Trying to interpret chlorine/CYA relationship article

[wes8398]

Hey guys. I'm curious about safe levels of free chlorine for swimming, given certain CYA levels. I read some of Chem Geek's article on the chlorine/CYA relationship and most of it was well over my head. I did gather some of the info I was after, I think, but I wanted to verify whether I'm interpreting correctly. It seems to me that FC levels can be VERY high and yet still be safe to swim in, given basically any CYA level within the suggested 40-80ppm. Is this correct? From one of the charts in his paper, I read that if my FC is 50% of my CYA level (FC=25 @ CYA of 50, for example), the equivalent FC without any CYA would still be less than 1ppm. This would mean that @ 50ppm CYA, I would have to have pretty astronomical FC levels for the water to be questionable to swim in. Am I correct with all this, or am I missing something?

The link for the paper/article I'm referring to is here: http://standards.nsf.org/apps/group_pub ... nt_id=5891

 

[jblizzle]

I think you got it. We generally say it is safe to swim up to the shock FC level for your CYA level (see Chlorine CYA Chart). And this still corresponds to a lower "active" chlorine level than is typical of public / indoor pools that do not have any CYA in them.

In reality, it is likely safe to swim with FC levels above shock level, but our thought is that you are above shock level for a reason ... and that reason (algae, bacteria, cloudy water, high CC, etc) is likely more of a reason not to swim than the actual active FC level.

 

[ROB N]

Your water is safe to swim in below shock values. The CYA level does have a dramatic effect on the FC levels considered safe or effective for swimming or sanitation... But as you approach higher CYA levels at or above 100 your effective FC threshold become difficult to maintain for sanitation.

 

[wes8398]

Here's the thing though: A FC of 20 is shock level @ 50ppm CYA. According to my example above though, those levels would be a relative FC of less than 1...

Jbizzle - you seem to see it the same way I do.

The reason I'm asking is just for convenience. I wanna know if I can add my liquid chlorine less frequently, meaning adding more at one time. If I can add 6 - 8 liters every 5 days, that's a lot easier than 2 liters every day or so...

 

[jblizzle]

Here's the thing though: A FC of 20 is shock level @ 50ppm CYA. According to my example above though, those levels would be a relative FC of less than 1...

I am missing your point / question?

Yes the "active" chlorine levels is less than 1ppm ... so it is less "harmful" to the skin/swimsuits/etc than a pool with 0ppm CYA and 1ppm FC.

Maybe you are missing that the FC is released from the CYA as required to continue to fight things in the pool?

 

[wes8398]

See my edit jbizzle - I didn't see your post when I posted.

 

[jblizzle]

The problem with doing as you suggest is that it will actually use more chlorine and cost more money.

The reason is that the CYA level only protects so much FC.
For example (likely not be TRUE numbers), with a CYA of 40ppm, you may loose half the FC to the sun each day. If you are dosing to 6ppm, you will loose about 3ppm per day. If you dose to 20ppm, you will loose about 10ppm the first day, then 5ppm, then 2.5ppm.

So adding 3ppm per day would use 9ppm of FC in 3 days
Boosting to 20ppm every 3 days, would use ~17ppm of FC in 3 days.

{Remember, my # are not exact, but simply represent the idea}

 

[wes8398]

10-4. Thanks for the clarification Jblizzle.