DPD vs OTO

What is the difference between DPD and OTO? If it's too in-depth to explain, then which one should I use for SWG? Just trying to make sure I use the right thing.

Thanks in advance,
Brad

Thanks to the folks in here, my water looks better than it has since we built the pool. Who'da thunk it - bleach in a pool.

OTO only tests Total Chlorine but can read chlorine at high levels (color is orange to brown when chlorine levels are very high)...problem is that most OTO kits only go up to about 5 ppm on the color comparator. It is good for quick daily checks on your pool or to make sure that you have chlorine if a DPD test bleaches out. OTO turns yellow if chlorine is present. Some OTO kits tell you it will read Free Chlorine if you do the reading within 15 seconds but I would not trust these results.

DPD can read both Free Chlorine and Total Chlorine. It's biggest drawback is that it bleaches out (turns colorless) at chlorine levels above about 10 ppm or so. This might lead you to believe that there is no chlorine in the pool when actually the level is high. DPD turns pink if chlorine is present.

Both OTO and DPD are colormetric tests. This means you compare the color against a color comparator block. This can be a source of reading error as many people have difficulty determining subtle variations of color intensity. (Men, for some reason, have more trouble with the pink colors of DPD than women do)

FAS-DPD is a titration test for both Free and Total Chlorine and is the most accurate but the most time consuming. You start with a pink liquid if chlorine is present and add drops of a titrant until it turns colorless. It is easy to read since the color change is distinct. It can test much higher levels of chlorine accurately and can detect Combined Chlorine to .2 ppm. It is the preferred method of testing if you want accurate results and is the test in Ben's kit (along with an OTO test for quick chlorine checks).

Any of these tests can be used with a SWG. I use OTO for quick checks every few days on my chlorine levels to make sure my SWG is operating and the chlorine is about where it should be and use FAS-DPD or DPD weekly to monitor my FC and CC. (I have several test kits)

If you are going to have only one kit I would recommend an FAS-DPD test. If you purchase Ben's PS234s (which I have and HIGHLY recommend...best value for the money that I have found!) then you will have both FAS-DPD and OTO along with every other test you will need for normal maintenance including salt!

Hope this helps.