Please help with pool chemicals

[chem geek]

As Ben noted, ORP is not a good absolute measure. It's best used for process control to maintain an FC level, but that means creating a "setpoint" and then retesting periodically where any long-term change may require a resetting of a new "setpoint". That is, you get the FC level to where you want it and then lock in that ORP reading as your target.

Your pool will have a greater tendency toward 'wobbly' pH than most large indoor pools. The reason is that your "acid" is a partially soluble gas that will tend to exit the pool water during aeration (ie, during warm-ups at a big swim meet!), causing the pH to rise. (Chem_Geek, will the TA also tend to rise?)

Injection of carbon dioxide will lower the pH with no change in TA. It is the exact opposite of outgassing of carbon dioxide. If not all of the gas dissolves and some outgasses, that's fine and won't have any affect on the water chemistry (i.e. the gas just passes through the water with no effect -- only that which dissolves into the water has an effect). I don't think the pH will be any less stable due to using carbon dioxide, per se, especially since the TA level is fairly low to begin with. If the TA were higher, then one could be fighting rising pH from outgassing while trying to lower it by injecting. Given the large pool size and implied poor circulation, that is probably the source of inconsistency since the feedback loop (measurement of pH to make further adjustments) is very time-delayed (hysteresis).

When using carbon dioxide for pH control one normally does see a rise in TA over time, but this is due to adding hypochlorite sources of chlorine that have some excess lye in them. Cal-Hypo has less than most, but it still contains some calcium hydroxide and calcium carbonate. For typical Cal-Hypo, for every 10 ppm FC it will increase TA by 0.5 ppm (and CH by 7.1 ppm) so the rise in TA is usually slow.

The other problem we have is trying to keep our Ca low, but the water we add is 400ppm, but below 500 is what the pool contractor wants, so I have been dumping and repalcing about 100,000 Gal, every week.

That's a real challenge, but one can run a pool with a higher CH level if one keeps other water parameters such as TA and pH somewhat lower. In fact, a near-zero saturation index would be at a pH of 7.4 with TA of 60 and CH of around 600. Does the pool contractor understand that a higher CH will be OK so long as the pH and/or TA are lower such that the saturation index is still near-zero? Also, if you are using a lot of carbon dioxide to try and keep the pH lower, then your pH target may be too low as well.

[PoolDoc]

Thanks, ChemGeek.

PoolDoc

PS. Mav., since I know who you are, I went ahead and bumped you to "Subscriber" so you can post w/o moderation.