Calcium = 650!

[jman311]

Hey everyone, first of all want to say what a great thing this forum is. I've been a "lurker" for a bit, trying to get an understanding of all that is pool. I've gotten a handle on the basics (pH, Chrloine, CYA) and upgraded my test kit, now I have a question on my high calcium levels...

My pool is 12,000 gallon PebbleTec with attached hot tub with an overflow into the pool. Aquarite SWG, located in Arizona.

Readings:
FC - 6
CYA - 80
pH - 7.3
TA - 110
Ca - 650
Salt - 3100

I recently drained and refilled half my pool to bring the salt level down (it was too high). Even after that I found my hardness to be at 650! I tested my fill water to be 200.

I have to keep some overflow on all the time, as it only runs on a single pump. Overwise I would get no circulation into the hot tub, or the hot tub drains out. Therefore, I am getting a good amount of aeration every day (pump runs 12 hrs/day) . I'm always adding muriatic acid to keep the pH down.

My understanding is, with that high calcium, I need to keep my pH and TA below average in order to avoid scaling, correct? I normally get a good amount of build up in the SWG in the summer and have to acid-wash. I also have gotten some scaling around the water line of the pool.

Am I correct? Am I ok with that Ca level provided I do this?

Thanks
Josh

[chem geek]

Josh,

Welcome to the forum! I will try and answer your questions for you. If what I say doesn't make sense, then hopefully someone else can help out and make the answers more clear.

You are technically correct that higher calcium increases chances for scaling, but your higher salt (TDS) level decreases these chances and in any event changing the calcium from 300 to 600 does not increase the risk of scaling enough to worry about. In other words, with your numbers you could still operate your pool at 7.5 if you wanted to and not risk scaling. Generally, your pool would start to look cloudy before the scaling occurred in any great amount and you are now far, far away from that situation. So, bottom line, don't worry about scaling.

If you drained and refilled half of your pool water with fill water that has CH of 200, then your original pool water had a CH of 1100! That's awfully high. Do you have any idea how it got that high? Did you ever use Cal-Hypo to add chlorine to your pool, even for shocking (since you have an SWG)? If so, you should avoid using that and use chlorinating liquid or bleach for shocking instead.

Now you said you have seen a lot of scaling around your water line and in the SWG unit, but that was probably before your half drain and refill. With a CH of 1100, you still would not be at risk of scaling unless your pH got up higher, say to 7.6 or so. With an SWG system and with your extra aeration, the pH probably wants to rise a lot so perhaps it did get up high and this combined with the high CH and probably higher TA as well all led to scaling. For example, a CH of 1100, pH of 7.6, and TA of 180 would possibly start to scale.

By the way, what is the TA of your fill water? This will tell me what your original TA was before the half drain and refill.

So now after your half drain and refill, your numbers look good. I assume you have your CYA at 80 because that is what the manufacturer of your SWG recommends and we know that most SWG units require higher CYA to operate efficiently. Your chlorine level is high enough to compensate for CYA's reducing the chlorine's effectiveness.

Now here's an alternative you can try to help lower the amount of acid you need to add. You can follow Ben's Lowering Your Alkalinity procedure to get your TA down to 80 or below (but no lower than 60). With the lower TA, you can definitely safely run your pH higher so can then operate at 7.5 (for a TA of 80) to 7.7 (for a TA of 60) where I'm pretty sure you will find your acid demand significantly lowered. Another thing you can try is to lower the power level on your SWG and increase its "on" time (to keep the same overall chlorine generation amount). The lower power level will give the generated chlorine more of a chance to dissolve into the water instead of outgassing (if the chlorine gas generated by the SWG doesn't dissolve into the water, then the net overall effect is a large rise in pH).

If you try this, let us know if it works (i.e. lowers the amount of acid you need to add). If this works for you, then you can always keep your calcium higher (where it is now) to stay in balance, though as I said before you are now far away from a risk of scaling (or corrosion).

Richard

[jman311]

Richard, thanks much for the reply!

Ok let me answer some of your questions here...

Not sure how my CH got so high. I know that the water here in AZ is notorious for having high levels, but as I stated I tested the fill water and found it to be at 200. I don't normally use Hypo-Cal or anything like that, only liquid chlorine or liquid bleach. However, I did shock the pool when I last drained and refilled it some time ago with something the pool store gave me (it was a powder shock agent, and to be honest I don't know what it was ... it was prior to my "education" I've received here on this forum and I don't know what was in it).

I test my fill water for TA today, it is at 110. I also tested mine pool water's TA again today, and it seems to have gone up to 130. I will of course test it again in a few days to see if it continues to climb.

The intense summer sun is letting up here in AZ so I have just lowered by SWG output anyhow, I will likely lower it again soon. I didn't realize higher levels for shorted periods of time could cause the pH to rise. I will keep that in mind.

I also tested my CYA again today and it's around 65-70. It's possible it's just variance in the testing, but I have had my CYA drop before. Interesting because I had read on here somewhere that CYA levels should never go down unless you drain water from the pool. Is that true?

BTW you mentioned acceptable levels of TA in relation to pH. Is there a chart posted anywhere with these values? It would be nice to have...

Finally, is there a good way to get rid of the scaling that now exists around my water line?

Thanks again for your help!
Josh

[chem geek]

My responses in bold below.

Not sure how my CH got so high. I know that the water here in AZ is notorious for having high levels, but as I stated I tested the fill water and found it to be at 200. I don't normally use Hypo-Cal or anything like that, only liquid chlorine or liquid bleach. However, I did shock the pool when I last drained and refilled it some time ago with something the pool store gave me (it was a powder shock agent, and to be honest I don't know what it was ... it was prior to my "education" I've received here on this forum and I don't know what was in it).

I'll bet the powdered shock was Cal-Hypo, though it seems strange that even this would lead to your very high CH level. If you used enough Cal-Hypo in your pool to add 10 ppm, then this would add 7.1 ppm of CH. So you can see that this doesn't explain your extraordinarily high CH.

I test my fill water for TA today, it is at 110. I also tested mine pool water's TA again today, and it seems to have gone up to 130. I will of course test it again in a few days to see if it continues to climb.

Because your fill water had a TA of 110, that explains why a half drain and refill didn't change your TA which was probably 110 earlier as well. As for the TA climbing, this can occur if the pH went way up, but otherwise it shouldn't climb unless you added carbonate to your pool (as with sodium carbonate soda ash or sodium bicarbonate baking soda). I'm guessing that the increase is a combination of a rising pH and measurement error.

I also tested my CYA again today and it's around 65-70. It's possible it's just variance in the testing, but I have had my CYA drop before. Interesting because I had read on here somewhere that CYA levels should never go down unless you drain water from the pool. Is that true?

That's not exactly true. The CYA does seem to go down very slowly over the summer (I've experienced that myself, even accounting for splash-out which affects CH levels), but goes down much more quickly over the winter in many closed pools. We don't know exactly why though there are bacteria that consume CYA (it's just that it doesn't make sense that such bacteria survive in pools with chlorine). Of course, winter rains overflowing the pool would cause a dilution, but the CYA seems to drop over the winter beyond such dilution.

BTW you mentioned acceptable levels of TA in relation to pH. Is there a chart posted anywhere with these values? It would be nice to have...

This chart will give you a rough idea of the relative outgassing of carbon dioxide as a function of pH and TA, but remember that this is also dependent on the amount of aeration in the pool so this is only a very rough guide. Your SWG will aerate when it is on due to the hydrogen gas bubbles that are produced. Also, the table assumes linearity of the outgassing when that may not truly be the case. Nevertheless, most people find that their pH will rise considerably when in the red region while most people will find that their pH is fairly stable or slowly rising in the green region. The orange region is mixed and varies with the pool (due to variation in aeration). Some people with SWG systems need to be well into the green region (some with TA down to 60) before they see a significant break in the pH rise.

Finally, is there a good way to get rid of the scaling that now exists around my water line?

The simplest way is to wash/wipe/rub it with a dilute acid solution. To be honest, I don't know how strong it would need to be for effectiveness, but I'd play it safe and use a very dilute acid solution and progressively increase strength in small steps until it seems to do the trick. I'd start by using white vinegar (acetic acid) which has a pH of around 2.4 -- the Muriatic Acid is WAY too strong even if you try and dilute it significantly (one ounce of Muriatic Acid in one gallon of pure water gives a pH of around 1.2).

Thanks again for your help!
Josh