OTO Accuracy and Difference from FAS-DPD

[tlhwxman]

Hi Everyone,

Got a few more questions that you folks will likely be able to shed light on. I have to the pool in a good balance. I have been testing TC with OTO daily during the week and then once a week I do a FAS-DPD for FC and CC. On the days I do FAS-DPD, I also do a OTO to compare. This is where the mystery starts. I notice that OTO is consistently lower than the FC from the FAS-DPD. Yes, I realise that OTO is testing TC and FAS-DPD is testing FC. However, the OTO results should be the same or higher than FAS-DPD. Now, I have read the extended instructions (thanks Jason) and noticed that OTO is not accurate above 2ppm (> 1ppm). But there seems to be a consistent low bias versus the FAS-DPD.

My other issue is one of color matching. I use the OTO to test during the week because of the convenience factor. The pool is in balance so testing with FAS-DPD once a week to test both FC and CC and using OTO to keep tabs during the week seems logical to me. I run a CYA level of 60 and as such my target FC level is ~4. The 3-4 range is precisely the two colors on the OTO that are nearly identical. It is so difficult to tell the difference. Given that your target chlorine level is so critical, I am hesitant to use the OTO for an extended period without FAS-DPD

BTW - I use a Taylor K-1000 for OTO and TF-100 for FAS-DPD

Okay here is goes

1) Am I crazy, or do others find that OTO always comes in lower than FC in FAS-DPD??

2) I want an easy way to keep tabs in chlorine daily, and keep FAS-DPD for periodic checks. Are there better ways than OTO (or the K-1000)?

Thanks
Todd

 

[jblizzle]

1 ... I have no idea
2 ... A DPD test could be used for quick FC check up to 5ppm ... although it again is a color matching exercise and quickly looses accuracy. The FAS-DPD is the best test.

Do you have a SWG? (adding pool info in your signature would help).
For a CYA of 60ppm, the MINIMUM for FC is 5ppm (without a SWG) or 3ppm (with a SWG). Just wanted to make sure you were staying ABOVE these.

 

[tlhwxman]

Yep, I have a SWG so my target FC is lower that 5ppm (4 according to the chart in pool school). Although, I am just considering keeping it at 5ppm anyway. This may help since the color scale seem to distinguish 5ppm much better.

I'll work on the signature. That is a good idea. Thanks

 

[Richard320]

Color matching is highly subjective.

I use the OTO test just because I have the reagent, and I like to see if I'm getting any better at distinguishing the shades of yellow. The answer is no.

What I think I'm reading can vary quite a bit depending on the light and the background and how far away I hold it. To me, 4 to 7 all looks like 5.

 

[CUTiger78]

Todd,

A bit off-topic, but are you the Tallahassee weather man?

Just curious, from your username.

Cheers!

 

[JasonLion]

I've never seen any problem with accuracy around 0-1.5. Above that things get kind of vague and anything 3-12 looks exactly the same to me.

The way I use OTO is for a quick check to see if FC is 3+ or near zero. If I actually want to know a number then I use FAS-DPD. Once every week or two, or any time I suspect extra chlorine demand, I use FAS-DPD to get a read on how much chlorine I should be adding each day. The rest of the time I use OTO just to make sure nothing has gone terribly wrong (i.e. to check that chlorine isn't heading rapidly towards zero).

 

[tlhwxman]

@ Jason and Richard - Thanks a ton. That is exactly the type of info I was looking for.

@CUTiger78 - Very perceptive. You nailed it. Go NOLES!!

 

[frogabog]

I find that the FAS-DPD is considerably easier to do every day and the reagents aren't that expensive that I'd choose OTO for monetary reasons. The OTO is just a PITA in my opinion. (I was gonna do an IMO there, but that was one acronym too many for one sentence, IMO - oh I'm crackin myself up now... stop that! )

Anyway, it's much harder to match colors, always a guess and then ask someone else to look to see what they think... blech! The FAS-DPD gives one a sure number every time and it's easy. 'nuff said...