Explanation for false negative free chlorine test-strip results?

[mikeyliny]

I was researching why there was a 4X difference between my Total Chlorine (TC) and Free Chlorine (FC) and after reading several comments about the inaccuracy of test strips on TFP, I decided to do an experiment. While the results shown below, I was concluding that the strips were providing false negative results for FC (that is they were showing a falsely low result). This relies on the assumption that Leslies new pool test (a version of the Lamotte WaterLink Spin Touch which is independently NSF-ANSI 50 Certified test) was calibrated and more accurate than the test strips, which makes sense given the circumstances. Please note, since these tests generally report TC rather than CC, I will be using the same conventions. I am wondering if there is a reason why these results are taking place related to the chemistry of my pool or the chemical nature of the tests?

Background: My FC was maintaining at around 3 ppm for a 20K gallon, fiberglass, new saltwater pool. Despite the anomaly of higher TC, suggesting higher CC, the pool water was clear.

Method/Experiment: Gather sample A & B, on two consecutive days. Test samples with 2 separate testing strips, then test samples at Leslies pool supply store.

Results & Conclusion: Suggest false negative result from test strips. They provide identical results with a TC that is nearly 4x of FC, compared to leslies which shows FC and TC are about the same on both days. Perhaps it is OK to conclude that the results of TC also accurately represent FC under similar conditions given the findings of test strips in comparison to leslies. However, understanding why this result is taking place will let me know what I should plan in the future for routine testing. I would like to have a home kit available to me rather than going to the store each time.


I was thinking it is possible since it is a new pool and some of the chemicals added (or their inactive ingredients) might cross react with test strips for FC? At different points in the past week, I've added sodium hypochlorite, sodium bicarbonate, calcium chloride, cyanuric acid, Sodium Bisulfate as well as hundreds of pounds of sodium chloride.

The remainder of the Leslie's analysis provides more data on the remainder of pool chemistry on these days. Perhaps some of the other chemistry results might have influenced the readings?



Regarding the accuracy of these tests, I contacted Leslies to find the manufacture is Lamaotte. A Lamotte technical support agent confirmed that

a) Leslies is using a branded version of the Waterlink Spin Touch which use Spectrometry for analysis
b) The devices go through independent certification for validation
c) the rep also stated he believes Leslies has only been using the device for a few months (meaning calbiration errors are probably less likely)

NSF Certification


In advance thank you for reading & responding.

 

[mknauss]

You need a proper test kit. I suggest the TF100 or the Taylor K2006C. A proper test kit is needed to get the accurate water chemistry results needed to follow the TFP protocols.

Test strips and pool store tests are not used for guidance on this forum.

 

[Katodude]

You are trying to compare the results of an ouija board with the information provided by a psychic.

Neither has any actionable information.

 

[mikeyliny]

You need a proper test kit. I suggest the TF100 or the Taylor K2006C. A proper test kit is needed to get the accurate water chemistry results needed to follow the TFP protocols.

Test strips and pool store tests are not used for guidance on this forum.

Appreciate the reply. I will look into the test. I do still believe my tests are providing false negatives. Not certain why.

With respect to innacuracy of pool store testing, I see many individuals providing anecdotal data about Leslie's in particular tests being inaccurate. I'm not going to dispute these experiences over the years. However, right now, as I called leslies pool to confirm they are using a Lamotte testing kit. In recent posts, several users shared experiences of this test having reliability. I'm assuming the manufactures are responsible for validating the test. If they used to lack reliability in the past, it does not seem to be the case any more. That is scientifically, the tests have repeatable results from store to store.

Does someone have independent lab testing or validity results available that can settle just how accurate each of the tests are? What is the gold standard? I can confirm that in bringing 2 days worth of samples to Leslies my results were very similar and changed for chemical items that were adjusted. But my anecdotes are inconclusive.

 

[cledee]

Appreciate the reply. I will look into the test. I do still believe my tests are providing false negatives. Not certain why.

With respect to innacuracy of pool store testing, I see many individuals providing anecdotal data about Leslie's in particular tests being inaccurate. I'm not going to dispute these experiences over the years. However, right now, as I called leslies pool to confirm they are using a Lamotte testing kit. In recent posts, several users shared experiences of this test having reliability. I'm assuming the manufactures are responsible for validating the test. If they used to lack reliability in the past, it does not seem to be the case any more. That is scientifically, the tests have repeatable results from store to store.

Does someone have independent lab testing or validity results available that can settle just how accurate each of the tests are? What is the gold standard? I can confirm that in bringing 2 days worth of samples to Leslies my results were very similar and changed for chemical items that were adjusted. But my anecdotes are inconclusive.

It comes down to who is doing the test, how accurate are they when entering the data, has the machine been calibrated recently, are the reagents still good, when you got your sample did you get it from the deep end away from skimmer and return lines. All of these points matter. How could a leslies test be wrong despite being validated by the manufacturer? for the exact same reasons I just gave you. There's humans involved doing the testing.
There's also pressure to make a sale.

 

[mikeyliny]

It comes down to who is doing the test, how accurate are they when entering the data, has the machine been calibrated recently, are the reagents still good, when you got your sample did you get it from the deep end away from skimmer and return lines. All of these points matter. How could a leslies test be wrong despite being validated by the manufacturer? for the exact same reasons I just gave you. There's humans involved doing the testing.
There's also pressure to make a sale.

Agreed which goes to my original point. Why is my TC =4XCC in a pool that is otherwise very clean. Should I conclude that Leslies result was inaccurate or try to understand a reason for a False Negative. Of note, all of the other parameters on the test strips with the exception of FC were similar to Leslies. I am going to modify my original question to discuss the Lesies test validation, as I just found out some info about that suggesting the current testing is more accurate than it was in past.

Regarding the accuracy of these tests, I contacted Leslies to find the manufacture is Lamaotte. A Lamotte technical support agent confirmed that

a) Leslies is using a branded version of the Waterlink Spin Touch which use Spectrometry for analysis
b) The devices go through independent certification for validation
c) the rep also stated he believes Leslies has only been using the device for a few months (meaning calbiration errors are probably less likely)

 

[cledee]

Agreed which goes to my original point. Why is my TC =4XCC in a pool that is otherwise very clean. Should I conclude that Leslies result was inaccurate or try to understand a reason for a False Negative. Of note, all of the other parameters on the test strips with the exception of FC were similar to Leslies. I am going to modify my original question to discuss the Lesies test validation, as I just found out some info about that suggesting the current testing is more accurate than it was in past.

Both test are probably off,
Test strips are guess strips. They give you a range, most of the time not an accurate one. A test strip is an indicator if you have chlorine but not how much FC or TC. Unless you test it with a good test kit it would be impossible to know if any of those numbers you got from Leslies or the test strip are accurate.

 

[Newdude]

With respect to innacuracy of pool store testing, I see many individuals providing anecdotal data about Leslie's in particular tests being inaccurate

Leslies just happens to be the largest chain with 930 locations so many of us have one or two nearby. They naturally take the brunt of the bad experiences. Time and time again, yes, many even recently, have proved them to be off from the Taylor drop based kits.

One recent poster went to 4 stores and got 4 totally different sets of read outs.

Can your store be accurate today ? Of course it can. Will it be repeatable week over week ? Probably not when the system is designed to be a tool for the management to sell chemicals.

 

[mikeyliny]

I don't really care enough about Leslies to argue to support them. In my case, I would say the same test was reliable in repeat tests and more consistent with what I believe to be the chemistry. The arguments provided about the lack of reliability of the Lamotte tests arent convincing. The fact is they are validated. I can't say they are calibrated properly in every state. But they are certified tests, using the same device relied upon by health departments that test drinking water. If you think that the dropper tests are more accurate... fine. But how are the tests carried by taylor validated or certified? Are there any objective analysis that proves one way or another the most accurate test?

 

[cledee]

I don't really care enough about Leslies to argue to support them. In my case, I would say the same test was reliable in repeat tests and more consistent with what I believe to be the chemistry. The arguments provided about the lack of reliability of the Lamotte tests arent convincing. The fact is they are validated. I can't say they are calibrated properly in every state. But they are certified tests, using the same device relied upon by health departments that test drinking water. If you think that the dropper tests are more accurate... fine. But how are the tests carried by taylor validated or certified? Are there any objective analysis that proves one way or another the most accurate test?

Dropper test are more accurate.
The Lamotte can possibly be accurate or close, if you are the one doing the testing. (There's just too many variables to take into consideration to trust it)

 

[mknauss]

The Lamotte can possibly be accurate or close,

For ranges it is designed for, excluding CH. CH is a known false reading with a ColorQ.

 

[Newdude]

But how are the tests carried by taylor validated or certified? Are there any objective analysis that proves one way or another the most accurate test?

I don’t have that information. I’m sure the manufacturer has it, but I myself take anything any manufacturer says with half a jar of salt.

However. What I can tell you is I have personally used somebody else’s Taylor kit, sometimes across the country and sometimes across the world, to guide them to invisible water. Never in my wildest dreams would I have thunk I’d be able to do that. And I am only one of oodles of oodles of folks here doing the same thing everyday.

 

[jimim]

Gold standard is using the kit we all use on here with us doing our own tests. Seriously. It's very simple and will avoid a lot of thinking and typing. Just buy the kit, do it yourself, and perfect water. 4th season myself doing all my testing and chemicals. Never came close to algae.

jim

 

[cowboycasey]

@mikeyliny You are trying to get us to accept pool store results.. we will not.. we have been doing this to long and we know they can not be trusted... it will be different from store to store, state to state and you name it.. to many variables that we just do not trust.. Just like we do not trust guess strips, they are worse than pools stores...

I know for a fact I use my test kit and it tests the same exact way day in and day out.. I can tell someone on the other side of the world using the same exact test kit and it comes out the same day in and day out... the pool stores can not do that...

If you want to use the pool store you have every right to do it... You could even follow TFP and have them test your sample every day.. I think after a week or 2 they would stop testing your water every day but they may not...

What happens when they try to sell you something you do not need, how will you know if you do not need it if you are not testing yourself??

We teach people everyday to take care of there pool and how to do it.. but it requires a great test kit and there are 2 to choose from.. We will gladly teach you how to do it once you get one..

Test Kits Compared

 

[mikeyliny]

Thank you to all. I purchased the Taylor K2006C. My results, several days after the Leslies test were very similar to the Day 2 analysis from Leslie's pool Lamotte test (copied below). Upon further review, Taylor's test is also certified by the same agency. While it may be moot, does anyone know why the false negative test results from the strips?

Taylor K2006C
FC=8
TC=8.5 (CC=0.5)
PH=7.9
ALK=90
Hardness/Calcium=90
CYA=50

 

[Orion7319]

I own the following test kits. Lamotte color Q, Taylor K2006c, TF-100. The lamotte color Q only seems to get ph readings correct on a consistent basis. The color q never even comes close on its CH and CYA tests. Chlorine and TA tests can be iffy at times. If your color blind like my son and have a hard time checking PH from the Taylor kits, the color q would work well for that. It’s better for checking ph then a cheap ph meter (which I also own) that needs to be calibrated every few days. It would be very expensive to buy a color Q just to use for a PH tester. In town there is a family owned pool store. For some reason they do not advertise their existence at all. I believe they started the store to support their pool building business. The only employees at the store are two sisters in law. They have a Lamont spin touch. It always jibes with the readings from my TF-100/k2006c. I do believe the spin touch’s when calibrated regularly and used correctly are very accurate. They are also close to $1000.00 and each disk for the tests is $1.50, so they don’t lend themselves for home use. Most pool stores probably don’t calibrate them very frequently and I wouldn’t trust their results, and I would typically expect them to be hit or miss simply for that reason.

 

[Bperry]

Thank you to all. I purchased the Taylor K2006C. My results, several days after the Leslies test were very similar to the Day 2 analysis from Leslie's pool Lamotte test (copied below). Upon further review, Taylor's test is also certified by the same agency. While it may be moot, does anyone know why the false negative test results from the strips?

Taylor K2006C
FC=8
TC=8.5 (CC=0.5)
PH=7.9
ALK=90
Hardness/Calcium=90
CYA=50

View attachment 361193

What did you do to make the CYA vary so much? That’s a huge difference.