bottle and doing the test again to recheck is valid. Another helpful aid is the "CYA Standard Solution" sold at TFTestkits.net... it lets you train your eyes to how the dot dissapears at a known value, in this case, 50 ppm. It's well worth it in order to be sure you are "reading" the test correctly.

    Good luck.



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