How much acid it takes to change either PH or TA depends on nearly all of your current water numbers. Changing TA in particular takes a different amount of acid for every starting TA level. If you post a full set of numbers I could make an estimate for you. Or you can figure it out experimentally, add the amount of acid you used to add and see what effect it has and adjust from there.
One thing to keep in mind when adjusting TA is that you are really aiming for specific PH levels and repeating the cycle until TA gets to where you want, rather than trying for a specific change in TA in one shot.
My current procedure has been to lower PH to 7.0-7.2 (which was accomplished with the 60oz acid) and then out gas back to 7.6 I spaced these 2 procedures 3-5 days apart and brought the TA down from 160 to 120 each one would drop TA 20ppm (this procedure by the way works great) The ideal thing would have been to monitor the PH rise and continue to add acid but in the real world time would not permit.
I discovered this site and the information on borates but missed the part about first dropping TA prior to adding borates (lucky me I'am sure HD will appreciate my increased acid quanity purchase) My thought was to do what you pointed out and repeat the above and note the results which I'am assuming I will now take more acid to drop into the lower PH range (Is this correct as I know what assuming does)
My current numbers for reference
Pool 11K marcite
Intermatic SWG :-D (Prior triclor pool)
TA 125-130 was 115-120 prior to borate addition
Flow rate 72 gpm
I am getting right around 60 oz to lower PH from 7.6 to 7.1. The amount of acid is lower because TA has gone down and higher because of the borates. Those two factors appear to more or less cancel out right now.
Thanks for the calculation starting point I will proceed and post my results for future reference.
Would I be correct to assume that out gassing will take longer for the PH to rise now with the borates in?
The outgassing rate is a function of the amount of carbonates in the water and the pH and amount of aeration. If you added borates but didn't lower the TA to reduce the carbonates, then the outgassing rate should be about the same. However, the change in pH from the same amount of outgassing will be less due to the additional pH buffering from the Borates. So the total amount of acid needed to restore the pH over a time period will be about the same, but such acid addition can be done less frequently since the pH won't rise as quickly. You can think of the increased pH buffering as creating "resistance" to pH movement and that resistance applies equally to slowing pH rise from outgassing as it does to resisting a pH drop when acid is added.
If you want to lower the amount of acid you need to add, you need to lower the TA (carbonates) level or target a higher pH or reduce aeration sources (such as by using a pool cover).
HOWEVER, in a pool with an SWG (such as yours), there is an additional effect from the algaecidal properties of the Borates. By keeping algae growth minimal, you may be able to lower your SWG output (on-time) and that will reduce hydrogen gas bubble production (i.e. aeration) which reduces outgassing. Since algae growth is dependent on multiple factors, this is why some people find that Borates let them lower their SWG output time while others do not. If you are able to lower your SWG output time and still maintain the same FC level you had before using Borates, then you will find you add less acid over time.
You can think of the increased pH buffering as creating "resistance" to pH movement and that resistance applies equally to slowing pH rise from outgassing as it does to resisting a pH drop when acid is added.
This is exactly what I was assuming would happen
Richard and Jason thanks again for your assistance.