Calcium Hardness rises rapidly

Total hardness and calcium hardness in pool water.
Scale, calcium buildup, hard water and scaling problems.
photogold

Calcium Hardness rises rapidly

Postby photogold » Fri 27 Mar, 2009 17:49

I have an ongoing problem with rapidly rising calcium hardness, and I have been refilling my pool every year. I never add chemicals that contain calcium. Our tap water is about 160, so it's a little hard, but not too bad. After refilling our pool last May, our CH was about 300, by the end of summer, it was 500. Now it's about 650, and I'm ready to refill it (again). It's interesting that the CH seems to rise even in the winter when I'm not adding tap water. In fact, it's getting filled with soft water from the rains, so you would think the CH would come down a little, if anything.

We have gunnite pool. I'm getting moderate calcium deposits on my tiles. The other chemistry is not too bad, but the TA tends to drop over time, and the pH tends to rise. Right now the TA is at about 60. I'm hoping that by keeping the TA low, it may reduce calcium deposits. I'm thinking of adding a chemical to reduce calcium deposits (a sequestering agent?)

Where is the calcium coming from? How do I stop it from building up?


chem geek
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Calcium Hardness rises rapidly

Postby chem geek » Fri 27 Mar, 2009 20:02

By any chance, did you have your pool replastered? Curing of plaster will cause both the CH and the pH to rise. This can continue for up to a year, sometimes even longer (though the first month or so is the most extreme). If you are adding acid or using an acidic source of chlorine such as Trichlor, then the pH won't rise as much (or could even drop) but the TA will drop over time. That sounds like what you are seeing.

Evaporation and refill will increase the CH and usually the TA as well. It essentially adds to the pool water whatever is in the fill water because evaporation only removes water and not calcium or bicarbonate. Evaporation occurs at night as well as day and also during the winter. It's a function of temperature and relative humidity so you lose water from evaporation even at night or in the winter if the pool water is warm. It's only if the water gets cold in the winter when such evaporation slows down significantly.

As for rains diluting the water, that only occurs if it rains enough to spillover pool water to a drain. If it only fills the water a bit, but then evaporation brings it back down, then this won't change the pool water chemistry (CH and TA should remain stable in this case).

Richard
Me...
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Calcium Hardness rises rapidly

Postby Me... » Fri 27 Mar, 2009 21:42

Just for fun, what chemicals do you use?
chem geek
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Calcium Hardness rises rapidly

Postby chem geek » Sat 28 Mar, 2009 02:01

I see where Me... is going. Specifically, what is your source of chlorine? By any chance, it's not Cal-Hypo is it?
photogold

Calcium Hardness rises rapidly

Postby photogold » Mon 30 Mar, 2009 13:55

Yes, the pool was replastered about 7-8 years ago, or a year or two before we bought this house. But the first few years, the CH seemed fairly stable. It is the last 4 years that we have noticed rapidly rising CH.

I know that some clorine has calcium (powder type), but I make sure that we don't use that. We use mostly the triclor tablets, and we use some liquid clorine (forgot the chemical name) when needed.

One curious detail. I just brought a water sample to Leslies Pools yesterday for a free test, and they measured only 380 for CH. That's much less than I expected. That would be strange, since Leslies measured 450 about six months ago, and I measured about 530 about two months ago.

Leslies said that some old chemicals may have given bad results. But even if the chemicals were old, they still showed rising CH over the summer. I'm going to take a sample to a different store and see if I get something close to 380. Since I bought the test kit from Leslies, maybe we both had bad chemicals.

Over the winter, I drained about 6 in from the pool from excess rain, and didn't add any tap water. But I wouldn't think that would lower CH significantly. That would be great if I didn't need to refill the pool this year. We have water rationing in northern California.
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Calcium Hardness rises rapidly

Postby chem geek » Mon 30 Mar, 2009 14:57

First of all, get your own good test kit so you can find out the truth about Calcium Hardness (CH) and other water parameters. Get either the Taylor K-2006 at a good online price here or the TF100 here with the latter kit having 36% more volume of reagents so is a little less expensive "per test". If the test kit from Leslie's was rebranded but uses Taylor drop-test chemicals, then your CH test should be OK. If it's test strips, then that's not good (and only measures Total Hardness, not Calcium Hardness).

If you have metals in the water, then the CH test can give you a fading endpoint. To avoid that, add some (say, 5) titrant (hardness reagent, R-0012) first and mix before you add the 20 drops of calcium buffer and 5 drops of indicator dye. You then add the hardness reagent as usual, but then add the count of those initial drops to your total count of hardness reagent you added.

If your pool is uncovered, then the evaporation rate (see this map ) in northern California (Bay Area) is around 50" per year. If your pool has an average depth of 4.5', then about 93% of your pool water volume evaporates in a year and gets refilled adding to the CH, but that would only explain an increase of around 150 ppm and only if you didn't have much water dilution, such as having a cartridge filter so no backwashing. The rains would only dilute the water if they overflowed the pool (if they just fill up a bit and then evaporate, then there is no dilution).

Well, you've got a mystery. It's possible for the plaster to be dissolving if your water chemistry is out of whack (very negative saturation index), but that seems unlikely unless your pH is very low.
photogold

Calcium Hardness rises rapidly

Postby photogold » Tue 31 Mar, 2009 13:22

chem geek,

Thanks for your comments. Yes, my test kit from Leslies uses Taylor reagents. I'm out of the CH reagents. How do I know that I'm getting fresh reagents? If I buy them from Leslies, I may ask them to double check that data from my reagents matches theirs before I take them home.

I didn't realize that there is that much evaporation in the bay area. I was thinking it was only about 24"/year, about 1"/week during the summer. Yes my pool is uncovered. It is a free form pool with a waterfall, so it is difficult to have a cover. but I was thinking of getting a few of those round, floating solar covers.

I do have a filter with a cartridge and it doesn't need backwashing. Can you explain how that can effect CH? Does backwashing remove some of the calcium? What about when I wash my cartridge filter?

Our tap water in San Jose comes from different sources, and they can change sources over the year.
http://www.sjwater.com/quality/report.jsp

The water can vary from 129-532. I measured my tap water at 170, but maybe during the summer, when I'm filling the pool, they are changeing to a harder water source.

I'm planning to try Scaletec Plus to remove and control the calcium buildup on the tiles. Does anyone have experience with that?
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Calcium Hardness rises rapidly

Postby chem geek » Tue 31 Mar, 2009 17:57

You can buy replacement reagents relatively inexpensively here or directly from Taylor online. I don't know how you can make sure getting them from Leslie's they are fresh except to read this though it doesn't say much.

Evaporation is actually higher than shown on the map since pool water is usually warmer than the pan evaporation tests they are using (that have water with temperature similar to average air temperature). The Bay Area doesn't have high humidity and evaporation is largely a function of the amount of water content in the air relative to its maximum (so a function of relative humidity and temperature so drier air creates faster evaporation). Wind also plays a significant role.

A waterfall is going to significantly increase the evaporation rate since the rate is also a function of the surface area of the air/water boundary. A waterfall is also going to tend to make the pH rise or if you are using an acidic source of chlorine then the pH won't fall as quickly (though the TA will drop over time in this case).

Backwashing removes everything in the pool including calcium. So it dilutes the water and the fill water then adds whatever it has in it to the pool. So if the CH is lower in the fill water than the pool, backwashing will lower the CH as will any other sort of dilution such as partial drain/refill. This is different than evaporation where only water gets removed so fill water adds CH directly to the pool (i.e. it always increases).

Richard

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