Monochloramine, Chlorine, CYA and pH

[waterbear]

Chem Geek,

Thank you for your reply.

I think I understand.

The whole point of the previous log was to get rid of algae so which of the three following methods would be preferable assuming a pool which has a CyA of 40. and a pH of 7.5?

1. Get the chlorine to the recommended shock level based on Best Guess CYA chart not adding any acid to lower it to 7.2.

2. Lower the pH to 7.2, then shock based on Best Guess CYA chart .

3. Raise the pH to 7.5 then shock based on Best Guess CYA chart.

4. Raise the pH to 7.5 then shock to a higher level say FC 35 or 40

For #3 and #4 why are you raising the pH to 7.5 when it is already at 7.5 according to the first paragraph of your post.

As to what is best the first thing to consider is what kind of algae. Green is easiest to kill, black algae and the bacterial pink slime and white water mold the most difficult. All must be taken into consideration.

Also, the pool surface needs to be taken into consideration. A vinyl pool can be bleached by too high a chlorine level (as can dark colored plaster and fiberglass but not as readily) so 'nuking' a pool is not always a viable option.

Bottom line is this. As long as the pH is within aobut 7.2 -8.0 and the pool is stabilized normally and not overstabilized then the FC needs to be raised high enough and kept there long enough to kill the algae. The best guess chart works for green algae and is on the border for mustard. Black, pink, and white require even higher chlorine levels. If you are going to use an adjuct such as sodium bromide or an ammonia compound then the pH is ususally raised to 7.0-8.0 before using the product and shocking to activate it. These are more useful at high stabilizer levels than at lower levels but can be useful at normal stabilizer levels with the more resistant algae types.