How can I avoid a pH crash?

[AnnaK]

Due to unavoidable circumstances my pool will be unattended for the next 3 weeks.

I've loaded the inline chlorinator with pucks and have adjusted the setting to 1/4. From past experience during the initial start-up weeks when the water is similarly cool, this will maintain FC at between 5 and 7 ppm. The pump is on a timer and will run 6 hours during the night. The pool will be covered with a solar cover.

Current readings are: FC=5, CC=0, pH=7.5, TA=60, CYA=30.

The TA of 60 is the natural state of my fill water. I use LC during the season and pH has stayed steady at 7.5 all summer.

I'm worried that the continued use of trichlor will crash the pH. This hasn't happened in the past but I've always been there to keep an eye on things. Murphy's Law pretty much assures that problems will arise.

My plan is to raise pH to 7.8 using borax and raise TA to 80 with baking soda. That would allow the pH to drift down over time and would add some extra buffering capability.

Is that a reasonable approach or is there a better one?

[chem geek]

If your typical daily chlorine demand is 2 ppm FC per day, then in 12,000 gallons that would be about 9 3" 8-ounce Trichlor pucks/tabs. Starting out at 7.8 pH and 80 ppm TA you would end up at the same TA but with CYA higher by 25 ppm (adjusted TA or carbonate alkalinity would be lower by 10 ppm) and the pH would be no lower than 7.0 and with carbon dioxide outgassing will probably be more like 7.2 so you should be OK.

[AnnaK]

Thank you very much for the projections. I expect the FC loss to be less than 2 ppm/day since there's not going to be a bather load and I'll have the solar cover on. I should be okay with the seven 3" pucks the chlorinator holds.

'Preciate your help!

[waterbear]

Just to be safe I would bump up the TA a bit higher. Murphy's Law and all that! I would shoot for 100 ppm. You can always lower it when you return if you find that your pH is rising too fast.

[AnnaK]

Well %$#@!

Murphy's Law kicked in. The day before we're to leave for Canada to help our very elderly parents the pump died. During its run time last night it got so very loud that we had to turn it off. Checked just now: skimmer clean, pump basket clean, pressure readings normal at low and high speeds, water coming out of the return as always. So I have normal inflow and outflow, no air leaks apparent. but the pump makes a hellishly loud noise.

There's no way we have time to disassemble it and check it out.

I'm going to pour in enough bleach to get to 20 ppm FC and mix it in with the net. Then I'll put the solar cover on and drive away. Whatever happens, happens. I know how to clear up a green pool.

But still: %$#@!

Pardon my language.

[waterbear]

Leave it to Murphy! Never seems to fail, does it?

[BigDave]

Pull the pump. Bring your tools. Let someone else drive. Take it apart in your lap on the way. Get parts in Canadia(if you can). Put it back together on your way home(someone else driving). It's all ready when you return. Plenty of time. ;-)

Warning: Do not attempt to rebuild your pump while driving.

[Watermom]

Anna,
You may want to leave a little bit of the pool uncovered to vent it some since you are shocking it up to 20 so the fumes won't damage your cover.