CYA results vary between different reagent lots

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S

Schorschi

0
Jan 10, 2015
16
Tampa Bay, FL
I got a new bottle of Taylor R-0013 reagent yesterday and noticed the CYA test result came in 10 ppm lower than with the last bit of the reagent left over from last season.

I have read several threads on the Taylor CYA test on this site and am aware of the subjectivity (and hence uncertainty and inaccuracy) involved with this test, but here I did a test with (what I believe to be) fresh reagent and compared it immediately afterwards with the results conducted with reagent I had been using and got distinctly different results.

So, the new reagent gave me a lower reading (20 ppm) than the older reagent (30 ppm). Actually, the 20 ppm was even lower, because the black dot was still slightly visible at the 20-ppm mark.

The new reagent has an expiration date of 8/2017 and was bought from PST Pool Supplies in California a few days ago. The older reagent I bought in July of last year from TFTestkits. I'm using a TF-100 test kit.

I'm currently in the process of raising my CYA level and am targeting 50 ppm. My problem is that the skimmer in which the sock with CYA granules is placed has almost no suction, so the four pounds of CYA I have placed (not all at once) in the sock over the past couple of weeks have now only gotten down to about 2 lbs. I think I will place the sock near a return very soon, because it's taking so long to dissolve.

Also, I believe my pool has a leak, but I'm not certain exactly how much it loses. I'm guesstimating a figure somewhere in the 1,000 gal area per week. So, CYA is somewhat of a consumable for me at this point. I'm currently only adding CYA and liquid chlorine to my pool. The other parameters stay in check without me doing anything about them, probably due to utility water being added regularly and in large amounts.

I've been a pool (and house) owner for a little over a year now with no prior pool care experience. The pool came with an autofill device that the previous owner must have installed.

Here is a link to a Google Spreadsheet with my recent pool test results:

Note, that if you're looking at the two charts in that spreadsheet that on the more complex one the scale on the left y-axis is to be multiplied by 10 for CYA and water temperature (in °C). So, a reading of 2 for CYA is actually 20. Also, test results are graphed as lines, chemicals added are graphed as columns. CYA added is in lbs, liquid chlorine (10.5%) in gallons.

The fluctuations in FC displayed are due to me testing immediately before and three hours after adding chlorine.

In case you can't open the above link to my pool test results, the most recent test results yesterday were:

  • FC 3.0 ppm
  • CC 0.25 ppm
  • CH 200 ppm
  • pH 7.7
  • TA 110 ppm
  • CYA 20 ppm
I am adding 0.5 gal of 10.5% liquid chlorine every evening. Have added about two lbs stabilizer five days ago.

It is a little disconcerting to me that the CYA test is so unreliable - besides the subjectivity. Two different lots from the same manufacturer give such different results? Can anyone explain this? Is one of my reagents compromised? If so, which one? Or is it normal that the reagent skews the results over time?
 
+/-15ppm is the best that test can do. As well, you are measuring at the lower limit (20-30ppm) of the test. The melamine cyanurate complex that form the precipitate has a 20ppm solubility limit at pool water pH and so the reagent has additives in it to shift the pH down to a lower value (more acidic) to avoid that. If the reagent was old or compromised in some way, it's possible that it did not buffer the solution properly. Finally, the mixing bottle for the water/reagent mixture is not incredibly accurate and one can easily add too much water or too little reagent.

It is indeed a subjective test but it is really the only test available on the market. Test strips are not helpful since their gradations are usually way too course to be useful. As long as you are doing the turbidity test the same way each time and verifying your read by pouring the sample back and forth several times and averaging the results, you should be fine.

By the way, if you are doing the tests correctly, you don't/can't report a CC of 0.25ppm. It's either 0.2ppm or 0.5ppm depending on the water sample volume size.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006
 
JoyfulNoise said:
+/-15ppm is the best that test can do. As well, you are measuring at the lower limit (20-30ppm) of the test. The melamine cyanurate complex that form the precipitate has a 20ppm solubility limit at pool water pH and so the reagent has additives in it to shift the pH down to a lower value (more acidic) to avoid that. If the reagent was old or compromised in some way, it's possible that it did not buffer the solution properly. Finally, the mixing bottle for the water/reagent mixture is not incredibly accurate and one can easily add too much water or too little reagent.
Click to expand...

I'm actually using a syringe to add pool water (and a different one for reagent) to the mixing bottle. So, I'm pretty certain the ratio between the two is equal.

JoyfulNoise said:
As long as you are doing the turbidity test the same way each time and verifying your read by pouring the sample back and forth several times and averaging the results, you should be fine.
Click to expand...

In this case I did them right after one another between the two reagents. So, I had exactly the same lighting conditions. The test result difference was so noticeable that I am certain a reading error or reading inconsistency can be excluded.

JoyfulNoise said:
By the way, if you are doing the tests correctly, you don't/can't report a CC of 0.25ppm. It's either 0.2ppm or 0.5ppm depending on the water sample volume size.
Click to expand...

Sure I can get a granularity of 0.25 ppm for CC if I start with a water sample of 20 instead of 10 or 25 ml.
 
I am not sure that test is capable of providing what you want.....absolute precision. The test does provide perfectly usable results for pool purposes, however.

The reagent is chemically inert so is one of the most stable (long-lasting) reagents used. We have never had a report of "bad" reagent that we can verify......in fact VERY few reports at all

Regarding the .25cc. ...

You can certainly use whatever sample size you want for the FC test but it confuses most all of us here when you post up something we are not used to. Did you have a purpose for that sample size?
 
duraleigh said:
I am not sure that test is capable of providing what you want.....absolute precision. The test does provide perfectly usable results for pool purposes, however.
Click to expand...

I'm not sure what you mean by "absolute precision".

Can one not expect consistent test results between samples of the same reagent of different lots (and probably ages) taken at the same time and hence identical lighting conditions? As I mentioned above, the difference was so significant that it is impossible to attribute the discrepancy to different lighting conditions, which could have been the case had the tests been conducted on two different days.

Especially, in light of the fact of what you write next..

duraleigh said:
The reagent is chemically inert so is one of the most stable (long-lasting) reagents used. We have never had a report of "bad" reagent that we can verify......in fact VERY few reports at all.
Click to expand...

..one should be able to assume that the reagents were intact in both cases. Having used up the first reagent in a little over seven months since delivery should not have exceeded its expiration date.

This reminds me of the thread Help with CYA and comparing to standard from last year, that seems to be still unresolved to this day. It showed a photo of a test tube with the standard at the 50-ml mark with the dot still visible.

In my case described above both lighting conditions and water sample were identical. Only the reagent R-0013 came from different lots. Yet the test results were different. This and my latest experience with the reagent surely doesn't instill a lot of trust in the product.

duraleigh said:
Regarding the .25cc. ...

You can certainly use whatever sample size you want for the FC test but it confuses most all of us here when you post up something we are not used to. Did you have a purpose for that sample size?
Click to expand...

Just a bit of extra accuracy, that's all. I didn't want to go overboard and take a 25 ml sample. I don't double the sample dose all the time.
 
I'm not sure I understand what you are concerned about here. In your first post you said you saw a 10ppm difference between the two different reagent solutions. The newer solution gave you a result of 20ppm and the older one gave you a result of 30ppm. I responded to you that the test has a variance of +/-15ppm. That variance isn't just in how you read the sample or how multiple people might read the same sample. It also covers variations in the reagent manufacturing itself. That is, if I ordered two bottles of R-0013 from Taylor Technologies and they came from two different production lots (lot id's are different), then I could experience as much as a +/-15ppm difference in the result I get when using the same sample of pool water. In my own use of the test I have taken the same test solution (pool water + R-0013) and poured in back and forth into the view tube and have made multiple reads. I have "read" differences as much as 10ppm between pours even while standing in the same position. Normally it's a bit better than that, but I have experienced read errors as much as 10ppm.

Let's say for the moment that one of those reagents is compromised and your getting a 10ppm error. What difference does it make? Over the course of entire swim season you might use a little extra chlorine because of the reagent error but that would amount to maybe a few dollars worth of extra chemicals. Your pool is not going to burst forth with an algae bloom over a 10ppm CYA variance.

What should be more of a concern for you is not the small testing error but the much larger problem of a pool leaking nearly a 1,000 gallons of water per week. That alone is going to cause a 5%/week change in chemical levels which is going to consume much more of your money to keep within the recommended levels than a test reagent being off.

I'm sorry you're not happy with the CYA test but, I can assure you, that it is the only test out on the market that has the consistency and precision needed to properly manage a pool's stabilizer level. You can buy a pack of test strips if you like but you will find them to be a lot worse and only give you ballpark ranges. There is the LaMotte ColorQ which tries to use colorimetric analysis of the various pool water parameters and those typically work well enough for the first few months and then quickly lose their calibration and/or become unstable. Unfortunately, that's about it for testing CYA unless you want to take your chances at the pool store (but experience has taught us that pool stores almost always get the CYA test wrong).
 

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I guess I'm just having a very hard time accepting an error of ±15 ppm. And I don't understand that it's not possible to create a test with higher consistency.

Must have to do with my background in computing where digital results prevail. Non-deterministic machines are a programmer's nightmare.

Oh, well...
 
Schorschi said:
I guess I'm just having a very hard time accepting an error of ±15 ppm. And I don't understand that it's not possible to create a test with higher consistency.

Must have to do with my background in computing where digital results prevail. Non-deterministic machines are a programmer's nightmare.

Oh, well...
Click to expand...

It's always possible to create a more robust and accurate test. Cyanuric acid can be measured photometrically to quite high precision. The problem is you have to make a test that is easy to use and affordable, in other words, it has to be suitable for the residential pool testing market. No one who owns a pool will be too happy run out and buy a $500 test rig for one specific chemical.

You could always try the Hach cyanuric acid test kit - Cyanuric Acid Test Kit, Model CY-3 | Hach USA - Overview | Hach

It costs roughly $1 per test and for the typical pool owner who only needs to test CYA once per month, it would probably last a while. Unfortunately for you, with the leak, your CYA is always changing and so you will have to test it frequently which would probably cause you to burn through the reagent packs a lot quicker.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006
 
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