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Thread: some tests agree; some tests disagree

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    some tests agree; some tests disagree

    Hi --

    1) I appreciate the high-quality information on this site.

    2) By way of background: I prepared a reference solution, aiming for 5 ppm chlorine, by diluting concentrated bleach with DI water. I observed:

    • FC+CC = 5.5+0 via FAS-DPD. So far so good.
    • TC/FC = 5/5 via test strip. So far so good.
    • TC = 9 via OTO. Not good. This is repeatable using different brands of OTO reagent. This result is not credible. There is no way that the store is selling product that is 1.8 times stronger than the label says.


    I read in section 2e here:
    https://www.hach.com/cms-portals/hac...neAnalysis.pdf
    that OTO is considered not very reliable. However it tended to give "lower" values relative to other methods, but that is not what we are seeing here. I cannot explain the high OTO reading.

    In contrast, there is a nice three-way agreement between the reference preparation, the test strip, and the FAS-DPD reading. This is evidence that I'm doing the chemistry right.

    3) We now measure some actual pool water. I have reason to disbelieve a great many of the results I see. For starters, I observe:

    • FC+CC = 3.0+0 via FAS-DPD.
    • TC/FC = 4/2 via test strip.
    • TC = 6 via OTO,


    At this point we have a three-way disagreement. The OTO reads higher than FAS-DPD by a factor of 2, so this part of the problem runs parallel to item (2) above. As a separate matter, the test strip cannot possibly be correct; if there were 2 ppm of chloramines you would smell it from a mile away, but the pool has absolutely no odor. I assume FAS-DPD is more reliable than the other tests.

    It must be emphasized that for each type of test, the results are consistent and reproducible. They just don't agree with other types of test. I have read the earlier threads about inconsistent and irreproducible test results, but that is not what we are talking about here.

    Tangential remark: The test strip consistently indicates zero CYA, even though the melamine turbidity test reads 35 or 40 ppm. This has been going on since forever. I can go through an entire bottle of test strips and never see a nonzero CYA indication, even immediately after adding some stabilized chlorine. This is reproducible across different brands of test strips.

    Another tangential remark: The test strip reports the pH as low (7.2) under conditions where phenol red reports it as high (7.8). I assume the latter is more reliable.

    Note that I vigorously aerate the pool. There is probably a great deal of molecular O2 dissolved in the water, along with rather low carbonate, low dissolved CO2, and low total alkalinity. Is there any chance that this could be interfering with the tests for CYA and/or TC/FC and/or pH?

    Note that the calcium hardness is high. I'm not sure exactly how high. Taylor chemistry indicates 625, whereas test strips consistently report offscale high, i.e. more than 1000. I mention this only in passing, since I don't worry about it, because there is no problem with scaling or cloudiness. You can see clearly from one end of the pool to the other underwater (more than 30 feet). Is there any chance that calcium could be interfering with the tests for CYA and/or
    TC/FC and/or pH?

    Are there other interferences I should be worrying about?

    Bottom line: What is going on? Why does pool water produce conflicting results from different tests (even though the reference solution behaves better)?

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    Mod Squad zea3's Avatar
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    Re: some tests agree; some tests disagree

    Hi, welcome to TFP! Test strips are not designed to give a precise measure, but a ballpark estimate within a prescribed range. Test strip efficacy can be affected by exposure to extremes in temperature, humidity, light exposure, and age. They are especially bad at measuring CYA. Strips measure total hardness and not just calcium, so it is expected to have a different result from a calcium drop based test.

    A drop based test kit will give more accurate results than a test strip. It is more important to develop consistent testing with your drop based kit than to try and match the results with a test strip. Try to test under similar conditions each time you pull a water sample. Consistency in lighting is important for reading the CYA test, for example. It should be read in full sun, with your back to the sun and the view tube held at waist level. If the test is performed inconsistently (high noon one day, dusk the next day) you will not have consistent, predictable results.
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    Re: some tests agree; some tests disagree

    I think zea has it covered - test strips live up to their name, guess strips. You never know the result you will get. While you may store them in a hermetically sealed vault at the perfect temperature there is no way of know,ing how they were handled before you got them.

    We agree, OTO testing is no where near as exact as some would think. The only time we recomend using the OTO is if you want to see if there is chlorine in the water. Want to know how much chlorine, use a FAS-DPD test.
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    Mod Squad pooldv's Avatar
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    Re: some tests agree; some tests disagree

    Any chance you are reading the bromine scale on the OTO test block? Chlorine should only read 1-5 and bromine reads 2-10 on the test blocks I've seen.
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    Re: some tests agree; some tests disagree

    Any chance you are reading the bromine scale on the OTO test block?
    No chance.

    Chlorine should only read 1-5 and bromine reads 2-10 on the test blocks I've seen.
    Which is why I dilute it 2x or 3x with deionized water before measuring, when I know it's high and want to quantify it using such a device. And yes, I checked that the DI water reads zero. And furthermore the reading of 6 was obtained two ways, once using dilution and once without, simply by extrapolating slightly beyond 5 on the scale.

    Test strip efficacy can be affected by exposure to extremes in temperature, humidity, light exposure, and age.
    None of that is relevant. As emphasized in the OP, the test strips are dead nuts accurate when applied to the reference solution ... just not when applied to pool water a few minutes later.

    Consistency in lighting is important for reading the CYA test
    As emphasized in the OP, consistency is not the issue. The CYA melamine test is entirely reproducible and consistent with itself. Also the test strip reading is entirely reproducible and consistent with itself. Improving the lighting cannot possibly increase the consistency or reproducibility of either test. The problem lies elsewhere.

    The problem is that the two CYA tests reproducibly disagree with each other by a factor of infinity. Can anybody explain this?

    Similarly, the FAS-DPD CC reading is entirely reproducible and consistent with itself. Also the test strip CC reading is entirely reproducible and consistent with itself.

    Again, the problem is that the two CC tests reproducibly disagree with teach other by a factor of infinity. Can anybody explain this?

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    Re: some tests agree; some tests disagree

    Welcome to the forum Dumbledore.

    What exactly are you trying to accomplish? The FAS/DPD is the proven, accurate, repeatable test for maintaining proper FC levels, and Taylor reagents are the industry standard.

    For 99.9% of our members here in real world application, the FAS/DPD chlorine test, as well as the Taylor reagents for the remaining tests have proven to be a safe, reliable, and repeatable method to maintain their pools, while test strips have not proven to be so. Just look at the "AquaCheck" result groupings:

    test strips.jpg

    How do I know if my CH is 250ppm or 350ppm, and how do I tell if the test is repeating when in that range? What about TA, how would I maintain 60ppm? The CYA is the worst of the group, how would I target 60-70ppm here? With my Taylor tests I simply count drops and do some simple math and I'm done, and I have repeatable results. Your average pool owner is not going to bring syringes out to do dilution tests on a daily basis, nor possess the proper lighting environment for color matching.

    The OTO test is really just a reference as it is opinionated for most (color matching). Dilution will increase your error margin.

    We are called "Trouble Free Pool" for a reason, and the FAS/DPD, as well as the other Taylor reagents have proven to be trouble free as well.

    Are you writing some kind of chemistry paper on testing methods? Seems like you are going through an awful lot of trouble to just test your pool water.
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    Re: some tests agree; some tests disagree

    the FAS/DPD, as well as the other Taylor reagents have proven to be trouble free
    When one Taylor reagent disagrees with another by a wide margin, that counts as trouble in my book.

    Seems like you are going through an awful lot of trouble to just test your pool water.
    Einstein said a theory should be as simple as possible, but not simpler. The same principle applies to experiments. When following the instructions that came with my test kit doesn't produce usable results, it seems natural to go looking for additional information, e.g. by cross-checking different methods, performing calibrations, and asking for help.

    What exactly are you trying to accomplish?
    Trying to solve the aforementioned problems, primarily to find a method that is both convenient and reliable, and secondarily perhaps to gain some understanding or at least some guidance as to when the standard methods can be trusted and when they cannot.

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    Re: some tests agree; some tests disagree

    This reminds me of a man trying to tell the exact time with two watches.

    Use the FAS/DPD test as the official reading. Use the OTO test only as a quick method of determining whether there is chlorine present. Throw the test strips in the trash.

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    Re: some tests agree; some tests disagree

    How old are your reagents? Taylor does recommend replacing them every year.

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    Re: some tests agree; some tests disagree

    I understand your goals to be choosing a suitable test, understanding why it's suitable and knowing what to trust. For Trouble Free Pool Care (TFPC), drop-based titration tests are the standard methods because they're easier to perform than anything involving color matching. Testing of pH and CYA are exceptions, and for many Salt and Borates have adequate test-strip tests.

    OTO is reliable and can be trusted as a "yes or no" test to determine if there is any chlorine in the water. It is not reliable for measuring the amount of free chlorine (FC), other than in very broad ranges. Some people like to just walk by, dip a sample, put a few drops in and know there's FC in the pool, so that's the circumstance when OTO can be trusted.

    Most members here use FAS-DPD because it adds less than a minute to total testing time, so FAS-DPD is considered convenient. In my experience with a 10 ml sample, FAS-DPD is reliable to about +/- 0.5 ppm FC when measuring from zero to 10 ppm FC, and about +/- 1 ppm above that. FAS-DPD can read FC levels up to 50, so it covers everything a TFPer might ever do. Dilution reduces accuracy, so it's not advised. The 10 ml sample size is recommended, but if you need to save on reagent when you're close to running out, you can use a 5ml sample when reading high levels of FC, e.g. during a SLAM. The only circumstances when this test can not be relied upon is if reagents are out of date or if the test is not done with reasonable care.

    Test strips are not suitable for day-to-day pool care because the ranges are too wide, color matching is subjective and affected by lighting, and test-strip chemistry is subject to various failures, or just plain doesn't work (CYA). For salt and borates, test strips are OK. Again there are many discussion threads available along with Pool School articles regarding reliable and convenient testing, and levels of tolerance suitable for TFPC.

    With respect to guidance on when the standard methods can be trusted and when they cannot, the articles and references here on TFP are regularly reviewed, debated and updated by experts, so you should not find any standard testing methods that cannot be trusted. One of the great things about this forum is that experts will correct or clarify anything written that is wrong or misleading, unlike many/most internet forums. This series of articles about testing is a pretty thorough starting point: Pool School - Extended Test Kit Directions

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    Re: some tests agree; some tests disagree

    I'll play.

    I have all three tests so I decided to give it a go and see if I reproduce same discrepancies between tests. Results:

    FAS/DPD
    FC 2.8
    CC 0
    CYA 40

    OTO
    TC 5 (maybe a hint darker)

    Clorox 6-way strip

    TC/FC 3/3
    CYA 0


    So my test agrees with OTO being about double reality on chlorine. The fact that the dispute is reproducible is comforting in a way. Take half of what you see on OTO for a quick chlorine read.

    Also confirmed that strips are useless for CYA (we all agreed there). When my CYA was 200 or so last year the strips did give me a reading somewhere close to darkest mark of 300 so maybe the strips can be used for a quick detect of a CYA overdose/need to drain and refill.

    Where I did not reproduce same discrepancy was on the strip test for chlorine. To my subjective eye, the results appeared to be 3 TC and 3 FC. In fact, how did you get readings of 2 and 4 for FC and TC on a test strip? Colors came in between 1-3 and between 3-5? I'm assuming that is how. And you continually reproduced this result with the pool water testing FC 3/ 0CC) with FAS/DPD? The strips overall are pretty tough to read with the water and color bleed taking place the entire time.

    My conclusion is that I stick with the FAS/DPD for the most accurate results. Don't get me wrong, this was fun.
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    Re: some tests agree; some tests disagree

    My conclusion is that I stick with the FAS/DPD for the most accurate results. Don't get me wrong, this was fun.
    Nice work! I agree with your conclusion!
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    Re: some tests agree; some tests disagree

    Maybe this thread belongs in the Deep End forum. My brain is spinning.
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    Re: some tests agree; some tests disagree

    I bet standard methods that can be trusted or not trusted has something to do with the global warming of the pool water.
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    Re: some tests agree; some tests disagree

    Thank you, troymeboy. It's nice to have somebody willing to do the experiment, rather than just telling me how stupid I am.

    Take half of what you see on OTO for a quick chlorine read.
    Yes, I've been doing a certain amount of that:
    • OTO divided by 2 on a day-to-day basis.
    • FAS/DPD every few days to make sure the story hasn't changed.


    This "might" be the right thing to do, but there are still some loose ends, as discussed below. No matter how many "experts" tell you otherwise, there are still plenty of things that can interfere with the FAS/DPD test ... maybe not quite as many as with OTO, but still plenty.

    Furthermore, there must be some situations where OTO gives the right answer without any factor-of-two fudge factors. So there must be some uncontrolled variable(s) still on the loose.

    FWIW the Taylor 0870 reagent contains many ingredients other than DPD, including KI, EDTA, phosphate buffers, and even secret ingredients, presumably to mitigate the interferences. You can make an initial check for interference as follows: Perform the FAS/DPD check using twice as much powder as you usually use, then immediately perform it again using half as much as usual. It's not a question of having more DPD strictly speaking, but rather more buffers and conditioners. If you get the same answer both ways, that's an encouraging sign. Chez moi I observe:
    • Double-powder DPD/FAS: FC+CC = 3 + 0
    • Half-powder DPD/FAS: FC+CC = 3 + 0
    • Test strip: TC/CC = 3/3
    • OTO: TC = 6


    So far so good. On to the next step. EDTA is used to gobble up metal ions. The Taylor 0012 reagent used in the calcium hardness test is just aqueous sodium EDTA, if the MSDS is to be believed. So the next experiment goes like this: Put 10 mL of pool water in the graduated cylinder, then add some 0012. Use about half as much as you needed in the last calcium hardness test. That is, knock the calcium down by a factor of 2. You can verify the new calcium level using a test strip if you like the belt-and-suspenders approach. Then add a scoop of 0870 and proceed with the DPD/FAS test in the usual way. One can chelate the OTO and strip tests also. I observe:

    • EDTA DPD/FAS: FC+CC = 1 + 0
    • EDTA OTO: TC = 1
    • EDTA strip: TC/FC = 1/1


    Those are reproducibly consistent with each other, but seriously inconsistent with the non-EDTA readings. It must be emphasized that a modest amount of EDTA was used, only enough to knock the calcium down by half.

    Obviously it would be nice to look into this more closely, but I reckon that preliminary information is better than no information.
    1. It seems near certain that somebody has been eating my porridge.
    2. Preliminary evidence suggests that metal ions are a nontrivial part of the problem.
    3. Preliminary evidence suggests that chez moi the true FC levels may be significantly lower than any of the ordinary tests indicate. This may have been going on for quite some time. This is less of a problem than it might appear, since I know from experience what sort of indicated levels are needed to produce more-or-less acceptable results. However, it's still somewhat of a problem on several levels, e.g. when communicating with other folks.


    ---

    Colors came in between 1-3 and between 3-5?
    Yes, it is possible to interpolate between color reference steps. This applies to liquids as well as to test strips. It's not rocket surgery. The OTO TC reading can even be extrapolated a little bit (whereas phenol pH cannot).

    -----

    This should go without saying, but I'll say it anyway: The main things that matter operationally are simple yes/no questions:
    Should I add half a jug of HOCl today, or not?
    Should I close the pool because it is dangerous due to low FC?
    Should I close the pool because it is dangerous due to high FC?

    This requires frequent testing, because the FC level changes depending on factors beyond my control, including bather load, windborne krud, insolation, and who-knows-what else.

    In contrast:
    • A test for "some chlorine" is not good enough. Knowing (based on OTO or whatever) that there is "some chlorine" provides too little information. It doesn't come close to answering the operational questions.
    • On the other side of the same coin, talking about the "exact time" or the "exact" anything else is equally impertinent, for the opposite reason. It demands infinitely too much information.


    To repeat: It doesn't have to be exact, and I never suggested that it did. There is a fairly wide band of tolerable FC levels. However, the tolerance band is not infinitely wide. A test that is off by a factor of 2 is not what I would call trouble-free.

    Caring about the answer and demanding an "exact" answer are not even remotely the same thing. I have been in situations where I cared about the time. Lives depended on it. There was a tolerance band of maybe 30 seconds either way, over a 40 minute span. I made sure the primary stopwatch was working correctly, and yes, that included checking against a secondary watch. Neither one was "exact", but both were within tolerances with an enormous safety margin. If they had disagreed by a factor of two -- or even by a few percent -- I would have noticed, and I would have made it my business to obtain a third vote.

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    Re: some tests agree; some tests disagree

    FAS-DPD works for maintaining chlorine at a safe level. OTO works for detecting the presence of chlorine. A test of, say, 6.5 ppm FC when tested with FAS-DPD can not be compared to a positive OTO result that simply indicates "yes, there is chlorine in the sample".

    If your goal is to double-check the Taylor FAS-DPD test, create a 'standard solution' with 1 ml of 10% chlorine in 10 litres of water. That should read pretty close to 10 ppm FC plus/minus the variability in chlorinating liquid, say 10%-15%, plus the chlorine in your source water. Or you could use some other test method of equal or better accuracy than FAS-DPD.

    It's not possible to add a jug of HOCl. You can add chlorinating liquid which contains sodium hypochlorite, and the added chlorine will join the pool's chlorine chems in equilibrium including additional HOCl. You can determine the amount of bleach or chlorinating liquid to add by entering your target FC, your test result, and your pool volume in PoolMath.

    Minimum FC can be found in the FC/CYA chart. Provided the pool is clear, and pH is between 7.2 and 7.8, FC up to shock level (per the same chart) is safe for swimming.
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    Re: some tests agree; some tests disagree

    I still don't understand the point to all this. I own a pool, not a science experiment, so I guess I'm just in the weeds here.

    I will not dispute that the FAS/DPD may not be flawless, but it is good enough to properly maintain a residential swimming pool.


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    Re: some tests agree; some tests disagree

    Dumbledore,

    No one has even implied that you are stupid, but several members have gone to some length to explain our position on strips. The wet testing we suggest for keeping pools here is more than adequate for that purpose, and they don't need verification with something else, especially multi-analyte test strips. They are inferior compared to the testing we suggest, so using them as verification is pointless for our needs. Nothing wrong with experimentation, but strips simply don't "cut the mustard" as they say.

    Lastly we are a teaching forum, not an argument or debate forum. Based on this thread, it seems clear you are here for that, at least on this point. You've made that point about strips, and several members have made our position just as clear. If you want to use them on your pool for maintenance or comparison purposes, you are perfectly in your rights to do so, but it isn't something we will ever endorse or suggest to provide the results we desire. This discussion is closed, and I kindly ask that you move on from this subject.
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