Accuracy of Test Strips

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Kmochem77

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Apr 1, 2011
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I couldn't find a more recent post about strip accuracy. So, I am bumping an old post. Sorry. :roll:
Split to a new topic. Please ask questions in your own topic, instead of reusing someone else's exsiting topic. JasonLion

I am doing "research" for my company on pool testing equipment. We supply dehumidifiers for indoor pools (mostly commercial). I understand that you guys are more experienced from a residential side but pool testing is essentially the same.

Anyway, according to PA state code, pool owners are required to test their pools atleast twice per day and set limits on the accuracy of acceptable tests to use. Residual (chlorine) to be accurate within 0.1ppm and pH to be accurate within 0.2 pH units.

From what I've read on this site, there is a lot of distrust in the strips. However, from what I've read (mostly from the sellers themselves), there's been advancements to make strips more accurate including handheld digital strip readers. There are now even iphone apps that will display readings of the sticks (e.g. howsmywater) from taking photos of the strip. In reference to accuracy, Aquachek makes claims that they are just as accurate as liquid tests and perhaps have even greater accuracy due to the simplicity of "dipping" and reading. I am widely skeptical that this is just propaganda.

Does anyone have more recent experience with strips, digital strip readers, or the apps and how they stack up with your tfp FAS-DPD tests? Any info would be helpful.
Thanks.
 
Test strips suffer from two different kinds of failure, lack of precision and just wildly wrong. Some of the better brands have made some progress on the just wildly wrong aspect. But the solution depends on the strips being stored very carefully to avoid having them absorbing any humidity out of the air. If you live in a humid area that is almost impossible to guarantee.

The lack of precision problem is inherent and no one has been able to improve it much at all. The amount of water that interacts with the indicator can not be accurately controlled. Small improvements are possible with substantial amounts of training, but even then they don't come even remotely near +-0.1. The electronic readers are great at reading out the color with precision, but the color is imprecise to begin with, so electronic readers simply give you the illusion of precision.
 
Ok, let me start by saying I wish there was a good test strip product on the market, and that some of these electronic color comparators may help in reading errors, however in my experience reading errors are a distant second to testing errors.

Here is a easy test for you to do yourself go buy a set of test strips, get some swimming pool water using half the test strips one at a time carefully dip them per the instructions, using a timer to wait the correct times, controlled lighting conditions, etc, read them and plot out your numbers. Now do the same thing again with the level of precision you may expect from the near minimum wage college student that you often find working at the front desk of a motel, estimate times by counting, etc. If you think you can repeatedly hit within a 25% window they have improved, remember this is with fresh supplies, no moisture contamination.

For a follow up take the remaining water sample to 3 pool stores that offer free testing and see what the spread is on readings done by their minimum wage employees (ask if they use strips or drops)

Ike
 
Here's an easy way to see for yourself how the readings can be wrong:

Get some aquacheck test strips. Follow directions. Oops, did you shake off too much water, or not enough? There's one. Now, do the most natural thing and set the strip on that plastic color chart to compare? See how the capillary action starts carrying the indicator from one test spot to the next? Oops. there's two. Finally, did you count right? Because the colors keep changing with time. Especially if the strip is sitting on the plastic color comparator. What you see at ten seconds is different from fifteen seconds and very different at a minute.
 
See this post comparing some common test strips vs. drop-based kits. Now this isn't using a photometer, but you can still get an idea of what can and cannot be tested and the ranges available. Test strips test for Total Hardness, not Calcium Hardness (CH) which is what you need to know to properly balance a plaster pool. The CYA test is notorious for being inaccurate with most test strips.
 
As someone interested in testing for indoor pools, CYA isnt an issue. Furthermore, I understand that there are "steps" of color matching (e.g. 0, 0.5, 1, 1.5, 2, 5, 10). There's no way this can be precise nor provide the resolution required for commercial pool testing per PA state code. However, if a digital reader/app is used, does this give better resolution? I would think the results would atleast be more precise. While the operator errors with sampling are still there, the color matching step is reduced to a digital display each time. This gives more than just an illusion of more precise results. Just wondering if anybody had any experience with these "readers".

Also, for the FAS-DPD test, this would not be approved for commercial pool testing either because one drop is 0.2ppm.

I'm at the point where I would believe your ordinary hotel desk jockey would be better with strips and a reader than counting drops. What am I missing?
 
FAS-DPD can achieve 0.1 ppm resolution by using a 50 ml sample. Titration is a much better and reliable method of testing.

Test strips are not as accurate or precise as titration.
(c) Testing kits shall be provided for making the necessary tests for residual disinfection and pH. The kit for disinfectant residual shall be accurate within 0.1 milligram per liter of chlorine or equivalent. The pH kit shall be accurate within 0.2 pH units. When the pool is in use tests shall be made at least twice daily or more often if required by the Department. http://www.pacode.com/secure/data/028/c ... 8_0018.pdf
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JamesW said:
FAS-DPD can achieve 0.1 ppm resolution by using a 50 ml sample. Titration is a much better and reliable method of testing.

Test strips are not as accurate or precise as titration.
(c) Testing kits shall be provided for making the necessary tests for residual disinfection and pH. The kit for disinfectant residual shall be accurate within 0.1 milligram per liter of chlorine or equivalent. The pH kit shall be accurate within 0.2 pH units. When the pool is in use tests shall be made at least twice daily or more often if required by the Department. http://www.pacode.com/secure/data/028/c ... 8_0018.pdf
Click to expand...
Click to expand...

:goodjob:
 
Thanks James. Good answer.

bk- "How does a test strip get you to 0.2ppm accuracy?"
I have no idea. I read somewhere that test strips were being approved for use for commercial pools. Of course, now I cant find where I read that. Most likely it was just some propaganda stuck in my head. If I find, I'll post it here.
 

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The strip readers will give you up to 0.01 ppm precision, as we sometimes see here when people post test results from a pool store (e.g. I have 7.41 ppm FC). That is a very accurate reading of the color on the strip. As Richard said above, it is the relative lack of correlation between strip color and reality that's the problem.

As for approval for commercial pools, it's for the same reason that the FC/CYA relationship isn't recognized. In this situation (as in many others), the people that make the rules don't really understand. If a strip manufacturer (or whoever) can convince them that a strip gives you 0.1 ppm accuracy, then by golly it's good enough!
 
You ask what are you missing, and as I suggested in my original post, try the hands on approach you will see soon enough, while reading the color with a digital meter may help, it does not solve the fact the if you do the test 10 times you get several visibly different colors, more so if you are at all lax in your testing procedure as would be expected of a motel clerk. This is not about trying to match a shade of orange to a reference color, it is about getting one shade or orange one time, andother the next and maybe a shade of red the third time. This will get worse with older strips that have been exposed to moisture (like what happens every time you unscrew the cap to remove a strip from the package)

Ike

p.s. it is suggested around here to keep some CYA (20ppm or so) in indoor pools to act as a chlorine buffer, since i started doing it with my indoor pool I have found it to be much more manageable
 
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