Recommended values for a SWG pool

[Charlie]

Hi everyone,
What are the recommended water results for a SWG pool? The reason I'm asking, because most values are assumed for a non-SWG pool.

My current readings:
FC: 4.8
CC: 0
pH: 8
TA: 100
CH: 425
CYA: 60
Salt: 3000
SI: ~+0.5

I know that my pH and TA are on the high side, and I started by lowering the pH down to about 7 or so in order to bring down the TA so I can lower my SI, but searching the net, I can't find any recommended values for a SWG pool.

[Watermom]

Do you have the manual that came with your SWCG? Most of the manufacturer's of those units have specific requirements. Your pH is high, but your alk is ok at 100. Your calcium hardness is pretty high at 425. I wouldn't use any products with calcium in them in your pool. Read through some of the posts in the SWCG section of the forum. Hopefully one of our members who knows more about SWCG requirements than I do will chime in here later today.

Go ahead and post some details about your pool ----- type of pool, volume, type of filter, what you have added to the pool.

[waterbear]

These values work well, minimize pH rise, and keep the water balanced as well in case you have a plaster pool:
FC 4-5 ppm
pH 7.6 (and not lower) to 7.8 (when pH hits 7.8 lower it to 7.6)
TA 70 ppm
CH 350 to 450
CYA 75, 80 or 100 ppm (whichever is the manufacturers maximum recommendation for your SWCG)
Salt, depends on the make of your SWCG. some are 3000, some 3200, some as high as 6000 ppm. It is generally better to run the salt a bit high (by maybe 200 pm rather than a bit low in terms of cell life (but not so high that the high salt shutoff kicks on if so equipped.)
Borates 50 ppm (optional but highly recommended. If you don't add borates then either keep the calcium at the lower end of the range, run the salt a few hundered PPM lower or keep the pH slightly higher to keep the water balanced)

If you would like a more detailed explanation of WHY they work well let me know and I will be glad to post it but I warn you it gets a bit technical.

[waterbear]

Do you have the manual that came with your SWCG? Most of the manufacturer's of those units have specific requirements. Your pH is high, but your alk is ok at 100. Your calcium hardness is pretty high at 425. I wouldn't use any products with calcium in them in your pool. Read through some of the posts in the SWCG section of the forum. Hopefully one of our members who knows more about SWCG requirements than I do will chime in here later today.

Go ahead and post some details about your pool ----- type of pool, volume, type of filter, what you have added to the pool.

Actually, using an estimated pool temp of about 85 deg (which gives a SI of about +.5 with these numbers) it would be better to leave the calcium alone, drop the TA to 70 ppm, raise the CYA to 80 ppm (both of which are much more doable than lowering calcium) and then maintaining the ph at 7.8 wihich would, at the same temperature, give a SI of .01, essentially perfect for a plaster pool and would also help minimize pH rise in a salt pool.

[Watermom]

See --- I knew someone who knew more about SWCG would be along soon. Thanks, Evan!

[Charlie]

Thanks Evan. BTW, my water temp is 86°F. I wouldn't mind knowing why those values will work well when you have the time.

I've been battling the pH rise since I have a SWG pool and a spa spill over. In one week, it can go from 7.4 to 8.

I have a Pool Pilot for the SWG. Its about 5 years old now and I'm probably going to replace the cell sooner or later. I believe the manual says they last about 5 years.

Pool size: ~14,000 gallons
type: plaster with pebble tech finish
filter: DE (using the alternate substance instead of real DE - sorry don't remember the exact name. Supposedly it filters better and when back washed, 95% of it comes out compared to DE).
what I add to the pool: normally only acid and sometimes choline if I measure it and its low.

[waterbear]

Thanks Evan. BTW, my water temp is 86°F. I wouldn't mind knowing why those values will work well when you have the time.

I've been battling the pH rise since I have a SWG pool and a spa spill over. In one week, it can go from 7.4 to 8.

OK, I will explain. These values will help minimise the pH rise and give you better pH stability:

FC 4-5 ppm
This one is easy. It has been found that keeping the FC at about 5% of the CYA in a salt pool avoids problems with algae blooms and the need to shock the pool so if the CYA is at 80-100 ppm (and I will explain the reason for that below) then adjusting the cell output to maintain 4-5 ppm FC keeps the FC high enough for the CYA level.

The next three are linked to pH rise from outgassing of CO2, which is the main cause of pH rise in a salt pool (assuming there is no new curing plaster). The recommended levels done together will slow pH rise from outgassing AND keep the calcium saturation index in acceptable range for plaster pools. Since you have a spillover spa the additional aeration it creates speeds up the outgassing of CO2 and the pH rise it causes so these recommendations are very beneficial for your pool to help slow pH rise.

pH 7.6 (and not lower) to 7.8 (when pH hits 7.8 lower it to 7.6)
The lower we put the pH the faster it rises because we convert more bicarbonate in the water (which is what TA really is) into carbonic acid (which, for our purposes, is the same as carbon dioxide dissolved in water. The operative word here is acid, so keep that in mind.) As the pH drops there is more carbonic acid in the water. Carbonic acid is essentially seltzer. If we let it go flat (CO2 outgasses) then the pH rises (less carbonic ACID). the more carbonic acid (carbonation) the faster it will go flat to reach equilibrium. So, the lower we put pH the more carbonic acid we form and the faster it leaves the water and therefore the faster the pH rises. Keeping the pH no lower than 7.6 and not letting it reach 8.0 slows the pH rise AND also keeps the calcium saturation index in an acceptable range for a plaster pool along with the TA and CH numbers recommended.
TA 70 ppm
Lower TA means less bicarbonate ions in the water to be converted into carbonic acid which means slower outgassing of CO2 which means slower pH rise.
CYA 75, 80 or 100 ppm (whichever is the manufacturers maximum recommendation for your SWCG)
Higher CYA means less cell on time is needed to maintain the FC at target level (lower output percentage/power level) which means less formation of hydrogen in the cell which means less aeration of the water from hydrogen gas generation which means less outgassing of CO2 which means less pH rise.
CH 350 to 450
This will keep the calcium saturation index in a balanced range for the above parameters.
Salt, depends on the make of your SWCG. some are 3000, some 3200, some as high as 6000 ppm. It is generally better to run the salt a bit high (by maybe 200 pm rather than a bit low in terms of cell life (but not so high that the high salt shutoff kicks on if so equipped.)
Borates 50 ppm (optional but highly recommended. If you don't add borates then either keep the calcium at the lower end of the range, run the salt a few hundered PPM lower or keep the pH slightly higher to keep the water balanced)
Borates, besides having a beneficial effect on water balance if the calcium is high, also add a secondary pH buffer system to the water that works in conjuction with the bicarbonate/carbonic acid buffer we call TA and works in the opposite directions. While the bicarbonate buffer wants to move the pH upward toward 8.2 ( because of outgassing of CO2) the boric acid/borate buffer want to move the pH down and these together tend to "lock" the pH at around 7.6-7.8 for an extended period of time. For example, when I applied these parameters to my pool my acid use went from about 24 oz. (3 cups) about every 10 days to 2 weeks down to about 6-8 oz every 6 weeks to 2 months! I have a fiberglass pool that is chemically inert so my results are more dramatic than in a plaster pool but even a plaster pool will have better pH stability follwing this guidelines than without.

I DID warn you it gets a bit technical.

I do want to add that since you have a plaster pool, you might have to tweak a few of the parameters to compensate for such factors as whether you have borates, your temperature (which often changes with the seasons if the pool is open all year or most of it), and your salt level. The easiest way to do this is by making small changes to where you put the pH.
For example, in YOUR pool, with a temp of 86 deg and a CH of 425 and salt of 3000, by dropping the TA to 70 ppm and maintaining the pH between 7.6 and 7.9 your SI stays between -.17 and +.11 (essentially perfect)
By adding borates to 50 ppm your pH range increases from 7.6 to 8.2 with a SI of -.25 at 7.6 to -.04 (essentially 0 or perfectly balanced) at 8.2 pH. At pH of 7.7 (where the pH tends to sit for an extended period of time) the SI is -.18 and as the pH rises to 8.0 the SI approaches ) so you will not have scaling conditions, which is excellent for SWCG pools and the very slightly negative SI is still well within acceptable parameters for plaster pools. Even with a + or - change in pool temperature the SI is still WELL within acceptable parameters for a plaster pool (+/-.3) so the water stays balanced.

[Charlie]

Thanks for the great info.
Today, the pH is now 7.2 and the TA is 80. I'm trying to get the TA down so when the pH is 7.6 to 7.8 the TA will be within range. This is probably going to take a few weeks to bring it down by lowering the pH?

[waterbear]

Not al all. Just aerate the water (your spillover is a great way if you turn on all the aeration your spa has such as venturis and bubblers and blower if you have one until the pH is up to around 7.6. It won't take long. Lower it back down to about 7.0 and test TA again and aerate to bring it back up. Repeat until TA is at 70 ppm. If you are at 80 ppm it might only be one more cycle. If you overshoot then add baking soda to bring it back up, wait about 24 hours for things to stabilize and then adjust pH down a bit if needed. In your 14k pool one pound of baking soda will raise your TA about 5 ppm so I would add 1 lb at a time (convenient since it comes in one lb boxes!) wait 24 hours and retest TA and pH until it it at 70 ppm.

Adding acid lowers TA (and pH) aerating brings pH back up with NO effect on TA.