Simplified equivalent FC calculation

Demegrad9

Well-known member
May 27, 2017
362
Coventry, CT
So I always see some number getting thrown around on the forum about equivalent FC when someone says they have X FC reading and Y CYA. I was wondering about how to calculate that so I searched the forum and found this page:

http://standards.nsf.org/apps/group_public/download.php?document_id=5891

Now I don't know if some better calculation has come along but there the equation is given as:

EFC = 2.06*0.31*(FC/CYA)/(1-(1.8*(FC/CYA)))

Just for fun, I was thinking I could come up with a rough scale factor for each level of CYA to multiple your FC reading and get EFC. Doing this I ended up plotting this CYA vs this scale factor. Then making a trend line of that plot to ultimately come up with a simpler (less accurate) equation.

EFC = 6.8*FC/CYA2

It's especially less accurate down near 20-30ppm CYA. But overall the error ranges between the models from <5% to ~285% and gets you in the ballpark. I was just thinking that WHEN someone on the forum starts asking questions about algae and quoting what they feel are adequate FC levels, this equation could be easily used to inform them about how little actual chlorine there is.
 

kimkats

Mod Squad
LifeTime Supporter
Jul 10, 2012
47,026
Tallahassee, FL
You will see them running for the hills when they see all of that ^^^^

This chart seems to work just fine:

Pool School - Chlorine / CYA Chart

We try to keep things simple so as to not overwhelm anyone. If they seem to want to go deeper into the hows and whys we can and will post the links to the papers and studies that have been done to support the above chart.

Kim TFP MOD
 

Demegrad9

Well-known member
May 27, 2017
362
Coventry, CT
I agree on the not trying to not overwhelm anyone. That's why this is in the deep end.

Really my thought was more that when someone new comes on here asking why the have a green pool and stating they keep FC up around 3ppm all the time, then naturally we ask what's the current CYA, and if they know it's something like 100ppm. Then the more informed people helping around here can talk about the FC/CYA relationship, point to the chart, and also quickly use this equation to give a rough order of magnitude of how much FC they have with no CYA, in this case the equation would result in 0.002ppm. The message being to the new person is 'you have so much CYA in your pool that you don't really have any active chlorine...'

So anyway, I was thinking this could be used by the more informed people around here, not really for the new folks to TFPC
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,764
Pleasanton, CA
Within the recommended ranges, you can also just use a simple percentage that is close enough. For SWGs, it is approximately 7.5% and for manual dosing it is about 15% of CYA.