Questioning repeatability of FC test

[Ronald U.]

Had some green algae spotting on walls. Water clear. Water temp=79, pH=7.3, TA=70, CH=225, Borate=40, CYA=29. I SLAMed yesterday. I did an OCLT. Prior night's results: FC=12.5, CC=1.0. I ran FAS-DPD test 4 times this am, same water sample: FC=10.5, CC=1.0; 12.0, 0.5; 10.0, 0.5; 10.0, 0.5. I called it a failed OCL and brought FC back up to 13 according to PoolMath calculator. My question is, how repeatable is FAS-DPD on 10ml sample? Especially in light of the </= 1.0 FC loss I am testing for. Should I do a larger sample for OCLT? Should I always see un-dissolved DPD powder in bottom of cylinder after adding and swirling DPD, to ensure saturation? Thank you folks for your time and attention. RU

 

[pabeader]

A SLAM is not a one day thing. It takes as long as needed to pass the 3 criteria. I'm not sure what the longest is but I've read of ones this summer that took over 19 days.
You need to start the SLAM again. Hold it at the proper FC level for your CYA, and monitor over the course of the day. Every hour or so you need to test, calculate and add as needed to maintain the FC level. You can stretch out the testing to every few hours once the FC starts to hold.
Only after there is no evidence of algae will you want to do the OCLT.

Variation in the test values can be for many different reasons. For me the big one was the sample size. You can get a pretty big variation, just within the difference in the size of the meniscus. i.e. If you test with it at the top of the meniscus VS the bottom. Also, droplet size can vary and will of course have an effect.

The greatest accuracy is with the 25ml sample. You might want to try that when it's time to do the OCLT.

Since you are using the same water and the same tube, the larger the sample the less effect from the meniscus.

 

[pabeader]

Also, how did you determine a CYA of 29? Just as something to note: The scale is not linear. For SLAM it's easier to just round up to the next number. SO i would use 30 in your case.

 

[JoyfulNoise]

The precision of the FAS-DPD is +/- 1 drop when the drop count is less than 10 and +/-10% of the final reading when the drop count is over 10 drops.

See this sticky for the precision of all tests - Extended Test Kit Directions


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[Ronald U.]

Thank you for your help. I have reviewed the details of the tests. Perhaps I was skimping on the DPD powder? My 29 CYA was the average of 3 readings, but for Pool Math I took the advice and rounded up to 30. BTW, my algae wall spotting seems to have abated and the pool passed the OCLT this morning. Ron