[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.