SLAM FC test variability question.

[n240sxguy]

I had a little cloudiness after a bunch of kids were in the pool a couple weeks ago. I added 2 gallons of 12% and it cleared up. A couple days later there was several kids in it again, but not as many as before. Started to haze a little like before. Still mostly clear, but just not quite right. I think my FC did drop too low one day, but the swg had it back up when I let it run overnight. I've been doing a legitimate slam since I think Friday. I bought some skimmer socks and was finding a surprising amount of dead algae. I never would've seen it otherwise. Those things are great. The algae has dropped to just a slight amount now, but there is still some, so I continue on. My numbers from before the slam were:
PH 7.5
TA 70
CH 200
CYA 65
I forgot to lower the ph before I started. How big of an effect does that have on things? The big question I have is about the FC test itself. How much inherent variability is there? I kept getting reading that I wasn't completely sure of because I didn't trust just eyeballing a miniscus in a tube to get it perfect. Last night and this morning I used a 6ml medical syringe to pull the samples. At 8:20 last night I got FC of 30,33,30 all tested from the same sample back to back. This morning at 5:20 I got 28,33,26.5,27.5. I assume I had to screw something up on the 33 test this morning. How much variability is there when running tests the same way back to back? The sample size was exact on all samples, bottle held upright, and all drops distributed as slowly as my sanity would allow.

 

[Patrick_B]

The differences you mention on FC are hard to explain, but you are over doing it. Take a 10 mL sample once and be done with it. If you want consistent sizes, order a sample sizer. It will eliminate variables on 10/25mL samples if you have that worry. On the pH, it's not a big deal at all. Having good Cya levels pushes open the pH range window a lot in terms of the effect of pH on FC. You didn't run the pH at that FC level did you? Over ten FC interferes with the phenol red test and it shouldn't be taken there.

 

[n240sxguy]

No. Ph was tested when FC was around 6 before the slam. Given the variability with my results I'm either done or far from it. That's my confusion. I did an exact 10ml, rinsed multiple times, etc. and still had varying results. I usually just take one, but I got curious and was surprised and disheartened by the results. From the decreasing amount of algae I believe I am nearly done. I turned the swg back on to help maintain the FC level while I'm at work today. Gonna shoot for one more OCLT and pray for the end. Also need to order more reagent as I've used over half a bottle testing now. A good bit of that was this morning and last night.


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