SI and Calcium Scale

[dvcavall]

Just thought I would voice my experience with using the SI and water balance to prevent calcium scale. Where I live in Southern California, pools can easily build to a CA hardness of 300-400ppm. I have read in more than one training certification manual that by keeping pH, TA in range (pH 7.2-7.6, TA 80-100 non-swg) and the water balanced by calculating the SI and keeping parameters to an outcome of -0.3 to +0.3, that scaling can be avoided even if CA is much higher than 400ppm.

My experience with my own pool, where chemistry and SI are both kept within range is that those statements are false if taken at face value. A typical 20k gal pool usually has areas that are exposed to a great deal of sun, and areas that are mostly shaded from the sun. Even though both pH and TA are kept within range within range, (pH 7.5 or under, TA around 80) and my SI is usually around 0.1, my pool with a calcium hardness level of 359ppm today, and usually around that number, can build significant scale on the sun exposed tile lines and water features within a month! As we all know, temperature is a factor of the SI, and I suspect that a contributing factor is what I will call micro-climates. I suspect that the sun (100+ temps) heats the surface water at theses areas to higher temps than the other areas of the pool, and that the greater evaporation of the water at these areas is a major cause. However, whatever the cause, at least in this area of the country, if you want to suppress calcium scale, you need to keep calcium levels as low as possible, that number being around 200ppm. Our source water here tests at a CA of 122ppm and a pH of 7.6.

So, IMHO, no matter what the books say, in areas where scale is a problem, keeping CA as close to 200ppm as you can will better your chances for avoinding scale. At high CA (300-400ppm) and sunny conditions such as what we have in So Cal and Arizona, you can do everything else right and you will still have scale problems. I have not tried using sequestering agents aimed at helping to prevent scale, but I think they may be worth a try as well.

 

[duraleigh]

My CH stays around 250ppm and I have never had scale in 8 years. Others will be along soon to describe MUCH higher CH levels over a long period without the use of sequestrants.

Typically keeping your parameters within the guidelines we suggest is all that is required.

 

[chem geek]

My CH is is at 500 ppm and I don't get any scaling, but my TA is lower and I keep the pH from getting too high as well. It's not just high CH, but the combination of CH, TA, pH and temperature (and TDS in the opposite direction) that cause scaling. What you are describing at the tile line and water features is more about what happens when there is evaporation which concentrates all chemicals. So it's still the CSI, but it's higher when the chemicals are more concentrated and when the temperature is hotter. So keeping the bulk water CSI a little lower might help, but you don't want to go too far that way or else you start risking the plaster that is constantly exposed to the bulk pool water.

Also, +0.3 CSI means that the product of calcium and carbonate is twice that at saturation. We have definitely seen spas with that kind of CSI that can develop scaling. It's in pools at lower temperatures where it usually isn't seen until +0.7 or so, but that's on submerged pool surfaces, not above water-line or areas that get evaporation where it doesn't take a high CSI. For areas of evaporation, the CH and TA levels will largely limit the amount of total calcium carbonate that can remain when the water evaporates and some of what remains may be other salts (mostly sodium chloride and calcium chloride). Even a negative CSI won't prevent salt precipitation (including calcium carbonate) from evaporation.

 

[dvcavall]

Thanks for the replies. The scale I am seeing is more than salt. Using strong acid cleaners or even pumice (which I don't like to use because it is harder than the tile glazing), barely removes it with a lot of elbow grease. As I said, all the tile lines in the shaded areas do not have the scale. Only the the areas where the sun shines all day long. I agree evaporation is a part of the issue, and my TDS is high by most measures (no swg) at over 2k ppm. If I had a CA hardness of 250, I don't think I would have the issue I am having. Since water conservation is big in So Cal during the summer months, I need to wait until the rains come to drain it down and see if I can get the CA around 200, lowering TDS at the same time as well. My TDS is mostly salt since I have been using liquid CL since before summer. I stay away from Dichlor and Trichlor since I don't need anymore CA. In this area pH rises, so keeping pH below 7.5 takes a lot of acid, but I am going to try to keep it lower, even though the guides say that a pH of 7.5, TA of 70-80 at around 80 degrees water temp should not be scaling.

I'm not surprised that some of you do not have scaling problems following the guidelines. As they say, every pool is unique, and there could be other factors at play, but I thought it was worth noting that there are always exceptions to the rule. After having the calcium removed from my tile through soda blasting, seeing the scale form again a month later in those sunny areas was a real downer. Since the tile cleaning, I check my chemistry every other day, and all parameters are always within "range", except my TA goes a little low now and then since I add quite a bit of acid. So until I can dilute my calcium and TDS levels, I think that keeping pH and TA toward the lower end of the scale may be best. However a lower TA will make it harder to keep pH low as well.

IF....scaling is a problem, then I believe keeping CA as close to 200ppm as possible is far, far better than letting it get higher and think you can prevent it with chemistry and the SI alone. A higher concentration will have a higher potential to form deposits than a lower concentration. Just seems like common sense.

 

[Richard320]

Try CH at 775, which is what I got a few days ago! No marks on the tile, and scale is visibly reducing. I keep pH around 7.3-7.4 and TA has been 50-60 for months.

I think you're seeing the end results of evaporation rather than scale forming from oversaturated water.

 

[chiefwej]

My fill water has a CH of around 350 and with Tucson desert temps and very high evaporation rates I've seen levels approaching 1000. I did have some scale before I lowered my TA down around 60, learned to keep a close watch on my pH, and keep my CSI very slightly on the negative side. Over a period of time all of the scale disappeared. I still get a line around the pool at the waterline, which I think is un avoidable but no scale on any surfaces below the waterline.

 

[dvcavall]

If we all agree that the scale I have (all of it is at or above the water line) is from evaporation, then I think that reinforces the strategy of keeping CA and TDS low. A lower amount of minerals in the water per volume, should translate into fewer mineral deposits left behind through a given amount of water evaporation. I am gong to try to keep pH and TA at the low end of the scale also, as my pool rarely sees a bather (great investment for my family). To do so I will probably go through at least four gallons of Miuratic a week and maybe more, so it could get pricey. Presently, I am using about two gallons a week to keep pH at around 7.5. TA goes down with the miuratic as well, and I need to add pH+ now and then so my pH is more stable and predictable. Not sure how pH will react to adjustments at a TA around 60? Sounds pretty low..at least according to the conventional wisdom?

 

[chem geek]

Let us know how it works out. It would be good to know which is more effective -- a lower SI (including pH and TA) or a lower CH with respect to evaporative scale.

 

[chiefwej]

The reason I lowered my TA was to control the constant pH rise. Then adding borates further stabilized it. Now I seldom add acid and the pH mostly stays in the 7.5-7.6 area With a TA below 90.

 

[onBalance]

I agree with those who believe it is from evaporation, rather than the heat of the water changing the chemistry. A deposit below the water line would generally be calcium carbonate scale, and that can be removed by acid. A deposit above the water line, and if acid doesn't remove the scale rather easily, then the problem is from evaporation. That is because a deposit from evaporation isn't just calcium carbonate. It would also include everything else water contains, like sodium and other stuff that isn't affected by acid. There isn't much that can be done to prevent that from happening. Is there something that continually sprays water on the tile?

Yes, lowering the calcium level and the TDS would slow down the process of a deposit forming on the tile above the water line a little bit, but it wouldn't stop it completely. My concern then would be that maintaining a lower TDS level and a negative SI would make the water aggressive, which isn't good for cement-based pool finishes.

 

[dvcavall]

If I was willing and able to scrub my tile line once a week, I believe that would prevent the build-up, but that just isn't practical for me and I am betting many others.

We have a combined pool/spa with a water spillway between them going from the spa to the pool. The sun shines directly on that spot and as you can imagine, thick scale builds there rather quickly. Since I agree that the above the waterline scale is a combination of minerals, including salt, I want to ask, has anyone found a cleaner that works well on it? I have used many, including some of the very expensive online/youtube offerings, without much success. They mostly target calcium. After a while there is no longer any reaction with the scale since the calcium is likely dissolved and what is left are non-calcium deposits. Other than soda blasting, the only thing that I have found to remove that scale, at least partially, is pumice. But at a mohs of 6.0, I know I am scratching my tile glaze in the process.

 

[n1server]

I am far from a pool expert as I have only had my pool for 2 years. However, I had some serious issues with CH this year 700+. I was constantly adding vasts amounts of acid to keep the PH low. With the tap water being at 300, I used this opportunity to buy a water softener. So now I fill my pool with nearly CH free water. The CH is still at 400, but the number continues to get lower. When it reaches an acceptable level then I will change my method.

 

[MattM]

For whatever reason, here in San Diego -- I've discovered that I need to keep CSI negative (between -0.1 and -0.2) to avoid calcium buildup in any of the equipment, including SWG. As long as water temps remain high, my PH never goes higher than 7.4 and spent most of the summer at 7.2. Whenever I let CSI get > 0, I start seeing white dust on the bottom of pool.

As far as regularly cleaning tile, I still haven't found a good solution to this. My dolphin robotic vacuum pretends to do it when I run it 2-3x/month, but I think manual cleaning is needed at least every 3-6 months to avoid it looking too bad.

fyi -- in just one season, water CH has gone from 225 -> 425....my current plan is to use a reverse osmosis service when it reaches 600.

 

[chem geek]

The SWG generally needs a somewhat negative saturation index to prevent scaling, though the use of 50 ppm Borates can help significantly (are you using borates? I thought you were). The reason is that the plate that is generating hydrogen gas raises the pH significantly near the plate so scale usually forms on that plate. This is why the plates periodically reverse in polarity to clean them and when that happens flakes or dust of calcium carbonate comes off and into the pool. That's probably what is settling into your pool water. With a more negative saturation index, such flakes redissolve in the water and there may also be less buildup on the plate as well. If you use 50 ppm Borates, then the pH rises only half as much at the plate as compared to not using borates.