Test discrepancy - OTO vs FASDPD

[bdrums]

Tested Friday evening and the FAS/DPD test showed FC at 1.5 and that I needed to add 68 oz of 8.25% bleach to get to my target of 4.0. I only had 48 oz so I added that and intended to get to the store Saturday morning.

Tested Saturday morning . Usually I skip the OTO test because I have a bit of difficulty distinguishing the deeper shades of yellow so I just do the FAS/DPD test. This time however I decided to give the OTO test a go as well.

The OTO test showed a light shade of yellow. Best my eyes could tell, not more than 2.0 TC.

The FAS/DPD test showed FC of 4.0 and CC of 0.

Hmmm... that doesn't mesh. I had used 2 scoops for the FAS/DPD test and it turned a deep red. So I decided to re run the test and used just 1 heaping scoop. Same result.

I wasn't expecting a FC result of 4 since I hadn't added enough bleach according to the calculator.

I bought the test kit last August. It stays in the 2nd bedroom closet. I'm low of the powder as I test about 3 times per week so I definitely need more.

Thoughts? Thanks!

 

[ewkearns]

I would trust the FAS/DPD test. If the FAS/DPD powder turns deep red, it means you have "more" chlorine rather than less. You might want to double check the size of your pool in the calculator.

 

[bdrums]

Thanks. I tend to trust the FAS/DPD test as well. Was just curious why the OTO test showed so different.
The volume I use in the calculator is what the builder provided from the CAD software. My guess is that it's probably within a few % of actual. Isn't that normally good enough?

 

[ewkearns]

My guess is that it's probably within a few % of actual. Isn't that normally good enough?

Yes, it is! However, I usually get myself in trouble trying to answer another poster's question and forget to change back from "their pool" to "my pool."

 

[Patrick_B]

OTO is just a color comparison, and doesn't specify TC or FC. If you want accurate specifics, just use the FAS DPD FC test.

 

[duraleigh]

As always, use the OTO to test for the presence of chlorine and maybe a rough idea of how much is there but you are mainly testing that there is chlorine in the pool.

When you need to know exactly how much FC is in the pool, you use the FAS/DPD.

OTO for presence - FAS/DPD for precision.

 

[Divin Dave]

^^^ yup. What they said.