FC tests higher than Pool Math estimates

[Kelly Gordon]

This isn't a big deal, but I'm curious. Lately, when I need to add chlorine to my pool, I calculate how much 12.5% Bosch to add using the Pool Math calculator. But when I test, about an hour later, my FC is much higher than expected - like 18 FC instead of 6. How is this happening? All my other levels are fine, and even the high FC drifts down in a day or so. Thoughts? Theories?

 

[Mr Bruce]

Are you adding in front of a return with the pump speed relatively high? What happens if you add the chlorine, brush the entire pool and test then?

 

[AftonJeeper]

A couple things can cause that, or a combination of them. Volume of the pool being off, pool math not set to 12.5% chlorine, error in testing.

 

[schatz]

I have also ran into this, my pool calculates to 33000 gals according to pool math calcs and have been told its 30-35000 gals. When I consult pool math it says I will need122 oz. of 8.25 bleach to raise ppm 2.5. But when I add 3/4 of gallon or roughly 90 0z. of the bleach I get 4 ppm raise. Very interesting. I was thinking my pool must be smaller than what I thought so me and my wife tape measured the pool again and entered the numbers in the pool math calculator and same 33000 gals came up so I am not sure if I should trust those #'s or use less.Don't mean to be critical of the pool math calculations but I think something is off?

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Or maybe I have been getting better clorox at Costco lately.

 

[Wasted0101]

It is all based on math so one of the numbers is off, the formula is fine as it works for everyone else.

 

[schatz]

One would assume. That is why I have never questioned it until I saw this thread, thank you.

 

[Wasted0101]

It's science, this forum didn't make up how this works off the top of someones head they just put it in an easy to use webpage.

 

[Kelly Gordon]

I think my pool measurements are accurate. I've done multiple tests to ensure accuracy, and I know the settings are right in the calculator. Until the last few weeks, the calculator was always spot on (or close enough) so this is a new development. Like I said, it's not a big deal. But I know it's science, so I was curious if there was some other variable that could make the FC test high.

 

[ping]

Could it be that you are using 25ml of water for testing instead of 10ml of water? The math is about right if you are counting .5ppm of FC for each drop when it should be .2ppm of FC for 25ml of water.

It wouldn't be the first or last time anyone did this.

 

[jblizzle]

If it just started happening, then I would suspect extra strong bleach, but there is no way it is 3x as strong.

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Oh, ping's thought is a good one too