My OTO test kit like all others has shades of yellow that correspond to CL levels from 0.5 to 10 ppm. I have found that at higher levels the result becomes a darker yellow, then orange and even to a brown. Since the OTO indicator is able to produce distinct colors corresponding to much higher CL levels, it would seem that it would be possible to simply supply a color match for these higher levels. Although I'm sure the accuracy might suffer at these levels, OTO testing is currently only a rough indicator of true CL level anyway.

So the question is why no one includes matching colors for these higher CL levels? It would seem that it could be of some value for quick testing when you need to maintain higher CL levels.

Ben Powell at The PoolForum has talked about the OTO colors at high chlorine level (see here, for example) and even thought about creating a color chart, but that never happened.

Trying to ACCURATELY determine FC levels with OTO is near impossible. When you have very high FC levels in your pool (like when you are shocking), it is usually important to know the FC level precisely so you can control the FC and not overdose.

Use the FAS/DPD test and the problem is solved. Trying to get the OTO test to function as a replacement for the FAS/DPD re-introduces "guessing" back into your pool management........BBB teaches that "guessing" is almost never a good solution.

I should also note that the PoolSolutions page I linked to is old and probably written before FAS-DPD became widely available. As Dave notes, FAS-DPD can measure high chlorine levels without bleaching out the way DPD does and it is generally accurate (to within 1 drop or 10% whichever is greater). I don't think the OP was suggesting to replace FAS-DPD with OTO since he said "OTO testing is currently only a rough indicator of true CL level anyway". I think he was just asking about a color chart as a rough guide, perhaps at high chlorine levels only with +/-30% accuracy or something like that. From what I can gather from historical posts, dark yellow is 5-10, light orange was 10-20 ppm, dark orange is around 25, brown is 35+. Even with a color chart it's doubtful one could estimate very accurately.

The current color charts for OTO test are not very accurate, since the ranges are listed as:
0.5-1
1-2
2-4
3-6
5-10
So, several CL levels fall into two different color bands. For example, even though 3 ppm falls right in the middle of the third color it is also at the bottom of the fourth color range.

It looks like a logarithmic scale and that the next color bands should be something like:
8-16 dark yellow
15-30 orange
25-50 brown

If a finer division than that were possible, (and I think it is) this could be a useful quick indicator for those who are for whatever reason running higher levels of chlorine.

Once it gets past the red brown stage it turns clear with falling red chunks

I hope I won't need to measure my chlorine level in the "red chunk" range.

Aren't the ranges on the OTO test actually the different values between chlorine and bromine?

i.e. level 2-4 actually is 2ppm TC OR 4ppm bromine.

frogabog,

I think you finally figured out what chiefwej was talking about. I couldn't understand why he/she was referring to a 5-10 range but what you say makes perfect sense.

After all that, I am reluctant still to post to this thread because trying to test FC accurately with an OTO test is a non-starter for me.

frogabog is correct. The left hand column of numbers corresponds to what the color reference indicates for TC and the right hand column is for bromine.

The main problem is that no one can reliably distinguish the difference between anything from about 3 to perhaps 10, even when they have a better color standard. A fair number of people can tell the difference between 3-5 and 8-10 most of the time, but even that is not completely reliably. So overall, OTO is fairly good from 0-2 and it will tell you the difference between 5 and 15 and 25 well enough (if you have a better color standard to compare to or enough experience to know what it should look like), but it dramatically lacks in precision in the 3-8 range where many people need a fair bit of precision to get their daily dosage figured out.

If your CYA level happens to be around 20-35 you can get by with OTO only most of the time. But that doesn't cover very many people.

Some day you can tell us how you know this...

I was just curious, I noticed how the test kept on getting more golden and darker. So took a normal sample in the vial and added a few granules of cal hypo shook it up until it dissolved and did the 5 drops and those are the results

So sad.. yes I know