Total Chlorine less than Free Chlorine ???

[TimboZero]

Hello All,

Yesterday I replaced my SWG and whilst I was at the shop I had my water tested. They used a automated precision pipette to measure water into individual test 'cells' that were pre-dosed with the testing chemical and sealed (obviously this seal was broken to add my pool water). Each 'cell' was then measured in a photometer. I would expect this to be as accurate as testing could be outside of a lab.

the results were :-

pH 7.4
FC 2.89
TC 2.08
TA 90
CH 170
CYA 40
Salt 4000

This is for a SWG chlorinated pool and I would consider that the CH and CYA should be raised a little

The thing that surprises me (and I dont understand) is how the TC is less than the FC? Surely TC=FC+CC and would always be slightly higher than FC ?
Is this an indication of another problem, a testing error or perfectly normal?

Thanks

Tim

 

[frogabog]

Not normal. A prime example of the error rate of such precise testing methods. Can't happen that way, the tester should have considered the whole set invalid when that occurs. The printout actually says this?

edit: Not invalidating the test so much as the results. The method can be precise, the test can be valid, but when you get such a "blaring in the face invalid result" it throws everything off. That's why scientific testing has rules that prevent invalid results as countable.

A CYA of 40 is the kind of number you get when doing the test yourself. I would have expected something like 39 or 41 before a solid 40. Just to prove it's accuracy. You do want to bring the CYA up to 60-80ppm for the SWG, if you trust this result. Try 10ppm at a time and see how the SWG reacts. Get your own test kit, then you'll actually know how your pool behaves if you change xx by xx quantity.

Out of curiosity, was the FC and TC tested with two different samples in their own tubes, or did they test for FC and TC from the same pippetted (is that a word?) sample?

 

[TimboZero]

Thanks for the reply frogabog,

The results mostly match the tests I have at home (pH meter , Aquachek Yellow strips and Aquachek TruTest digital) in as far I they will test (only get FC and no CH with them).

The shop tests were all individual tests samples and the only thing that concerned me was the TC. Maybe she swapped the TC and FC tests but, they were clearly labeled sample tubes and my FC tests all show the 2.8-3.1 sort of result (not the 2.0 result that I would expect if she had swapped TC and FC).Maybe the TC test wasnt given enough time to give a 'true' result.

I have just added 1kg of StabiSel (granular cyanuric acid) which for my 42000liter pool i would expect to raise the CYA from 40 to 70. Hopefully that will be about right but will mean I need to use calhypo to shock instead of the stabilised stuff the shop says to use.

 

[frogabog]

Or bleach. It's cheap and it works fine adding nothing but a trace amount of salt.

Works great for many, most... nearly all of us.

 

[Richard320]

That's impossible.

Let's go back to grade school, Venn Diagrams, sets and subsets, shall we?

Is it possible for the number of boy students in a classroom to be greater than the total number of students in the same classroom? No. At most, the number can be the same, meaning, no girls. And so it is with chlorine.

Free Chlorine + Combined Chlorine = Total Chlorine. The only way that test result is valid is if your Combined Chlorine is -.81. Impossible. Combined Chlorine can be no less than zero.

Those testers look impressive, and probably sell a lot of chemicals, but they need calibration, and they can just plain be wrong, as you prove.

 

[duraleigh]

I would expect this to be as accurate as testing could be outside of a lab.

A perfectly normal expectation but, the more you read on the forum, the more you realize it's not true.

Pool store testing is designed to LOOK precise and accurate. Unfortunately, operator error and some other factors make it quite a bit less reliable than doing YOUR OWN testing with drops based (seems sorta' old fashioned, doesn't it?) technology.

It's really one of the eye-openers when you first find this forum. Naturally, one would assume the pool store people are the experts. However, with just a little bit of knowledge and the test kit to apply it, it turns out YOU are the single best expert on your own pool.

 

[TimboZero]

Totally agree with what has been said

The mere idea of TC being less than FC concerned me but, with my lack of experience I had to check that such a result wasn't a sign of something other than a testing error.

I also agree with the norm being a lack of user ability in some stores and so was pleased to see a system that removed as much user error as possible. Shame that something managed to creep into the 'fool proof' system :lol:

I am currently accepting all the results except the TC one as they match my own results.

Many thanks for the replies and help.

 

[loop_pea]

Doesn't the way TC is tested using DPD3 reagent in a photometer normally require a bit of time for the reaction to complete before the reading is taken? My test kit instructions say 2 minutes. If they didn't wait before taking the reading on the TC, then it could throw the result off.

 

[frogabog]

The TC test in my Hanna meter also instructs to wait after mixing before running the test itself (pressing "read" button). 2 minutes 30 seconds. It also requires a clear sample to calibrate (zero) before every test no matter what it's testing for. Every sample is calibrated, then reagents/powders added, and finally the test is performed. FC can be tested immediately, as can PH, iron and hardness.

FWIW... all my Hanna tests are consistent with my drop tests. As long as FC is below 2.5ppm and TC below 3.5ppm, of course. I do wish it was not limited so. Occasionally it's fun to get it out, spend two hours locating scissors, wipe off the glass test vials in case the eye gets confused, try not to tighten the cap too far and break it, attempt to pour all the powder out of the tiny foil packet w/o dropping any, run a calibration test for every sample, pause to prevent a kid from killing the other kid... find my way back to the meter... and run a nice precise test.

Hmmm... Seriously, drops are so much simpler.

 

[TimboZero]

I am convinced that the TC test was done too fast , the other tests concurring with my own.

If I could get one , I would by a TF100 kit but they wont ship to the EU. Then I would have a full set of tests of my own.

Discarding the TC result in the first post , would you all be happy with the other results (I have already added 1kg of CYA to raise it a little) or would the rather low Langelier Index concern you ?

 

[loop_pea]

frogabog, you said "so long as FC is below 2.5ppm and TC below 3.5ppm" the tests are consistent. What sort of errors do you get above these values?

 

[frogabog]

Too high for the meter. It just doesn't test higher than that. It was purchased by my husband for his work with the intent to test potable water after sanitizing with mixed oxidants (like SWG). It didn't have to be the model that would test any higher than that. Most drinking water has less than 1ppm chlorine at the tap.

The meter just happens to live at my house, H doesn't represent the manufacturer (Miox) anymore. Tis the way of mfr's representatives...

 

[TimboZero]

frogabog,

If you wanted to double the range of your tester you could :-

1. Take water sample and mix it with distilled water 50/50 (a plastic syringe from your local pharmacy would do this accurately)
2. Perform the test
3. Double the result

You could also triple or quadruple with the same method but , the intrinsic error of the way we test our water would also multiple as you diluted. Better than nothing though

 

[zea3]

I also agree with the norm being a lack of user ability in some stores and so was pleased to see a system that removed as much user error as possible. Shame that something managed to creep into the 'fool proof' system :lol:

Unfortunately, better "fools" are created all the time!