Raising TA while maintaining salinity

[carp6318]

My first question here, but I have not found any information anywhere that has been helpful. I have an ingound plaster pool that is now 1-3/4 years old. It is a saltwater pool that holds 19,500 gallons and I have not had any issues with it to date. No algae, no cloudy water etc. I have a Taylor K-2006-C test kit that i use to moitor the levels. The equipment is all Pentair with a 520 cartridge type filter. The pump, an intelliflow, runs 12 hours a day and I also have an intelliph acid injector that maintains the ph at a constant 7.4.
My trouble is that my TA is constantly falling. I have to add about 5 to 10 lbs of sodium bicarbonate every month to month and a half. I have not added salt to my pool since it was first filled and I constantly show high salt levels. I am making the assumption my salt levels are increasing from the constant additions of sodium bicarbonate. We have two decorative sconces and a 4' sheer decent waterfall that run a fair amount on the weekends only, and I am thinking this might be causing my TA to drop like it does. Can anyone confirm my assumptions made above and does anyone have any suggestions?

Plater Pool
19,500 gallons
FC 3.4
CC 0
PH 7.4
TA 90 (currently)
CH 450
CYA 60
86 degrees

[BigDave]

How much Acid are you putting in the feeder? I'd guess a lot, here's why:
The aeration from the water features and the SWCG raises the pH.
The acid feeder responds adding acid to compensate for the increase in pH. The acid also reduces the TA.
You measure reduced TA and respond by adding sodium bicarbonate. The increased TA causes the aeration to raise the pH faster.
And we go around again.
To reduce the acid and baking soda usage, I suggest reducing the TA target to 70 and raising the pH target to 7.6-7.8. Can you adjust the acid feeder?
I'm not so sure that any of this has any effect on the salt level.

[waterbear]

I agree, drop the TA to around 70 or even 60 ppm, maintain the pH between 7.6 to 7.8 and consider adding borax or boric acid to maintain a borate level of 50 ppm (which will introduce a secondary pH buffer to maintain the pH at around 7.7 for an extended period compared to without the borate and also has algaestatic properties and some other desirable effects on the water). You might also want to bump up the CYA to 80 ppm. This might allow you to lower the cell output or pump run time a bit which would create less CO2 outgassing (and therefore slower pH rise and less acid demand) from the hydrogen production in the cell (which produces tiny bubbles that aerate the water).
Also, by raising your pH setpoint while dropping the TA and adding the borate you will maintain almost exactly the same calcium saturation index that your currently have (very slightly negative, which is ideal for a salt pool since it helps limit scale production in the cell while still not being aggressive to the plaster surface. By my calculations the CSI is right around -.15 at your current levels and stays right there if you make the adjustments suggested (you didn't state the make of your SWCG or the salt level so I assume that high salt means aroud 3400-3500 ppm--even if it is as high as 5000 ppm your CSI is still well within acceptable range on the negative side and if you don't add any borate it moves closer to 0--however, the borate IS a worthwhile addition to a salt pool!)

[carp6318]

Thanks for the replies. My acid injector is the intelliph and I set how much acid it is to inject every 60 minutes. It, as well as the SWCG, is on a timer that runs 12 hours per day in conjunction with the pool pump and it currently injects 1.5 oz every 60 minutes. My SWCG is a Pentair IC40 and it is set at 5% on the intelliph control panel. My pool has never really demanded a lot of chlorine. As for my current salt levels, they have progressed up from 3500ppm in early 2012 to 4400ppm today. I have added zero salt since the pool was filled and the builder put the salt in (30 days after filling). That is what has me searching right now is how can my salt level be climbing without adding salt - my guess was sodium bicarbonate but I dont know. Thanks for the suggestion of the boric acid.

[chem geek]

Your salt level is increasing because you are adding hydrochloric acid (HCl) to control the pH so that is where the chloride is coming from. The acid lowers both pH and TA so when you add sodium bicarbonate (NaHCO3) to increase the TA, you are adding sodium. So between the acid and the baking soda, you are increasing sodium chloride -- salt. The following table shows you what is happening:

......................................... pH ...... TA ....... Na+ .... Cl-
Carbon Dioxide Outgassing ..... + ......... 0 ......... 0 ........ 0
Adding Acid ......................... - .......... - ......... 0 ........ +
Adding Baking Soda .............. 0 ......... + ......... + ....... 0
---------------------------------------------------------------------
Net Result ........................... 0 ......... 0 ......... + ....... +

Na+ is sodium, Cl- is chloride, and sodium chloride is ordinary table salt and is what is used in the pool at an elevated level for the SWCG.