1. Chlorine test accuracy

Daughter is doing a science project. Her project is on sanitzing pool/hot tub water.
On the testing for chlorine levels, found that:
For OTO:
Chlorine test is 0 to 5 PPM (Parts Per Million)
Precision is @ .5 PPM + or - 1 and @ 2 PPM + or – 2
For DPD:
Chlorine test is 0 to 5 PPM.
Precision is @ .5 PPM + or - .5 and @ 2 PPM + or – 1 and @ 3 to 5 PPM + or – 1.5
Cannot find the precision figures for FAS-DPD.
The web site that I got the other numbers from I lost. I know that test is 0 to 20 PPM but cannot find the rest of the numbers.
Anybody know or know where I can find them?
Thanks
Mike

2. Re: Chlorine test accuracy

I consider those OTO and DPD accuracies a little optimistic. They are assuming you have excellent color vision and are experienced at reading the colors. For most purposes, the OTO test is useless above 3 ppm, as very few people can distinguish the colors between 3.5 and 8 or so. The DPD test is better but gets strange right around 5 ppm, as anything from about 4 to 10 (or 12) will read as if it is 5 and then above that it suddenly starts reading as zero.

The upper bound on the FAS-DPD test is well above 20, there isn't a clear limit but we generally call it good up to 50.

The FAS-DPD test is +-1 drop and an additional +-1 drop for every 10 drops used during the titration. Drops are normally done as 0.5 ppm, so it is +-0.5 from 0 to 9.5, +-1.0 from 10.0 to 19.5, +-1.5 from 20.0 to 29.5, and so on. There is also a way to do the FAS-DPD test so that drops are 0.2 ppm, which requires adjusting those numbers appropriately.

3. Re: Chlorine test accuracy

Originally Posted by JasonLion
...There is also a way to do the FAS-DPD test so that drops are 0.2 ppm, which requires adjusting those numbers appropriately.
Yes, instead of using a 10 ml sample size, you would need to use a 25 ml sample size. Then, each drop would be 0.2 ppm.

4. Re: Chlorine test accuracy

Thanks!
She got her report/project done last night. Bringing it to school this morning. I think that she has read the entire Pool School and about 1/2 of the the posts in Testing and Balancing Your Water.
I now have a companion in my pool an spa care!
Mike

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