OCLT and FAS-DPD Accuracy Question

dtarantino

Active member
Jul 15, 2016
34
Florida
I see that for an OCLT the overnight FC loss should be 1.0ppm or less. However, when doing a SLAM with FC in the ~20-25ppm range, how accurate is the FAS-DPD testing? I read that it is +/- 10% accurate. Accurate enough for the lower FC levels, but at 25ppm that can be +/- 2.5ppm. Therefore, the testing accuracy is less than what the OCLT test requires. Any thoughts?

I've seen some info on this, but nothing definitive.

Thanks
 

atttech-2

Bronze Supporter
TFP Guide
Jul 24, 2015
1,182
Central Valley CA
Never had to think about it, so if someone with SLAM experience chimes in I would taker their advice over my guess, but absent a better idea I think I would mix 100 ml of pool water to 100 ml of non chlorinated drinking water and use that mixture for my 10 ml sample. Test results should be 1/2 of true levels but still give you the accuracy you are looking for.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,885
Tucson, AZ
This has been discussed both internally among the TFP experts and mods as well as externally in the threads. It is true that, at higher FC levels the test's reporting accuracy starts to get a little worse than the +/-1 drop tolerance you get when you are below 10 drops of R-0871. But, it should not be that far off that you can't be confident of SLAMs completion. Also, one could wait for the FC to come down a bit and get a better idea of the OCLT at lower FC levels. In 99% of the cases, a person is going to have no overnight loss of chlorine and low CCs fairly quickly in the SLAM process. Achieving clarity is usually the biggest problem faced when trying to complete a SLAM - you'll often see most threads ending with "my OCLT is less than 1ppm and I've had zero CCs since day three of the SLAM but I just can't get the pool clear...."
 
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