FC Levels

[bxcrwlly]

I've read in this forum that a salt pool with a CYA of 70 should have a target FC level of 5. Everything I've read about chlorine levels indicates anything above an FC of 3 poses a health risk. Can someone please comment or advise?

 

[Richard320]

pool-water-chemistry-t628.html

 

[Bama Rambler]

There are lots of posts here about that very issue. Surmize it to say that the 3 ppm recommendation doesn't take into account the effect of adding CYA to the water. In fact the industry still doesn't recognize the FC/CYA relationship.

 

[UnderWaterVanya]

I've read in this forum that a salt pool with a CYA of 70 should have a target FC level of 5. Everything I've read about chlorine levels indicates anything above an FC of 3 poses a health risk. Can someone please comment or advise?

Just to add a minor note to this:

My water routinely is between FC 5-8. My CYA is around 45-55 depending on how I read the test. I had one kid who was a friend of my daughter's ask me how I kept the pool so clean without chlorine. He thought he was swimming in fresh water.

Our tap water has a much stronger smell of chlorine than the pool. Which is pretty much rational - since the tap water active chlorine levels are higher than the pool. The tap water has 1.5-3ppm depending on when and where I measure it - but zero CYA. The pool is consistently at 5ppm or higher. There's a chart around here that shows the actual "active" chlorine levels in various CYA/FC concentrations - and if I recall correctly the 5ppm+ I have is more like 0.1ppm active. The 1.5-3ppm tap water is much much higher than that.

 

[y_not]

Just to add a minor note to this:

My water routinely is between FC 5-8. My CYA is around 45-55 depending on how I read the test. I had one kid who was a friend of my daughter's ask me how I kept the pool so clean without chlorine. He thought he was swimming in fresh water.

That is just TOTALLY AWESOME!!!
Yay for TFP

Our tap water has a much stronger smell of chlorine than the pool. Which is pretty much rational - since the tap water active chlorine levels are higher than the pool. The tap water has 1.5-3ppm depending on when and where I measure it - but zero CYA. The pool is consistently at 5ppm or higher. There's a chart around here that shows the actual "active" chlorine levels in various CYA/FC concentrations - and if I recall correctly the 5ppm+ I have is more like 0.1ppm active. The 1.5-3ppm tap water is much much higher than that.

Yup, it's all about the sequestering properties of the CYA. It's certainly a beautiful thing!!
The chart you're referring to is in the link that Richard320 provided.
ChemGeek's HOCl to CYA Chart

UWV, at an FC of 5 or 10ppm at a CYA of 50ppm. Your active Hypochlorous Acid (HOCl) would be 0.043 & 0.102 ppm of active, pure CL2. So you're somewhere in the middle.
Can you imagine maintaining that with no CYA? WOW!
You'd have to live by the pool. HAHA

 

[chem geek]

Also see this post which has links to some FC limits for commercial/public pools in different states that vary from 3 (Massachusetts) to 10 (Florida). The EPA limit for drinking water is 4, but that doesn't have any CYA. The state limits are higher mostly because of outdoor pools that use CYA, though the states don't regulate for an active chlorine level. If they did, then they'd be regulating for a minimum and maximum FC/CYA ratio since that's an approximation proportional to the active chlorine level.