shocking with cal hypo/bleach

[poolbee]

Do these 2 deplete at different rates? I have shocked my pool once this summer with bleach(to 15 ppm) and it was down to 5 in about 24 hours.
I had to shock yesterday because my cc was 2 following 7 days of a patio installation-tons of dirt, grit and who knows what else in my pool. I shocked to 15 with cal hypo yesterday and after 28 hours it is only down to 9. My CYA is down to 35 now after draining 6 inches last week due to excessive rains. I am waiting for the Fc to go down before I add more stabilizer.CC is 0 now.

[chem geek]

Assuming your cal-hypo was fully dissolved in your pool (and it should have been unless you saw some clumps that didn't dissolve for some reason), then the chlorine that is produced is exactly the same as that from bleach or liquid chlorine. So, they should break down at exactly the same rate. Of course, how quickly they break down depends mostly on the amount of sunlight since the greatest rate of breakdown comes during the day from the sun.

The other "consumer" of chlorine would be the stuff that is in your pool, but generally even a CC of 2 would only consume 2-3 times the amount of chlorine (so 4-6 ppm).

So my educated guess is that you don't have the same amount or intensity of direct sun exposure as you did the last time you shocked. The half-life of chlorine under direct noontime sun at a CYA of 35 is about 6 hours so getting down from 15 to 9 in 28 hours is about right (since you probably don't have more than 6 hours "equivalent" of noontime sun). What is not explained as readily is how the first time your chlorine dropped from 15 to 5 in about 24 hours unless you had something else seriously consuming your chlorine such as algae. Otherwise, I can't explain what you saw the first time.

Richard

[poolbee]

Hmm. My pool is in direct sun dawn til dusk. The weather has been much cooler the past days than when I shocked in early July. My CC then was only .5. I don't know. I never had any algae that I was aware of.
Thanks for your input. I was just wondering about the cal hypo. I didn't buy it. It was left here by the pool tech after he did some work. I'll stick with the bleach in any case.