How can TC be less that FC

[adl6009]

ALK=140
PH=7.5
CYA=40
FC=3
and now for the part I don't understand
TC=2

It's been doing this for last few days. I opened the pool last week of June and the TC has been = to FC until now. It's my first time using bleach so I am not familiar with all the variables that may occur which is way I am asking.

[CToon]

Might you be using strips to test ?

The sticky post in the Testing section of the forum says if that happens the strip is bad...

[CarlD]

It can't. If you are using Ben's kit, there is the FAS-DPD test for FC and CC--TC = FC + CC always and forever.

Ben's kit also has an OTO test for TC. This may come out lower. DISREGARD IT! The FAS-DPD is the most reliable test.

One of my objections to test strips is they ALWAYS seem to show TC lower than FC. So one of the pads MUST be wrong--Experience tells me the TC is, so I NEVER look at it.

[adl6009]

I use a taylor dpd kit. It is about 2 years old maybe i need new agents?? What does dpd oto fas mean? Is dpd the "best" kind to get? Thanks.

[CarlD]

somebody spelled out what FAS-DPD stands for.

OTO is the simplest, is fairly reliable, but only measure TC and only goes to 5ppm (some kits stop at 3ppm). You can dilute it with distilled water to extend the range but you'll lose precision. Sometimes it's all you can do.

DPD usually uses a tablet. It CAN go up to 15, but there it bleaches out and looks like 0. It can measure FC and CC. I'm not a fan of DPD because it's as much a pain as FAS-DPD (actually, even more of one) but rarely do the kits go beyond 5ppm in what they can measure.

FAS-DPD is the most powerful, the most accurate and the most reliable. It takes a little longer than OTO, but can measure both FC and CC levels up to 50ppm! Is it the "BEST"? Well, by most standards, yes, it's the best.

[waterbear]

Ok, I'll post it in this thread also

The DPD test uses 3 reagents. The first is a buffer solution, the second is DPD (N,N-diethyl-p-phenylendiamine) which turns pink in the presence of free chlorine. The color is compared on a comparator block to get the FC reading and then the third reagent, potassium iodide is added to get the TC reading once again using the comparator block. The FC reading is then subtracted from the TC reading to get the CC. The biggest drawback to the DPD test, besides the inaccuracy of the human eye and color comparator blocks, is that DPD will bleach out at chorine levels above about 10 ppm which could lead you to believe that there is NO chlorine when, in fact, the chlorine levels are very high. The sample can be diluted to overcome this but you will lose precision in the reading.

The FAS-DPD test (it stands for Ferrous Ammonium Sulfate- DPD) is performed by adding a buffered DPD powder to the water sample (10 ml). It will turn pink if chlorine is present. The sample is then titrated with the Ferrous Ammonium Sulfate solution until the pink color disappears and the sample turns colorless. The number of drops is counted and this is multiplied by 2 to get the FC readings (each drop= .5 ppm FC). Potassium iodide is then added and if any CC is present the solution will turn pink again. The sample is then titrated again and the drops counted...once again each drop= .5 ppm CC. If you want a TC reading you add together the FC and CC. This test will not beach out like the DPD test. If the sample bleaches out when the DPD powder is added you simply add more until the pink color stays. This way it is possible to test well over 20 ppm (possibly up to 50 ppm) FC. Also if the water sample size is increased to 25 ml the test becomes accurate to .2 ppm per drop instead of .5 ppm per drop.

Hope this explains it without confusing you too much.
Check out this page on the Taylor Technologies website for more info
http://www.taylortechnologies.com/Ch...P?ContentID=11

OTO (orthotolidine) is a test for TC only. It will turn yellow in the presence of chlorine. It is not the most accurate test in the world but it is 'bullet proof'. It won't bleach out and can give you a rough idea of high chlorine levels when you are familair with it. It's great for a quick daily test to make sure there is chlorine in your pool. The yellow colors of the OTO test are a bit easier to differentiate than the red colors of the DPD test but you cannot test FC with OTO (and it is not permitted for testing chlorine in commercial pools in most, if not all, states)