CH = 400 too high for a new gunite/pebble pool?

[joelq]

Hi all - we have a new 12000 gallon IG with SWG, started on August 1st. I've been testing at least once each day with a Taylor K-2005, and below are my numbers as of 30 minutes ago:

FC: 4
CC: 0
TC: 4
pH: 8.0 (as it always is in the morning, so I have to add 40 oz of muriatic every morning).
Alk: 90
CH: 400
CYA: 55
Salt: 2900 (according to the Aqualogic PS-8), 3200 (according to Taylor salt test kit)

I noticed that my CH has risen since start-up (was 140 at startup, 350 ten days later, and now 400.) My Aqualogic manual says the range for my CH should be 200-400, so now I'm at the high-end of that.

I've read many posts saying it's bad to have too low calcium for a gunite pool, but what happens if it's too high? Is 400 too high? What's causing it to keep rising?

Thanks everybody!

[KurtV]

It's the plaster dust coming off the new plaster that's making your CH rise. It's probably not a problem at it's current level but may become one (can cause scaling) if it goes much higher (especially at higher pHs like you're seeing). It sounds like your fill water is relatively soft though, so if need be, you'll be able to do a partial drain and refill to lower CH once your plaster stabilises.

Edit: Joel, I just re-read the title of your thread. Do you have actual pebble-tec or one of the knock-offs? It was my understanding that pebble-tec does not leach calcium.

[joelq]

Hi Kurt - thanks for your reply. We have Stonescapes (and not PebbleTec).

[KurtV]

Joel,
I'm not familiar with that particular product but I'm guessing that it is plaster based and is the cause of your rising calcium hardness. Have you asked the builder or plasterer about it?

[dawndenise]

Any chance you were using calcium hypochlorite as your chlorination source prior to your SWG going online?

[joelq]

Kurt - I haven't asked the builder yet, but I guess that's the next logical step. I'm guessing though that Stonescapes isn't plaster-based as they have taken the pebble all the way out of the water for our beach entry. They also gave us an option not to do waterline tile specifically because of this reason (although we went with water line tile to have that "finished" look). Wouldn't plaster have to remain underwater?

DawnDenise - I was using bleach exclusively prior to firing up the SWG, so no calcium hypo.

Thanks!

[KurtV]

Joel, According to NPT's website (http://www.nptgonline.com/nptg_site_west/nptg_site/finishes_folder/stonescapes_contractor.html), StoneScapes is plaster based.

[Watermom]

I've read many posts saying it's bad to have too low calcium for a gunite pool, but what happens if it's too high? Is 400 too high? What's causing it to keep rising?

Thanks everybody!

Scaling can be a problem as was mentioned above and also you may have trouble with cloudy water.

[chem geek]

A few things don't make sense to me about what's going on here. If the source of the rising calcium in the pool water is coming from the pool surfaces, then this should slow down and stop when it reaches equilibrium. With the numbers posted, the pool water should be in perfect balance at a pH of around 7.6 and at 8.0 the pool water should already be over saturated with calcium carbonate. I can understand the initial rise from 140 to 350, but do not understand why it is continuing to rise.

The other piece of information that does not makes sense is that adding 40 ounces of Muriatic Acid to a 12000 gallon pool with a TA of 90 and CYA of 55 should result in a drop in pH from 8.0 to 7.12 -- is that what he is seeing? So the pH rises from 7.2 to 8.0 over 24 hours and creates a daily acid demand of 40 ounces of Muriatic Acid every day? This seems extraordinarily excessive to me and since the TA is already somewhat low (a good thing to avoid carbon dioxide outgassing), it would appear that there is an additional strong source of base in this pool. I suspect that the SWCG system is somehow faulty and possibly not dissolving the generated chlorine gas into the water properly, though it would take generation of 6.5 ppm of such chlorine per day to explain the numbers (and that seems too high).

Are the posted numbers correct? Especially the 12000 gallons and 40 ounces of Muriatic Acid daily?

Richard

[PatL34]

I think there is a combination of things occuring here that are creating high pH and high CH.

The aggregate is still curing, and causing the pH to rise. With a pH of 8, the SWCG is at its minimum output regardless of the output setting. Therefore I can well understand the 40 oz of acid having to be added each day at this stage of curing, but in this case it does seem excessive. However the SWCG is still producing 4 ppm Chlorine, so as Richard says, something is out of whack.

I have just been going through this with my Diamond Brite replastering in my 20,000 gal pool, which may not be quite the same as Joelg's Stonescape. Not being familiar with Stonescape, it may be that the leaching occurs faster with this product.

Another set of numbers from a pool store might help to qualify the initial set.

Pat

[chem geek]

Thanks Pat. I forgot about the curing process which produces calcium hydroxide, Ca(OH)2, which of course adds calcium to the water and is highly alkaline. Doh! Thanks again.

Richard

[joelq]

Hi all - thanks so much for all the help. I should have paid much more attention in Chemistry class! :)

Here are my numbers as of 15 minutes ago:

FC: 3
CC: 2 (had 7 teenagers in the pool last night so will need to add some bleach! :eek: )
TC: 5
pH: 8 (as is usual for my morning water)
Alk: 90
CH: 425 (although using a different method from yesterday - I followed the high-calcium procedure for the Taylor K-2005)
CYA: 55
Salt: 2900 (according to the Aqualogic PS-8), 3200 (according to Taylor salt test kit)

A couple other details on how I've been testing: when I test the pH first thing in the morning, it is always at 8 or slightly off-scale. I put in 40 oz muriatic, and I re-test around 4 PM and consistently get a pH of 7.4. Does this make sense? As for the water volume of 12000, I'm pretty comfortable with it as I read the water meter before and after the pool was filled, and that's how I derived the figure of 12000. I'd say give/take around 1000 gallons to take into consideration our normal household water use.

I've also done some tests with Lamotte test strips, and the numbers are consistent with my drop test kit. I'll take some water over to Leslie's as well.

Assuming my CH readings are correct, however, should I do something to lower it now?

Thanks everybody!

[joelq]

Hi all - just went to Leslie's and got the following results:

FC: 5
TC: Not tested (didn't notice until they dumped all my water out)
pH: 7.4
TA: 100
CH: 380 (using the 10ppm/drop method)
CYA: 60
Salt: 3300

I also ran a test with the same batch of water I took to Leslie's and got the following results:

FC: 3
CC: 2
TC: 5
pH: 7.2
TA: 70
CH: 375 (using the 25ppm/drop method)
CYA: 60
Salt: Didn't test

Notice the drop in pH after I put in 40 oz of muriatic at 1.5 hours ago.

Maybe I need to tighten up my testing? Specially the CH testing?

[PatL34]

When you backwash, see if you can use softened water to refill. It is about the only way to bring it down, other than using chemicals used for that purpose. CarlD or Waterbear may help you here.

Pat