Struggling with acid consumption

[brettc]

Hi folks,

I've been reading and learning a lot here already, and I've really enjoyed your discussion board.

I've had my pool for about 6 months. I had a bit of trouble getting the salt levels and SWG sorted out to begin with, but finally that seems to be working ok. Things seemed to roll along ok for a while, although I was adding acid, probably not regularly enough. Then I got to where my chlorine levels dropped, and I couldn't get them back up with the SWG. I supplemented with tablet chlorine and ran the pump longer, and it would come up but then drop down again.

I bought a taylor k-2006 test kit about a week ago. I've added a lot of CYA. The chlorine seems to be holding well now. I am surprised by how much acid I'm actually using now that I'm more closely monitoring and measuring. I'm also running my SWG at 50%. So, I want to talk about my water balance and acid usage. Also from what I've read, CO2 off gassing is an issue. I want to discuss this in context to my large rock water fall, flagstone stepped spillover spa and other water features.

My latest numbers:

FC 6
CC 0
PH 7.4
TA 90
CYA 55 (plus I added 2 pounds yesterday)
CH 260
Salt - it's been 3100-3300 hundred but today with rain this week, it's low 2900-3000
pool temp has been about 89 in the late afternoon, outside of all the clouds and rain last week.

In the last 7 days I added 1 gal 1pt of acid. It rained heavily two days, and one day the waterfall got a lot of use.

So my question is first on chlorine, I assume based upon what I've read that I'll shoot for 70-80 on CYA and see what this does for my 50% output on the SWG. I suspect lowering that may improve my acid use. I'll just see what that does, and if it's not a good idea, rain and splash out here will probably bring it down soon enough. What ever there was at start up in January was completely gone just a couple of weeks ago.

I've read that lowering my alkalinity will help. Well, how low is low? What about the PH? What about CH? What should I be targeting here? What impact does my waterfall have on all this? If I'm constantly fighting a rising PH, and I'm also running that big waterfall shouldn't I continue to expect to fight falling alkalinity as well? It's dropped from 110 to 90 in the last week with rain and one good day of running the waterfall.

 

[chem geek]

So my question is first on chlorine, I assume based upon what I've read that I'll shoot for 70-80 on CYA and see what this does for my 50% output on the SWG. I suspect lowering that may improve my acid use. I'll just see what that does, and if it's not a good idea, rain and splash out here will probably bring it down soon enough. What ever there was at start up in January was completely gone just a couple of weeks ago.

I've read that lowering my alkalinity will help. Well, how low is low? What about the PH? What about CH? What should I be targeting here? What impact does my waterfall have on all this? If I'm constantly fighting a rising PH, and I'm also running that big waterfall shouldn't I continue to expect to fight falling alkalinity as well? It's dropped from 110 to 90 in the last week with rain and one good day of running the waterfall.

The higher CYA may let you lower your SWG output and if that happens, then you will have less hydrogen gas bubble production from the SWG (you can see the bubbles at night with a pool light and the SWG on) which means less aeration which means less of a pH rise and less acid addition.

If you get your CYA up higher, that will raise TA since CYA is a component to TA (it will raise it by about a third of the CYA amount). At 70-80 ppm CYA, you can lower the TA to 80 if not 70 but probably not lower than that. You can also target a pH of 7.7 since a lower TA and higher pH result in a lot less outgassing as seen in this chart that shows relative outgas rates (it's relative because aeration isn't readily quantifiable). The rate of pH rise won't change that much by lowering the TA though it will change by having a higher pH target as seen in this chart that shows the relative rate of pH rise.

Your waterfalls just compound the problem of aeration and you can readily see the contribution they make to the pH rise and acid amounts by turning them off for a time and comparing acid usage. By the way, rain is a great aerator of water as the water drops splash, break surface tension, increase surface area, etc.

You can consider running at a somewhat higher CH level to compensate for calcium carbonate saturation with the lower TA, but if you run at higher pH then you should be fine. If you run at a pH of 7.7 and TA of 80, then a more optimal CH is 400. If you run at a pH of 7.7 and a TA of 70, then a CH of 500 is a little better but not at all a big deal. I would say you really only have an issue if you intend to run at a pH of 7.5 and a TA of 70, but since the lower pH just aggravates the pH rise and acid addition problem, I wouldn't recommend that.

Another option that can be used in conjunction with lower TA is to add 50 ppm Borates to the pool. This adds another pH buffer that will slow down the rise in pH, but it won't change the amount of acid you have to add UNLESS the algaecidal properties of the Borates kill algae instead of having chlorine do that. If the Borates reduce the chlorine consumption, then you can lower the SWG output even more and THAT will reduce aeration and improve the situation greatly.

Another option is to use a pool cover since that reduces aeration -- eliminating it from the rain and virtually eliminating the natural surface outgassing and reducing it from the SWG though with mixed results (depends on where the SWG bubbles end up -- remember that they are mostly hydrogen gas though there may be a small amount of oxygen bubbles as well).

Richard

 

[brettc]

Keep in mind I've only been recording my acid consumption for about a week, and part of the 9 pts of acid I added was catch up from letting PH get too high.

So, I think my plan is to continue watching this. I'll measure CYA again in a few days. It will probably be around 65. I'll see if that does anything to my chlorine levels. I'd expect to see them rising which will prompt me to turn down the SWG setting. I still don't really understand whether it should be 65, 70, or 80. What's the criteria for picking a target in that range?

Other than that, I'll try to hold the water balance where it is and continue to get more realistic measurements on what I'm actually using. That way I'll know better what impact anything I change has on acid consumption.

 

[itabb]

I run my SWG at 10% for 6 hours per day, for now. With use and exposure to sun, I seem to lose about 1FC every 2-3 days, but I am at 4.5FC now and my target is 3FC. So we'll see if it holds at 3FC at 10% or if I have to use 15% or if I have to increase my run time to 8 hours a day. I have an opaque automatic cover which covers the pool when it is not in use.

Just remember that the ammount of chlorine your SWG generates is a function of a number of variables. One of those variables is run time. Another one is water temperature. I recall a chem geek response where PH was another, though I can't recall what was discovered (Mark test?). And of course, salt level has an impact. Oddly enough, CYA does not seem to impact the amount of chlorine generated, just the amount of FC retained in sunlight.

One other thing you can do is suppliment your chlorination with bleach, especially if you need to shock. PH rises less from adding bleach than from running your SWG, and will PH will drop the same amount as FC drops from either bleach or SWG. Based on chem geek's help, I add about 1 or 2 cups of acid a week, right now, and only because my TA is high from the CYA I added (which has since vanished and left a higher TA).

Tablets are stabilized chlorine and will add CYA to your pool as you use them. If you raise your FC by 3ppm with tablets alone, you will raise your CYA by 1ppm. The FC will get used up, the CYA will not.

 

[brettc]

double post

 

[speed151]

I have a solar pool cover on my pool with swg for about 2 months now and My ph is holding at 7.4 to 7.6 with TA 80 so hardly ever have to add acid to the pool now. have not had to add water to the pool as well so its all good with the cover. I do uncover pool a few hours a day to breath just thought that would be a good thing to do.

 

[chem geek]

Just remember that the ammount of chlorine your SWG generates is a function of a number of variables. One of those variables is run time. Another one is water temperature. I recall a chem geek response where PH was another, though I can't recall what was discovered (Mark test?). And of course, salt level has an impact. Oddly enough, CYA does not seem to impact the amount of chlorine generated, just the amount of FC retained in sunlight.

Mark found that the SWG efficiency dropped by 15% going from a pH of 7.5 to 8.0. He did not see a change from 7.0 to 7.5 even though technically I would expect one. Essentially, the SWG should be more efficient at lower pH and less efficient at higher pH, but the difference isn't huge as you can see. Keeping your pool at 7.5 to 7.8 should be fine -- most people don't run it at 8.0 anyway.

 

[brettc]

Here are some new numbers:

FC 3.5 - dropping from 7.5 on 6/24
CC .5 - measured for four days
PH 7.8 - rising from 7.4 on 6/27, added 1pt acid on 6/29
TA 80 - dropping from 110 on 6/23
CH 240 - dropping from 260 6/21
CYA 55 - Stable despite adding 2lbs on 6/26
Salt 3000 - added 35lbs since 6/26
SWG @ 50%
pump run time 8hrs
It's rained heavy for two days straight. High probability for rain every day this week.

I'm not making any progress on the CYA, plus after only three tests, I'm out of reagent. Where's a good place to get more? I've got two more pounds, and I'll add that as soon as it stops raining.

I'm not keeping up on the chlorine. It was dropping a lot faster before I brought up the CYA to 55, but at this point I was hoping it could keep up. I'm running the pump for 24hrs today to bring it up. Why do I have CC, and what should I do about it? Once it gets back up, I'll run it at 60% and see if it can hold it there. Is the rain affecting this too, or is it chlorine use, or SWG output?

The TA is dropping. This is what I was saying before. It seems that if I'm constantly adding acid and running all those water features, I'll continue to fight falling TA. Will this continue after reaching targets of ph 7.7, TA 80, and CYA 80?

CH seems to be dropping as well, although my tests don't seem to be all that consistent. I had 260 on 6/21, 230 - 6/28, 250 - 6/30, and 240 today. Chem Geek said I should target ph 7.7, TA 80, and CH 400. I don't understand this. I look at the slide rule that came with the Taylor test kit. With the temperature at 89, ph at 7.7, TA at 80, it says CH should be 155. Can someone explain Chem Geek's target of 400?

So, bottom line, what should I be doing right now?

 

[Guest]

The Taylor Watergram is a Langelier Saturation Index (LSI) calculator. You did it backwards. You are supposed to set the CH and TA and then read the temp and pH. It doesn't work in reverse. Also the LSI is not to be taken as gospel but only as a rough guide. Chemgeeks suggestion is sound and is based on a different saturation equation.

Also, the watergram uses corrected alkalinity which would be about 60 ppm for you (not exact but close enough)

 

[JasonLion]

I wouldn't worry about CC of 0.5. Given more sunny days it will eventually go away on it's own. If CC goes above 0.5 then worry about it.

The CH test has some variability in the result. Even if you test the same water sample two times in a row you will often get slightly different results. Your level has probably been stable, or nearly so.

Chem Geek is trying to optimize the CO2 level in the water to reduce CO2 outgassing, which is what causes some, hopefully most, of your PH rise. If you get to the targets he has suggested the CO2 outgassing will either slow down dramatically or stop. There is additional rise in PH from plaster curing that we can't do anything about but which will go away on it's own over time.

You can order CYA reagent directly from Taylor or from TFTestkits.com. Leslies web site has a sale on a CYA test kit right now, but it is not interchangable with the Taylor/Ben/TFTestKits version.