Free Chlorine loss vs UV index

[rjledoux]

has anyone investigated the relationship of FC loss to the UV index?

I was calculating the balance point for my SWG based on 60% loss due to UV and I wondered if the relationship is linear to the UV index.

My pool should be stable at 5ppm with the cell set to 63% with a 10 hour pump time if the 60% loss holds true.

 

[linen]

I am not sure if that correlation has been studied. 60% loss is really high though. What cya level are you maintaining?

 

[rjledoux]

It was 45 yesterday. I added a gallon of liquid CYA this afternoon. I just finished SLAMming this morning. I didn't want to crank the CYA up until after I had finished. It's possible that my loss will drop now that I'm not feeding my FC to organics.

 

[rjledoux]

UV index today was 11 (Extreme)

 

[linen]

With cya up in our recommended range of 70-80 ppm (and no organics), you should see much less loss (I see about 10-20%/day).

 

[rjledoux]

With the CYA I added this afternoon I should be ~60. I'll confirm tomorrow and add another gallon. I'm adding slowly because I really don't want to overshoot.

 

[chem geek]

The UV Index calculation is described in this EPA link where you can see that the amount of UV radiation by wavelength is weighted by its effects on human skin. The UV index is overweighted at 400 nm compared to chlorine loss since the molar extinction coefficient is low at that wavelength. Their 320 nm peak is similar to that for chlorine but they assume more UV gets through at 290 nm than is actually the case according to standard sun irradiance.

At any rate, the UV index would be a reasonable number to use to assess predicted chlorine loss from the UV in sunlight -- not perfect, but not horrible either.