Shocking with CYA at zero

[JesseWV]

Split off of this topic. JasonLion

Thank you for correcting me Jason, I'm still a grasshoppa! I didn't think about the FC dropping to 0 during the day. I figured it would be so quickly used up oxidizing that it wouldn't matter.

So hypothetically with CYA at 0, if the shock process was started with bleach right after sundown and maintained for about 8 hours wouldn't that be much more effective at killing organics considering the higher hypochlorous acid levels? Or is the FC/CYA equilibrium aggressively maintained enough to kill everything just as fast AND have the UV protection?

If it IS more effective at 0 CYA, then doing the initial round of shocking after sundown would speed up the conversion process, right? Before sunrise, shocking could continue with dichlor into the daylight hours, yes? One could easily figure out the amount of dichlor needed to bring the CYA up to 30ppm and then stop using it once you've added that much and switch back to bleach.

Another thing to worry about is bleaching the liner the first night with the higher levels of hypochlorous acid. Reading chemgeek's Chlorine/CYA Chart I see that at 0 CYA the shock level is technically only 0.64ppm but that leaves no reserve. So, what is the upper limit safe level of FC at 0 CYA without bleaching the liner?

 

[JasonLion]

With CYA really at zero, shock level is 5. You can go somewhat higher than that without damaging the liner, but in most cases doing so isn't a good idea (ie you usually want a reasonably wide safety margin).

There are all kinds of problems with talking about shocking with CYA at zero in an outdoor pool. You can't simply lower CYA to zero when you want to shock, so it almost never comes up. Also, CYA really and truly at zero is not at all the same thing as the CYA test reading zero. Most of the common CYA tests can read zero with CYA as high as 20, sometimes higher, which is nothing like CYA really and truly being zero.

One of the big issues about CYA being lower is that the amount of chlorine you can add to the pool at one time is way less than it is when CYA is higher. Killing algae requires two different things, first that the FC level be high enough that algae is getting killed faster than it is growing, and second and more importantly, that there is enough total chlorine to kill all the algae. It really doesn't matter how "effective" the chlorine is, if it all gets used up killing only 1/10th of the algae.

A major algae bloom can take 30, 50, even 100 ppm of chlorine to kill off. With CYA at zero you can't add all of that chlorine at once. You have to add it 5 ppm at a time. With CYA at 30 you can add chlorine 15 ppm at a time, meaning one third the number of chlorine additions, and more chlorine added per hour, so less algae growing back.

When CYA starts very low, shocking with dichlor for a while, and then switching to bleach can work very well. The main issue seems to be that finding dichlor at the store isn't always all that easy, though that isn't really a serious problem.