pH constantly rising, normal or not?

[alpha1]

I have a 22k gal SWG pool that is 6 months old. I typically need to add half a quart of muriatic acid per week to keep the pH in the 7.4-7.6 range. Is this normal?

I understand that with a SWG, there is also production of sodium hydroxide which raises the pH, and that new plaster interacts with the water in releasing calcium carbonate and calcium hydroxide which also raises the pH. Is the amount of pH rise that I'm seeing and the muriatic acid correction that I need within the normal range?

On a related topic, during my weekly brushing of the pool there is still a noticeable amount of "plaster dust", even after 6 months. For my pebble finish, how long should I expect for it to completely "cure", and does the presence of plaster dust at 6months indicate that the finish is still not completely cured?

Today's chemistry: FC=1.8; CC=0.2; pH=7.4; CH=260; TA=100; salt=3,600

 

[JasonLion]

Some PH increase is unavoidable during the first year after the plaster/pebble surface is applied. In addition to that you are getting excess PH increase because your TA is too high. You should let TA come down to around 60-70, which it will tend to do naturally over time due to the acid additions required to maintain the PH.

What is your CYA level? We recommend CYA be between 70 and 80 for use with a SWG. Lower CYA levels can aggravate the PH increases and shorten the lifetime of the SWG (because it needs to work harder to keep up with the extra chlorine demand).

By the by, it isn't really worth measuring FC and CC in steps of 0.2. You don't need the extra precision, measuring by 0.5s is fine, and it uses up the reagents fairly quickly.

 

[waste]

Welcome to TFP!!

Your situation sounds pretty normal

How many returns do you have in the pool? The SWCG might constantly cause a pH rise if there are only a few of them.

I'm not the expert on this ... but, the SWCG produces hydrogen bubbles and the airation that they make causes the pH to rise. One answer is to lower your T/A (total alkalinity) to ~ 50.


Your curing plaster is exacerbating the issue, especially while you see plumes of dust while brushing.

The folks who are experts on this will chime in with more info, but I've given you the basics :-D

{edit} Jason responded whilst I was, please listen to him! But also PLEASE post your pH, TA, calcium hardness and CYA #s {/edit}

 

[alpha1]

Jason -- Thank you very much for your reply. I will let the TA come down. I was told by my pool builder to keep the TA between 80-120ppm, so I thought right in the middle would be good For my education, why the difference (lower TA) for a SWG pool?

My pool builder also told me that I don't need CYA for an indoor pool. Would you disagree? I tested for CYA and it read less than 30ppm (no cloudiness at all). I was on trichlor for a month prior to converting to SWG, so there's probably a little bit of CYA in the pool, but not much. The pool pump runs 24/7 (in conjunction with the Dectron dehumidifier) with the Intellichlor set at 4%, giving me a pretty stable FC of 1.5-2.0.

I tested at the 0.2 increment this time because my first test this morning at 0.5 increments gave me FC=1.0, which is very low for my typically stable settings. So I tested again at the 0.2 increments at got my more typical FC level (usually 1.5-2.0). I must have done something wrong with my first test. You're right that I don't need to use up my reagents two-and-a-half times faster!

Why is it that for either the 10cc or 25cc test volumes, the Taylor K-2006 kit says to add the same two scoops of R-0870 powder reagent? It seems as though I should be able to use less than half as much for the 10cc test volume?

 

[chem geek]

See Water Balance for SWGs. As was noted above, get your TA down to 70 ppm, get your CYA up to 80 ppm [EDIT] WRONG! not for an indoor pool which should have around 20 ppm CYA or no more than 30 ppm CYA [END-EDIT], target FC at 4 ppm and a pH of around 7.7. Also, if you have any waterfalls, spillovers, fountains, etc., try turning them off since aeration increases the rate of pH rise. Once you get things stable, you can see if you need to increase your CH level to balance the water, but don't worry about that right now.

As for your explanation of sodium hydroxide produced from the SWG, that is not the reason for the pH rise since the chlorine gets consumed/used and that is an acidic process. See this post for a technical explanation if you are interested.

 

[alpha1]

Ted - Thanks for the welcome and for the information. Your post came up while I was replying to Jason. I have 6 returns, but 4 of them seem to have the greatest amount of flow. I will let my TA come down, as you and Jason both advised. Why is it that a SWG pool should have a lower TA?

Would you give it about an entire year for the plaster/pebble finish to cure, and for the plaster dust to "go away"?

 

[alpha1]

Thanks chem geek for the links and advice. Good reading.

 

[chem geek]

Total Alkalinity (TA) is mostly a measure of the bicarbonate in the water. Pools are intentionally over-carbonated in order to provide pH buffering and to provide carbonates, which along with calcium, protects plaster surfaces. However, such over-carbonation leads to carbon dioxide outgassing and that causes the pH to rise with no change in TA. As shown in this chart, you can see that the water is further out-of-equilibrium with the air when the TA is higher and the pH is lower. Even with a TA of 70 ppm and a pH of 7.7 there can still be carbon dioxide outgassing, but it will be a lot slower. The rate of outgassing is actually proportional to the square of the TA (the table doesn't show that since it shows the over-carbonation amount, but not the outgassing rate).

Think of a carbonated beverage. If you were to stir it up causing aeration, you would make the drink go flat. That is because carbon dioxide is outgassed more readily. If you were to measure the pH, you would find that the carbonated beverage was low in pH, but when you stirred it up the pH would rise.

 

[alpha1]

Chem geek -- thanks again!

 

[ping]

My pool builder also told me that I don't need CYA for an indoor pool. Would you disagree? I tested for CYA and it read less than 30ppm (no cloudiness at all). I was on trichlor for a month prior to converting to SWG, so there's probably a little bit of CYA in the pool, but not much. The pool pump runs 24/7 (in conjunction with the Dectron dehumidifier) with the Intellichlor set at 4%, giving me a pretty stable FC of 1.5-2.0.


Why is it that for either the 10cc or 25cc test volumes, the Taylor K-2006 kit says to add the same two scoops of R-0870 powder reagent? It seems as though I should be able to use less than half as much for the 10cc test volume?

Your CYA DOES NOT need to be raised to 70-80 for an indoor pool. I believe it should be 20-30 for an indoor pool. The others here should comment on this.

As for the plaster dust, it should have been completely gone by now. The plaster should be cured enough to not make any more dust after the first month. Since your pool is 6 months old I think you need to use a manual vacuum to get rid of the dust. The drains are not powerful enough to suck it all in and the dust just goes into suspension and settles again after each brushing.

You need enough R-0870 powder to lock up the chlorine in the sample. If you don't have enough powder it wont lock up all the chlorine for the test, but having extra powder in the test does no harm.

 

[chem geek]

ping is right. Sorry for not catching that you had an indoor pool. An indoor pool not exposed to the UV in sunlight doesn't need a higher CYA and should instead have a lower CYA more like 20 ppm. You generally need a somewhat higher active chlorine level for an indoor pool so 20 ppm CYA should let you do that (30 ppm is OK, but I wouldn't go higher than that for an indoor pool). Your target FC can be in the 2-4 ppm FC range (use the higher end of the range if you find that Combined Chlorine readings tend to be higher). Note that indoor pools often need supplemental oxidation since they are not exposed to sunlight. Some people find that UV systems help in this situation.

 

[alpha1]

Thanks ping and chem geek. I have been troubled by the plaster dust for a while. I have vacuumed the entire pool floor once, two months ago, in hopes that it would decrease the dust. No change. I see a cloud of dust as I brush the walls from the top down -- seems to be coming off the wall, rather than from the floor.

I've read and re-read JasonLion's thread on chemistry testing. Very helpful - thanks.

I appreciate all the advice for you knowledgeable folks. I'm just 6 months into pool ownership, trying to figure it out as best I can, to keep the pool in good condition for as long as possible.

 

[supertune]

Hi alpha1,
Do a search or it might be in Pool School that you could use some DE in your cartridge filter to catch finer debris for when you do a vacuum to help insure to catch it up in the filter and not have it just go back out the returns. :idea:

Chuck

 

[alpha1]

Thanks Chuck. I will.