Phosphate problem?

[CarlD]

Thanks Evan. My knowledge of SWGs is limited so I can only provide what I've learned from the SWG users.

I know that pH of 8 is very bad if you don't have an SWG because the chlorine is far less effective, but I didn't know it's even worse if you do have an SWG because it will stop producing chlorine!

I knew that a CYA of 80 is appropriate for an SWG, but not therefore, the correct FC--I would normally recommend an FC of 5-10, and, while it CAN be lower, clearly it can't be more than a PPM lower than the minimun non-SWG pool.

I also knew the lower T/A was better--but, again, not at a level even below the normal minimum.

I DO know that rising pH is very common with SWGs, and that the lower T/A can inhibit that. It's a bit counter-intuitive but it does work.

Again, thanks to Evan for helping me with that.

I just came back from vacation to VERY low pH--it took a full box of Borax and a full box of Washing Soda (soda ash--same as "pH Up!", but cheaper) to get pH back into the safe zone. It wasn't unexpected as I used Tri-Chlor tabs to help maintain my FC while we were away for 2 weeks. CYA isn't very high as the constant rain has washed it out--but with the solar cover one there's been no aeration to reduce the increase in acidity the tri-chlor generates.

Still the water is now fine and warm enough to swim, but the days are getting shorter and the nights are going down to 50 or lower so my solar panels are working VERY hard to keep the water over 80. I'm hoping for a dry, sunny September after a rainy summer so we can stretch the season a little--we haven't swum much this year, and the water only reached 90 once--usually in early August it passes 96 and reaches 98--like a giant hot tub!

Soon, it will be time to close again as another season ends....I'm just not that big a fan of autumn!

[waterbear]

Thanks Evan. My knowledge of SWGs is limited so I can only provide what I've learned from the SWG users.

I know that pH of 8 is very bad if you don't have an SWG because the chlorine is far less effective, but I didn't know it's even worse if you do have an SWG because it will stop producing chlorine!

This is only really true in an unstabilized pool. Once you have CYA in the water the effects of pH on chlorine's effectiveness (whether it's in the form of hypochlorous acid or the much less effective hypochlorite ion) become much less important since most of the chlorine is now in the form of chlorinated isocyanurates and pH has much less impact on the activity of the chlorine. Chemgeek knows the actual numbers but bottom line is that the old 'chlorine is more effective at a lower pH' is another one of those industry TEKTATs (Things Everone Knows That Aren't True) when you are talking about a stabilized pool.
http://www.poolsolutions.com/tips/tip72.html

[chem geek]

I have graphs here in The China Shop. However, the fact that the active chlorine concentration without CYA drops so dramatically at higher pH is a bit of a moot point since its absolute concentration is so much higher to begin with because there is no CYA to moderate it.

As Evan wrote, the effect of pH on active chlorine is significantly reduced when CYA is present. With an FC of 3 ppm and no CYA, going from a pH of 7.5 to 8.0 goes from an active chlorine (hypochlorous acid) concentration of around 1.5 to 0.7 or roughly a 50% drop, but with 30 ppm CYA in the water it goes from 0.042 to 0.036 or a drop of around 14%. CYA is like an active chlorine buffer just like the carbonates (and borates) are a pH buffer.

As for a lower TA having the pH be more stable when using hypochlorite sources of chlorine, the simplest way to understand this is that the carbonates portion of Total Alkalinity (TA), which is mostly a measure of bicarbonate, has TWO effects: 1) it is a pH buffer so higher TA is more resistant to changing pH from outside influences and 2) it is a SOURCE of rising pH itself where higher TA results in faster carbon dioxide outgassing that causes the pH to rise. This latter effect outweighs the former so as ironic as it may seem lowering the TA leads to greater pH stability when the reason for that rising pH is the TA in the first place (if the reason is primarily from something else, such as plaster curing, then a higher TA might be helpful up to a point).

Richard