a potential aid to calcium scale?

[JamesW]

We have salt chlorination Cells used in sea water conditions and they rarely foul up with hard crystalline deposits.

Sea water has very high levels of salt, which significantly lowers the CSI.

I should be noted that sea water contains about 1700 ppm of calcium and up to 400 ppm of magnesium.

Can you provide a reference for these numbers? Most references show calcium to be closer to 400 ppm (calcium) and magnesium to be about 1280 ppm (magnesium) in seawater.

 

[duraleigh]

If we get too far into the weeds on this, I get lost. I shouldn't have brought up (yet) the overall effect on pool wall scale.

The initial idea that Mark W has is if you have an SWG, it is a good idea to add a certain amount of magnesium (somewhere around 20ppm but the exact amount is not critical) to your pool water.

What results is that calcium scale either tends not to form on the SWG plate or it forms as a mushy, easy to clean scale. Is that benefit worth everyone on the forum doing it? Well, not if you don't have an SWG. If you have an SWG and don't get scale, then it's not worth it either.

If you get scale on your SWG, are you keeping your pH controlled properly? If so, then the benefits of magnesium would seem to be helpful.

There is a potential side benefit of a better "feel" to the water but I find that VERY subjective (borates) and would prefer not to focus much on that.....it's too individual.

If someone decides to do this in his/her pool, I agree with mas985 that a more definitive ppm sould be refined and the cumulative effect over the years need to be thought out. Mark W reports NO accumulative effect but, as we know, each pool is different in many, many ways so establishing a guideline (and an easy, inexpensive test) seems like a logical step.

 

[JasonLion]

I've read around enough to be reasonably confident that this is helping some people. I have two concerns. One is that it isn't clear just how many people will benefit. The conditions where this could make a large difference, CH and TA both forced high enough that you have little choice but to get scaling in the SWG, don't seem to be all that common. Two, it isn't at all clear what the dosage really is. I'm wondering if the real dosage goal is some calcium magnesium ratio, rather than an absolute level of magnesium. The second point can be cleared up with some experimentation. However, if the audience is too small that may not be worth the effort.

 

[onBalance]

As JamesW points out, sea water has about 400 ppm of calcium (which is equivalent to 1000 ppm when expressed as calcium carbonate as we always do in pool water chemistry), not 1700 ppm (don't know if this is actual ppm of calcium or the calcium carbonate equivalent) as stated by Watermaid (Mark). And the content of magnesium is about 1280 ppm (and about 5200 ppm expressed as calcium carbonate), not 400 ppm as stated by Watermaid.

So the ratio is about 1 to 5 (calcium to magnesium and expressed as calcium carbonate) for sea water. The ratio for typical tap water is generally anywhere from 1 to 1.5 (Ca to Mg) to 10 to 1. Just the opposite of sea water.

Based on this, if sea water doesn't scale (or makes scale less hard) a salt unit because of the high magnesium content, it would simply require a lot more "magnesium chloride" to be added than the 20 ppm suggested by Watermaid.

I also agree with Chem geek comments. Hard tap water always contains a lot of magnesium. And to change the ratio toward magnesium sufficiently, it would require large amounts of magnesium chloride. In addition, due to chlorine santiziers being added, pool water also has a lot of chlorides present. I am hesitant to accept the above claim.

 

[JasonLion]

Adding magnesium in some form or another is apparently very common in Australia. Zodiac and Pool Magic both suggest adding Salt Cell Protector at the rate of 1Kg of magnesium sulfate per 10,000L of water once every six months. That is 35.27 oz for 2642 gallons or 8.34 lbs per 10,000 gallons. Various others companies in Australia recommend against magnesium sulfate because of the sulfates and instead propose a liquid of some kind that appears be based on magnesium chloride.