FC different with 3 consecutive tests?

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Vito1

Vito1

0
Jul 24, 2016
23
Bristol, CT
I tested my FC and thought the number was a little low. For the heck of it, I tested again and got a higher number, then tested again and got a number in between the two. I'm using a TF-100 Test Kit and testing at 10ml. My CYA is around 60 and I'm trying to finish my SLAM by keeping FC at 27. I'm now in the OCLT and even that 1ppm makes a difference so I can stop testing so much. I need between 45-55 drops at a time. Anyone?
 
Two notes:
Drop size can vary from test to test and drop to drop. This can lead to variations like you are seeing.

You could switch to 5ml sample size and each drop is 1ppm. Just don't want to use that for the OCLT.
 
That sounds more like a testing error. My guess is that you are not seeing the meniscus clearly.

I use a sports squirt bottle to get my sample. Then I add water and eyeball the level/meniscus in the vial. I then set the vial on a flat surface and bend down so that I am looking at the vial perfectly horizontal. I add a drop or two of water as needed to get the bottom of the meniscus lined up with the 10ml mark.

It also could be your drop size, but try the above first.
 
Vito1 said:
1st reading 21ppm, next 25ppm, next 22ppm?
Click to expand...
Try not to go too fast with the drops, let them form and drop off naturally. I never drop faster than one per second. Also try to keep the bottle as vertical as possible.

Most of the time I see issues like this is when there are many drops involved (42, 50, & 44 in your case), making drop size even more important. I know it's tedious, but don't rush it.

Dom
 
^Dom hit the nail on the head.
when you are testing in the high range there are more drops(variability) + variability in the sample size. Add in any fluctuations from bleach mixing with the pool water and you could see a +/- 4-6ppm possibly higher as the CYA level increases(FC target higher)

I've seen the same thing trying to hit 18FC for a 30CYA during mustard algae shock.

If during each test the CC reading remains constant at zero trust your test results from the first run - and you're not going to destroy the vinyl going over a few ppm on FC, provided only liquid bleach is added.
The liner has a life expectancy, keeping it pretty and original blue will be less of a concern as the years go by
 
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