As an experienced professional who has tested and evaluated water in thousands of swimming pools, I can say without a doubt that DPD tablets are far more accurate than OTO drops for testing chlorine levels.
Both DPD and OTO are colormetric tests – which means that you compare the color against a color comparator block. This can be a source of errors, as some people have difficulty determining subtle variations of color intensity. However, if you have a good eye for color variations, this should not be a problem.
For those who are unfamiliar with these acronyms, I’ll give the definitions along with some basic information. DPD stands for diethyl-p-phenylene diamine. DPD is a little more time consuming and complex, but when properly done you’ll find this system of tests is the most accurate. OTO stands for orthotolidine. My biggest problem with OTO is that the reagents in the liquid have a shelf life – although no expiration date is ever printed on the bottle. As a result, you will not know if the reagent has gone bad – and that causes problems by giving you inaccurate results. On the other hand, you can usually tell if DPD tablets have gone bad because the color of the tablets will change. DPD tablets are normally yellow – so if you open the package and see that the tablets have lost their yellow coloring, it’s probably time to buy some new ones. My personal recommendation for brands of DPD test kits is Jack’s Magic – which offers kits for around $100.
There are other methods of chlorine testing on the market that are often heavily advertised. My advice is to stay away from these. One of the most common is an electronic meter with an ORP electrode. This is not only very expensive, it is also extremely unreliable. I advise you to go with DPD tablets for accurate chlorine testing that has been proven to be consistently reliable.