I would like some help in identifying some possible sources of acid demand in my pool. I have a salt pool with a SWCG. First my most recent numbers:
FC 3.0
TC 3.2
CC 0.2
pH 7.6
TA 80
CH 370
CYA 80
Borates 30
Salt 3100
Temp 90F

I have installed an acid feed pump that maintains the pH more or less constant. Anyway I am adding about 32 oz of 31% acid per week. Some sources of acid demand that I know of:

1. CO2 out gassing. This is a possibility. However my TA has not changed 10 ppm in over a month. It is my understanding that CO2 out gassing at a constant pH will tend to decrease the TA. Using my Colorq it appears that the TA has changed by less than 5 ppm in a month.

2. Plaster curing. Another possibility. However the pool was filled at the end of October. It is now August so the plaster is about 10 months out. I would expect that the acid demand from curing plaster to be rather small.

3. Rain. I live in west central Florida. In the summer we have very high rainfalls. I am pumping out 1 to 2 inches of water from the pool each week. I don't know what effect this would have on the acid demand, but is causing me to add salt and CYA

I am open to any more ideas.

Thank You,
Donald

More to add:

4) Fill water - This may not impact you as much given the amount of rain you are seeing. I have very high TA fill water which tends to keep the TA high and raises the PH.

My guess is that the plaster is still adding TA to the water and adding acid only keeps it steady. The SWG will cause the PH to rise unless the TA is kept really low.

Rain aerates the water and will cause CO2 to outgas, btw.

Good point, completely forgot about that.

I forgot to add my SWCG settings. I have an AquaPlus unit with the T-15 cell. The current SWCG setting is 10%. During the week I run the filter from 10 am to 7 pm, and on weekends from 8 am to 7 pm. The AquaPlus has a two hour cycle time. So on weekdays I have five cycles of 12 minutes each (one hour total run time) and on Saturday and Sunday I have six cycles of 12 minutes for a runtime of 72 minutes each day. The T-15 cell is rated to produce 1.45 lbs of chlorine per day, so for a weekday the daily production is 0.06 lbs of chlorine, and on weekends is 0.073 lbs. Total weekly production is 0.45 lbs of chlorine.

What other processes other than plater curing can raise pH and TA?

The pH can rise from outgassing of carbon dioxide with no change in TA. The TA can rise from evaporation and refill of water since fill water has some TA in it and usually the fill water doesn't affect the pH very much. So a combination of the two can lead to both pH and TA rising over time.

As you mentioned, the curing of plaster can increase both pH and TA. Corrosion of pool plaster (from a low saturation index) can likewise lead to a rise in pH and TA.

Other than the above, the only other thing I can think of is chemicals (e.g. pH Up or Alkalinity Up or Borax or Lye) or stuff that gets into the pool of sufficient quantity that is alkaline.

Since rain can cause sufficient aeration to increase pH levels, I would assume the aeration provided by a spillover spa or sheer descents would have a similar effect. I have lots of wind where I live, and would assume this would have a similar effect, as well.

You are correct. Increasing the rate of outgassing of carbon dioxide will increase the rate of pH rise. So a higher TA, lower pH, or more air-water mixing such as aeration from rain, wind, spillover, fountain, waterfall, shower, splashing, returns turned up, will all increase the rate of pH rise.

Well it sounds like I am up the creek on reducing my acid demand. I have not filled the pool from tap water for over two to three months. The only fill is from rain. I also have not added anything to the pool expcept salt and CYA.

Try dropping your TA to 60 ppm and increasing the borates to 50 ppm.