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Thread: How (im)precise is the OTO test and when does that matter?

  1. #1
    Administrator JasonLion's Avatar
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    How (im)precise is the OTO test and when does that matter?

    The standard OTO chlorine test is simple to use, inexpensive, and easy to find at the store. Still, it has a couple of disadvantages, it measures TC instead of FC & CC and only goes up to 5 (or 3 in some versions). Both of these disadvantages can be mitigated to some extent. The range can be extended with dilution and some sense of FC vs CC can be gotten by watching for a color shift.

    I can use OTO to measure TC levels from 0 to 2 to the nearest 0.5 fairly reliably. 3 can usually be made out, but anything from 4 to 8 looks much the same to me. Somewhere above 8 it starts to turn more of an orange, and I have been told that at sufficiently high levels it turns some kind of red. Using a 2 to 1 dilution and measuring levels around 15 that means it would be +-6 or so.

    For normal day to day chlorine testing this level of precision is usually just fine. Half of what you want to know day to day is "did my chlorine drop to zero?" and for typical CYA levels around 30-50, a simple below 3, around 3, higher than 3 determination is usually plenty.

    I am wondering what other situations OTO testing will work for and which situations OTO testing won't work for. Also, does anyone have experience distinguishing values around 4, 5, or 6 reliably with OTO?
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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  2. #2
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    Re: How (im)precise is the OTO test and when does that matter?

    I can't tell the difference between 3, 4 or 5.
    Helpful links: Pool School; CYA/Chlorine Chart
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  3. #3
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    Re: How (im)precise is the OTO test and when does that matter?

    Quote Originally Posted by frustratedpoolmom
    I can't tell the difference between 3, 4 or 5.
    I have no problems distinguishing up to 5 ppm with a comparator, the deep yellow that appears around 8-10 pp, and can recognize the orange color which indicates about 15-20 pm and the brown color that appears at around 35 ppm.
    Then again I have been using OTO for a long time.

    Interestingly enough, Ben Powell from Pool Forum thought OTO was a useful test and he was working on a color chart for high chlorine levels since it can 'accurately' test them. He was having problems getting the color chips to print out correctly last I spoke to him about it (shortly before he became MIA at PF) but I do have one of his preliminary charts. The colors according to Ben are roughly as follows (his own words):
    really dark yellow (darker than the darkest on the PS200) => 5 - 15 ppm
    light orange => 10 - 20 ppm
    dark orange => 15 - 30 ppm
    brown => 30+ ppm

    He felt the main benefit to OTO (and I have to agree with him) is that it is a 'bulletproof' test. If you have chlorine it WILL turn yellow to brown. If you don't it will stay clear. This cannot be said for DPD and even FAS-DPD testing. Both can suffer from 'bleachout' at high chlorine levels. For DPD testing often you will see a 'flash' of pink as you add the reagent then it turns colorless. With FAS-DPD it might stay colorless or it might develop a yellow off color, particularly when the potassium iodide reagent is added for the CC test. Granted FAS-DPD will test higher chlorine levels without dilutions but Taylor says it's only good to about 25 ppm FC before problems can develop (although I have used it successfully at much higher chlorine levels).

    Ben had this to say about OTO testing over at PF:


    "The fact that those colors EXIST is one of the major reasons I LIKE having the OTO kit in the PS23x series -- the OTO test is quick, easy, meaningful, and almost bombproof.

    Most of them, you can operate quite successfully with the following verbal scale:

    * Yellow to Dark Yellow => Ideal
    * Light Yellow => Add chlorine
    * Orange tinted => Swimmable, but too high, unless you are shocking or have high CYA
    * Orange => Old swimsuits only! Ideal for algae cleanup.
    * Brown => Too high for people and vinyl pools.
    (Not dangerous, but it will fade things, and sting eyes. You might get a little chapped where the sun doesn't shine, too.)
    Great for concrete pool cleanup.

    Now, you can get meaningfully better results using the DPD-FAS, and tracking things more closely than that. But, using that scale alone, probably 1/2 the pool owners in the US could do better than they are now!

    Ben"


    I have to say I agree with him.
    I am not saying throw out the DPD and FAS-DPD tests, just that OTO really is a lot more useful than people realize. It can quickly give you an idea of whether your chlorine has dropped below shock level and can tell you when you have a large amount of CC (over 1 ppm) because the color will darken after about 2-3 minutes. It just won't give you precise numbers. Then again, we ARE talking about a swimming pool so precise numbers really are not THAT necessary nor are they possible in most cases. Most people, unless they have a chemistry background, are not really aware how precise or imprecise our testing procedures are. This can be seen by the number of people who will post a CYA number of 28 ppm using a TF100 or Taylor kit because it's "a bit below the 30 mark" (call it 30 and call it a day--the test is NOT that precise) or who get upset because their TF100 tests their TA at 100 ppm on Tuesday and at 110 ppm on Wednesday. These are the same results since this particular test has a precision of +/- 10 ppm!

    As far as the ease of use, a better kit such as the Taylor one in the TF100 is far superior to the cheap two way testers often found at pool stores. Most of these use very diluted reagents for both the OTO and pH tests and are next to impossible to read. One only needs to compare one of the comparators that come with these to the Taylor one to see the difference and see that the colors ARE distinguishable in a better quality kit and that the colors on the comparator actually DO match the ones that the reagents create in your water sample.

  4. #4
    Member CRG_80cc's Avatar
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    Re: How (im)precise is the OTO test and when does that matter?

    You can't beat the OTO to see if you have ANY chlorine. It's fast and easy. I know this won't sound good to most of you, but for the past year, I have just used strips (for CYA & TA) and an OTO and pH test to manage my pool. I didn't get TF-100 kit until last month. Based on what I learned here, I was able to just use those cheap test to keep my pool clear. To say the cheaper test are worthless is not accurate. I do agree, if you're gonna get an OTO/PH kit, get the Taylor kit. What I learned with the OTO kit was that since my CYA was at 30 ppm, if I could get/keep the color yellow to dark yellow, I was probably going to be good in terms of keeping my pool clear. I didn't really care what my actual ppm number was for either FC or CC.

    Would it have been easier with the TF-100? Absolutely. No argument there. I think the analogy that comes to mind when comparing the cheaper test to the TF-100 is to think of the cheap test as using a magnifying glass to look at your water and the TF-100 as microscope.

    Great post Waterbear.
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    Administrator JasonLion's Avatar
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    Re: How (im)precise is the OTO test and when does that matter?

    One situation where the OTO test isn't good enough is measuring FC loss overnight to decide if you have any algae left or are done converting from baquacil. The +-0.5 precision of the FAS-DPD test is important in these two cases. In both cases it is possible to continue shocking for a few additional days to be sure you are done. The higher precision of the FAS-DPD test saves time and chlorine in these cases.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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  6. #6
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    Re: How (im)precise is the OTO test and when does that matter?

    Quote Originally Posted by JasonLion
    One situation where the OTO test isn't good enough is measuring FC loss overnight to decide if you have any algae left or are done converting from baquacil.
    You are comparing apples and oranges here. OTO only tests TOTAL chlorine so why would you even consider using it when an accurate test of FC is needed? This is a misuse of the OTO test and in no way negates it's usefulness!
    This is like saying a phenol red pH test is not good enough to determine TA. Use the right test for the right purpose.

  7. #7
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    Re: How (im)precise is the OTO test and when does that matter?

    I can usually make out when it's 4ppm. But it really depends on an individual's ability to discern very slight changes in color hues.

    I don't need OTO that much. It's only use is to tell me if TC is above or below 5ppm.

    If it's below 5ppm, time to add another jug. If it's 5ppm or above, I need to break out the FAS/DPD test anyway to find out what FC is.
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    Re: How (im)precise is the OTO test and when does that matter?

    For about the first year after I purchased Ben's test kit, before I tested with the FAS/DPD, I conducted the OTO test and estimated the total chlorine level. By the end of that time, my reading of the OTO test was as accurate as the FAS/DPD test when TC was at 4 or below, and within 0.5 up to 5.
    Conclusion: You can train your eyes so that the OTO test is quite accurate at these levels of TC.
    22,000 gal, IG vinyl, 3/4 hp pump, 300lb sand filter (switched from DE), converted from baquacil (thanks to Ben), BBB since 2004

  9. #9
    Senior Member Shelley N's Avatar
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    Re: How (im)precise is the OTO test and when does that matter?

    With a CYA of 50, I need to keep my FC at 4-8 with 5.7 being the goal. I have run OTO along with FAS/DPD for this entire year with the exact goal of "learning" the OTO like others have, and am, alas, no better at determining the amount of CL with OTO than when I started...it's always the same color yellow because typically its 5+/-. I believe I'm best with "learning" my pool and figuring out how much it likes to eat on a regular basis (currently about 3ppm per day) combined with the FAS/DPD done on a daily basis, until I really learn how much it looses. I to it this way because I never ever want to see cloudy water, heaven forbid green! and it's worth it to me to test and add CL daily. As a result, I never have seen cloudy or green water.
    Shelley
    Denver, CO

  10. #10
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    Re: How (im)precise is the OTO test and when does that matter?

    Quote Originally Posted by CRG_80cc
    You can't beat the OTO to see if you have ANY chlorine. It's fast and easy. I know this won't sound good to most of you, but for the past year, I have just used strips (for CYA & TA) and an OTO and pH test to manage my pool. I didn't get TF-100 kit until last month. Based on what I learned here, I was able to just use those cheap test to keep my pool clear. To say the cheaper test are worthless is not accurate. I do agree, if you're gonna get an OTO/PH kit, get the Taylor kit. What I learned with the OTO kit was that since my CYA was at 30 ppm, if I could get/keep the color yellow to dark yellow, I was probably going to be good in terms of keeping my pool clear. I didn't really care what my actual ppm number was for either FC or CC.
    This is a very good point. For daily routine use, and your comfortable with your routine, it's quick and simple and does what it needs to do.

    I agree Waterbear's post was quite informative.
    Helpful links: Pool School; CYA/Chlorine Chart
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  11. #11
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    Re: How (im)precise is the OTO test and when does that matter?

    Quote Originally Posted by Shelley N
    With a CYA of 50, I need to keep my FC at 4-8 with 5.7 being the goal.
    There seems to be a LOT of misconception on the accuracy and precision of the testing we are doing in our pools. It's really a good estimate at best for several reasons. First there is a difference between the precision of a test (What is the actual test number?) and the accuracy of a test (Is the test results repeatable within a specific margin?) As a matter of interest, test strips ARE relatively ACCURATE but they are by no means PRECISE! This is the problem with using strips.

    Let's look at the CYA test since it is the MOST subjective. It's accuracy is very good for CYA levels between 30 to maybe about 80 ppm but it's precision is really only about 10 ppm at best and most likely higher because of the subjective nature of this test. This means if you repeat the test on the same sample the dot should disappear in about the same place BUT what is the actual reading? You can't really interpolate between numbers since the scale on the view tube is not linear nor is it truly logarithmic. For example, this means that just because the dot is disappearing midpoint between the 30 and 40 mark it does NOT mean the CYA reading is 35 ppm. It's an invalid assumption. It just means that the CYA as you are testing it is somewhere between 30 and 40 ppm! Someone else testing the same sample might come up with a different reading because of the subjectiveness of this particular test. This does not invalidate the test by any means because it gives you a baseline to see if and how your CYA level is changing. Realize that the $1000 LaMotte Waterlink colorimeter used in many pool stores has a precision of +10/-25 ppm for this particular test (and this is much more accurate than the human eye and a disappearing black dot). That means that you CYA reading might be 35ppm off from the measured value in a worse case with this setup.

    Now let's look at the TA and CH titrations. It's precision is +/- 10 ppm but it's accuracy will change with the actual level of TA or CH since the total number of drops used to reach endpoint changes.
    Ditto for the FAS-DPD titrations which have a precision of either +/- .2 ppm for a 25 ml sample or +/- .5 ppm for a 10 ml sample. In this last case it is possible for your reading to have a precision that is off by up to 2 ppm, believe it or not! As FC increases the accuracy also changes because of the increase in number of drop of titrant needed.

    We also need to remember that there are a lot of factors that can affect the drop size (angle of the bottle, static charges, amount of reagent left in the bottle) so there is no guarantee that the drop size is going to be uniform each time we test. If we would want to increase the precision of these titrations (get a more precise number) the we could use a graduated glass burette (a piece of laboratory glassware used for titrations) so the actual AMOUNT of reagent used for the test can be read off the burette (usually in ml.). The use of dropper bottles introduces more error into the precision of our testing. However, it is certainly precise enough for our puposes of testing a swimming pool!

    We have to keep this in mind when we test a pool and make changes to the chemical levels. Also, unless you metered the water when filling the pool you really DON"T know how many gallons it actually holds, you just have another 'estimate'.

    So when you say your CYA 50 so you need to maintain your FC at 5.7 you are really just deluding your self. Saying that you need to keep your FC between 4-8 is much better and then in actual pracitice you might find that 4 or even 5 ppm as you test it is too low and you really need to keep it at 6ppm, for example.

    The main advantage to testing your own water is that your testing procedure is going to be done the same way each time so your results are more likely to be accurate (repeatable) but if you think they are precise to the degree that test results are talked about on here (or most places for that matter) then you are sadly mistaken.

    Anyone who has had college level Analytical Chemistry will understand what I am talking about.
    For those that don't just remember, we are talking about swimming pools, not laboratory experiments!

    Going back to OTO, let's say that you have found out that when the OTO is a deep yellow your pool is fine, then you know when it's a pale yellow you need chlorine and when let's say when you shock it turns a deep orange, then you know when it's not that deep orange and it's yellow again you are no longer at shock level. A pretty handy test if you ask me (and certainly accurate enough for daily use).

  12. #12
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    Re: How (im)precise is the OTO test and when does that matter?

    Waterbear, you lost me at the phrase....

    "There seems to be a LOT of misconception on the accuracy..." Just kidding!

    I just thought of a new drinking game. Everytime someone says the following:

    Accurate (or any form of the word thereof)
    Help
    Bacquacil
    Normal
    Pool store
    Bought some
    Threw In
    trichlor
    pucks
    Calcium

    ....We take a drink!

    I thought about "Shock/Shocked" but that's used too much and we'd all be drunk.

    (I apologize if I offended anyone and was insensitive to those dealing with addiction issues. I'm in a goofy mood.)

    Have a great Saturday. I lounged in my sparkly pool (thanks to TFP) for 3 hours. I think the sun got to me, or maybe the LIIced Tea.....
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
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  13. #13
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    Re: How (im)precise is the OTO test and when does that matter?

    Frustratedpoolmom...

    Could you pass the popcorn???

    If you know the EXACT measurements of your pool, you can calculate quite precisely how many gallons you have. Since my pool was a customized version of a FantaSea, the deep end has to be calculated by both them and me. Therefore, due to basic High School Geometry I was able to compute my volume for a given depth.

    If you have a 52" oval and the dimensions are pretty accurate, and you probably have a TRUE 48" of water, you can very easily compute the volume. Just remember to multiply the cubic feet by 7.48gallons. Of course, if you figure your pool in liters, your job is easier as there's no conversion.
    Stay ahead of your water!

  14. #14
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    Re: How (im)precise is the OTO test and when does that matter?

    Quote Originally Posted by CarlD
    Frustratedpoolmom...

    Could you pass the popcorn???

    If you know the EXACT measurements of your pool, you can calculate quite precisely how many gallons you have. Since my pool was a customized version of a FantaSea, the deep end has to be calculated by both them and me. Therefore, due to basic High School Geometry I was able to compute my volume for a given depth.

    If you have a 52" oval and the dimensions are pretty accurate, and you probably have a TRUE 48" of water, you can very easily compute the volume. Just remember to multiply the cubic feet by 7.48gallons. Of course, if you figure your pool in liters, your job is easier as there's no conversion.
    Carl,
    That's great for an AGP but what about a free form gunite pool, which is probably one of the more common types of IGPs? What about pools with swim ledges and beach access? What about negative edge pools? What about water features? Add all that up and the pool volume becomes difficult to accurately determine.

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    Re: How (im)precise is the OTO test and when does that matter?

    Quote Originally Posted by waterbear
    Quote Originally Posted by CarlD
    Frustratedpoolmom...

    Could you pass the popcorn???

    If you know the EXACT measurements of your pool, you can calculate quite precisely how many gallons you have. Since my pool was a customized version of a FantaSea, the deep end has to be calculated by both them and me. Therefore, due to basic High School Geometry I was able to compute my volume for a given depth.

    If you have a 52" oval and the dimensions are pretty accurate, and you probably have a TRUE 48" of water, you can very easily compute the volume. Just remember to multiply the cubic feet by 7.48gallons. Of course, if you figure your pool in liters, your job is easier as there's no conversion.
    Carl,
    That's great for an AGP but what about a free form gunite pool, which is probably one of the more common types of IGPs? What about pools with swim ledges and beach access? What about negative edge pools? What about water features? Add all that up and the pool volume becomes difficult to accurately determine.
    Yeah, the geometry gets more and more complex---my AG starts simple--16x40. But the deepend is a flat-bottomed hopper with sloping ramps on all 4 sides---but the ramp from the shallow section is longer, so figuring our the volume of the hopper was tricky. Yet it STILL was all just high school geometry. Swim ledges are usually easier to figure. What you have to do is break the pool's shape down into basic units: A simple example: An 18x33 oval AG is really an 18' round and a 15x18 rectangle--but the 18' round is cut in half and each half is stuck on the ends of the rectangle. (9'+9'+15'=33').

    You'll have to remember the solid geometry rules for volume of spheres, cylinders, trapezoidal shapes (like my hopper) and it can get complicated...but it's do-able if you are persistent enough...or you could use a water meter! (Waterbear pounding on CarlD )
    Stay ahead of your water!

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