More FC = More UV Chlorine Loss?

[MarkTX]

Hello,

I have a simple question that I have been struggling with. Lets say my target FC is 6. If I put in enough bleach to get to 10, will I lose (or waste) more bleach to UV during the next 24 hours, than if I had put in enough to get only to 7? (assuming pool is clean, clear, and CYA = 40)

Put another way, can a fixed amount of UV break down a fixed amount of chlorine, regardless of chlorine ppm? Or does the break down increase with higher chlorine concentrations?

I hope my question makes sense. I know I could experiment and come up with an "answer," but I would still not be sure. Too many variables involved. This is a real brain teaser to me.

 

[Richard320]

You do lose more faster at higher FC levels. But CYA helps buffer that.

Wrap your head around this thread, especially the third graph.

 

[jblizzle]

The way I like to state it (while maybe slightly over-simplified) ... the CYA will protect a % of the FC in the pool. Assume that 50% is protected and 50% is lost (for some CYA level). Starting at 7ppm, you will lose 3.5ppm. Starting at 10ppm, you will lose 5ppm. So the higher you raise the FC above the recommended levels, the more ppm of FC will be lost to the sun.

 

[MarkTX]

You do lose more faster at higher FC levels. But CYA helps buffer that.

Wrap your head around this thread, especially the third graph.

That is a great thread!

This part I understand:
You do lose more faster at higher FC levels. But CYA helps buffer that.

The other part:
Trying to understand. But there is no "mind blown" emoji

Thanks, you answered the question

- - - Updated - - -

The way I like to state it (while maybe slightly over-simplified) ... the CYA will protect a % of the FC in the pool. Assume that 50% is protected and 50% is lost (for some CYA level). Starting at 7ppm, you will lose 3.5ppm. Starting at 10ppm, you will lose 5ppm. So the higher you raise the FC above the recommended levels, the more ppm of FC will be lost to the sun.

I can understand that, too. Thank you!

 

[JoyfulNoise]

FC loss to UV light is a function of the active chlorine concentration (HOCl/OCl-). The hypochlorite ion (OCl-) is most susceptible to UV photolysis and, as UV uses it up, the HOCl/OCl- equilibrium shifts to create more OCl- since the ratio of HOCl to OCl- is fixed at a given pH (at pH 7.5 the ratio is roughly 1:1).

CYA binds up most of the chlorine and lowers the overall active chlorine level. As you increase the FC/CYA ratio by adding more FC to a fixed CYA level, you are creating more OCl-. So the FC loss rate goes up since photolysis is a 1st order reaction rate, the rate of photolysis is proportional to the concentration of the species reacting to the UV light.

 

[MarkTX]

FC loss to UV light is a function of the active chlorine concentration (HOCl/OCl-). The hypochlorite ion (OCl-) is most susceptible to UV photolysis and, as UV uses it up, the HOCl/OCl- equilibrium shifts to create more OCl- since the ratio of HOCl to OCl- is fixed at a given pH (at pH 7.5 the ratio is roughly 1:1).

CYA binds up most of the chlorine and lowers the overall active chlorine level. As you increase the FC/CYA ratio by adding more FC to a fixed CYA level, you are creating more OCl-. So the FC loss rate goes up since photolysis is a 1st order reaction rate, the rate of photolysis is proportional to the concentration of the species reacting to the UV light.

Oh my, I have much book-learnin to do. Thank you JN! It seems unanimous.