Weird test results for Chlorine

[howbout]

I used to have a simple test kit for chlorine and pH. The chlorine test was just a matter of adding 5 drops of a liquid to the water and it would turn a yellow color you would match to a color chart to check levels. With the new kit, you add drops to test for free chlorine then more to test for total chlorine. You match it to a color and then subtract FC from TC to get CC. Problem is, the colors have been showing as EXACTLY THE SAME every time I test which would put the CC at 0. I know it is NOT. We have had a lot of rain and yesterday I added chlorine to offset that. I also have at a minimum 3 pucks in the pool. Can anyway explain why this is happening and what it might mean? I don't want to do anything and mess the chemicals up (and we had it tested at a pool store and everything was perfect, which makes me think something is wrong either with what I am doing, or the kit itself...most likely me!). Oh - the test water IS turning a pink color...with both tests...could it be that the water color differential is so small I just can't tell (in other words, one IS pinker than the other but I can't tell with the test colors that I match it with).

Thanks for any advice!

[waterbear]

DPD testing is the only way to test for FC and TC. If the CC is small then the color differential between the FC and TC will not be apparent. It is hard to detect less than 1 ppm CC with this test. If you want to be able to detect CC at concentrations as low as .2ppm then you need a different test, FAS-DPD titration test.
The OTO test you mentioned that turns yellow only tests for TC.

[howbout]

[QUOTE=waterbear]DPD testing is the only way to test for FC and TC. /[QUOTE]

What is DPD testing? Was my old simple test sufficient for testing?

Actually, my test kit says "Complete Poolcare DPD Test Kit"....

[AmigaDude]

Howbout -

It took me a minute to understand what you were getting at, and I think that you are doing fine. Here is my novice take on it, and I'm sure the experts will slap me around for any inconsistencies..

a) FC = Free Chlorine *Good Stuff doing sanitation duty*
CC = Combined Chlorine *Not so Good Stuff locked up and not working*
TC = Total Chlorine *Total of Good Stuff (FC) and Not so Good Stuff (CC)*

b) Best case, as almost all of my readings have been, is that your FC = 2+, and CC = 0 ! That means the TC=FC so the colors will be the same.

c) My best advice (again novice) is to use the FC number to determine if you have the chlorine count where you want it. Only worry about the CC number if you have some reason to check, AND it is anything but 0 then you probably need to shock the pool.

At least that's my view of the world...

[Bleach=Chlorine?]

Rain water is not necessarily going to consume Cl so your results may be correct. Have a pool store nearby? Take them a test and compare results.

[waterbear]

[quote=howbout][quote=waterbear]DPD testing is the only way to test for FC and TC. /

What is DPD testing? Was my old simple test sufficient for testing?

Actually, my test kit says "Complete Poolcare DPD Test Kit"....

DPD is a reagent that turns pink when chlorine is present (usually 2 different reagents are added and you compare the color to a color comparator block). Then you add another reagent to test for TC and if the test gets darker then there is TC. It reaally won't show less than 1 ppm increase (1 ppm CC). There is another kind of test callled FAS-DPD which is a titration test. DPD powder is added to the sample and then the reagent is added by drops until the color turns from pink to colorless. The number of drops deteremines the amount of FC. You then add another reagent and if the color turns back to pink you have CC so you titrate again until colorless to get that reading. It can detect CC as low as .2 ppm and will work at FC levels up to about 500 ppm. A DPD test can bleach out at FC levels above 10 ppm leading you to believe there is no chlorine in the pool when, in fact, it is high. The third test is OTO which turns yellow and is what is found in most of the cheapie test kits. OTO only tests for TC but it will not bleach out so it is good for quick checks and to see if your chlorine levels are very high. OTO is always reliable.

[howbout]

I'm sorry all, but NOW I am really confused! So....if I do the first test with the first set of drops and the color shows about "3" on the color match (it IS pink) then am I o.k. and I can skip the second part (adding the other 5 drops and then doing the math?). Sorry, but this thread totally confused me....

[VOLDADDY]

Order Ben's kit from this site and your confusion will disappear.

[howbout]

Order Ben's kit from this site and your confusion will disappear.

I'm actually hoping for an answer I can use TODAY. Ordering his kit won't help me TODAY. Thanks anyway.

[waterbear]

I'm sorry all, but NOW I am really confused! So....if I do the first test with the first set of drops and the color shows about "3" on the color match (it IS pink) then am I o.k. and I can skip the second part (adding the other 5 drops and then doing the math?). Sorry, but this thread totally confused me....

in a nutshell, no. The first part of the test will tell you your FC level. This is the 'good' chlorine that you want in your pool. the second part then measure the total chlorine, which is your FC and any CC (the 'bad' chlorine) in the water. If you have CC this test will darken. Usually you cannot detect CC under about 1 ppm with this test so if it does darken , even a bit, you should shock your pool to get rid of the combined chlorine. If the test darkens enough to be able to read the level on the comparator you can get a reading for your CC. For example, you add the first two reagents and get a FC of 3 ppm, then you add the last reagent and the color deepens to 4 ppm TC. You then subtract the FC from the TC to get your CC....TC-FC=CC which in this example would be 4-3= 1 ppm CC.

Also, the results for TC with a DPD test might not match up with the number for an OTO test for TC (turns yellow). This is normal. OTO is a very rugged test and will work with very high clorine levels but it is hard to get the precision at lower levels because the color changes are subtile and more difficult to read. Think of OTO as a good 'quick and dirty" test for your chlorine for a daily check just to make sure nothing is wrong. An OTO test should be allowed to develop for a minute or so before you read it. DPD tests for FC needs to be read immediately and the TC needs about 30 seconds to develop fully.