Cl with R871 not logical- Is reagent bad?

[Joe3]

We had a LOT of rain in Austin for several weeks. CH dropped. CYA dropped. Salt dropped. Cl by FAS/R871 was 20 which made no sense. Cl by DPD was 0.5, which does make sense. The solution turned pink with DPD powder, but took a surprisingly large number of drops of R871 to go clear. The R871 is clear. It’s been stored in a cabinet at room temperature. It’s less than 8 months old. Before I replace what I have on hand, should I be looking for a different reason for the discrepancy?

 

[Texas Splash]

Can you clarify the math formula you used and the water sample size?

 

[jblizzle]

Simple ... lots of clouds and rain and colder water means that you were losing less FC to the sun and if you had not turned down the SWG output, the FC could certainly have built up. Just found mine up to 13ppm 2 weeks ago myself.

When you say the DPD registered 0.5ppm, let me confirm that is a pink color matching test right? And not the yellow color matching test that is OTO and included in the TF-100?

The pink DPD color matching test will actually bleach out at higher FC levels and make you think the FC is low when in fact it is high ... the yellow OTO will not do that and will get darker yellow and start to turn orange at higher FC levels.

 

[Jimrahbe]

Joe,

I agree with Jason.. I just type toooo slooowly..

Jim R.

 

[Joe3]

Oops, my bad - the Cl 0.5 reading was from the OTO in the TF-100 kit. If it had been dark yellow/orange, I wouldn’t question the FAS/R781 result. Volume tested was 10 ml. Turned pink with scoop of DPD powder. Ran test twice- stopped at 20 drops of R-871 the first time without clearing. Second test cleared at 24 drops. Should I believe the Cl is 12 or that the OTO result of 0.5 is correct?

 

[Texas Splash]

I'd go with the R-0871. The FAS-DPD kit is more accurate. The OTO checks for the presence of chorine, and that includes total chlorine. More on the Pool School - OTO Chlorine Test page.


One final thought, just make sure the sampling container and FAS-DPD testing tube are clean and sanitized as to not skew the results. We've seen that happen before.