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Thread: a potential aid to calcium scale?

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    a potential aid to calcium scale?

    I am taking the liberty of publishing these words from Mark @ Watermaid Canada. At my invitation, Mark has joined the forum but I cautioned him about direct advertising, which some might construe this info to be......so I'll do it.

    The chemistry is over my head but it seems to stem from a good bit of practical application so I'd like to see the subject batted around here in the Deep End a bit.

    Watermaid recommends that customers add 50 lbs. of magnesium chloride to their pools every year. Calcium is attracted to the cathode plate of a salt chlorinator and can form hard scale. Adding magnesium softens the scale and makes it mushy. Normal water flow tends to flush the scale off of the cell plates. It also adds hardness to the water without using calcium. This once again reduces scale formation. Magnesium chloride will not harm a Watermaid cell. In fact it will increase the cell life. If you let a cell scale up, water flow will reduce and the cell will overheat. This will damage the cell
    What do you think?
    Dave S.
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    Re: a potential aid to calcium scale?

    In many cases this is not needed. My CSI is solidly negative and there is essentially no chance of scale forming on the cell plates, with or without magnesium. However, for people with high CH fill water using magnesium can be a wonderful thing. Unavoidably high CH levels tend to lead to frequent cell scaling which magnesium can mitigate, potentially increasing the cell lifetime significantly.
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    Re: a potential aid to calcium scale?

    Is there a chart on how much magnesium chloride to add per gallons of water? 50 lbs. would be drastically different for a 5,000 gallon pool compared to a 25,000 gallon pool. They say to add it yearly, does the magnesium get used up or will it build up over time? My water is more like Jason's, - CSI, and I will not be adding this to my pool, but I would hate for someone to put the wrong amount in their pool. Also, is there a test that measures it?
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    Re: a potential aid to calcium scale?

    I've seen magnesium > 20 ppm recommended in a couple of places. Magnesium test kits are available for reef aquariums, though several of them seem to target higher ranges than we would want.
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    Re: a potential aid to calcium scale?

    Watermaid recommends that customers add 50 lbs. of magnesium chloride to their pools every year.
    Much like CYA, for those that do not backwash and do not refill, this could potentially build up over time. I am not saying it is a bad thing only a target level would be more helpful than the above statement.
    Mark
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    Re: a potential aid to calcium scale?

    Hi Everyone

    I guess this is where I should jump in. We have salt chlorination Cells used in sea water conditions and they rarely foul up with hard crystalline deposits. The soft mushy deposit in these cells can always be cleaned with a strong jet of water or a dilute acid solution. I should be noted that sea water contains about 1700 ppm of calcium and up to 400 ppm of magnesium. We have seen that after the addition of Magnesium at about 20 ppm, Cells begin to mimic the sea water experience and the effect can last some 6 to 9 months. Watermaid recommends the use of Magnesium Chloride at the rate of 25 Kg per year as a means of changing the chemistry balance of the water when salt Cells foul up with deposits that are hard to clean and the Magnesium content of the water is low.

    Mark_Watermaid
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    Re: a potential aid to calcium scale?

    25 kg per year, independent of pool size and splash out? Doesn't seem logical.

    A 5000 gallon pool will have 10x the concentration as a 50000 gallon pool. Wouldn't a ppm target make more sense as with most additives?
    Mark
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    Re: a potential aid to calcium scale?

    The “logic” goes something like this. I said that a level of “about 20ppm” is good but that is often a little difficult to figure out. Magnesium is good for the chlorinator and really good for the bather so a little extra doesn’t do any harm. In fact the more the better. The other reality is that Mag usually comes in 50 lb. bags so if you just get into the routine of dumping a bag in every spring it save a lot of unnecessary calculating. I am talking about an average in ground home owners pool. Yes you are right there is a bit of difference between a blow up pool and the olympic pool at the University. I hope that most readers of the post would realize that the average pool is between 80 and 100K litres. If your pool is outside of that reality a little common sense needs to come into play

    Mark_Watermaid
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    Re: a potential aid to calcium scale?

    A 50 lbs bag in a 20k gallon pool adds 300 ppm and nowhere close to 20 ppm so I am still confused.
    Mark
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    Re: a potential aid to calcium scale?

    I guess you missed "the more the better" part of my last post. My wife uses bath salts to up the magnesium in her bath water. I am sure that if you measured how much mag you get from a bath bomb it would be really really high (way more than 600 ppm.) She does it because it is really good for your body. Mag cost about $35.00 per bag so is it really worth calculating how much you should add when the side effect of putting too much in the pool is, "it feels better"? I didn't throw in my two cents to get into a head banging contest. I thought that people would be interested in my personal experience with magnesium. If this has some how offended you, I am sorry. That wasn't my intent.

    Mark_Watermaid
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    Re: a potential aid to calcium scale?

    I wasn't questioning the merits of using mag, I was just trying to figure out exactly what you were recommending as a level. 20 ppm sounded reasonable but 300 ppm sounds a bit high and would add to total hardness not to mention that it makes testing for CH much harder.
    Mark
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    Re: a potential aid to calcium scale?

    You are correct. It would make CH testing harder. Then it begs the question. What problems do you see with a high mag hardness reading as long as SI its correct?
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    Re: a potential aid to calcium scale?

    I should note that most fill water that has water hardness has both calcium and magnesium in it. Usually the calcium is higher in concentration typically 4-to-1 (where I live it's 2.6 to 1). If 20 ppm magnesium (is that measured as ppm CaCO3 as is the case with most water hardness?) is all that it takes to protect cells, then that's pretty easy to do and I'd suggest people just look at the Total Hardness of their fill water, subtract out their Calcium Hardness of the fill water, and assume the rest is magnesium hardness. If it's less than 20 ppm or so, then they can add more to their pool.
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    Re: a potential aid to calcium scale?

    I'd suggest people just look at the Total Hardness of their fill water
    Don't test strips test Total Hardness? Is there a more accurate way?
    Dave S.
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    Re: a potential aid to calcium scale?

    Taylor K-1503, though it is a little pricey for this application, around $50.
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    Re: a potential aid to calcium scale?

    Doesn't magnesium scale just like calcium at higher levels?
    Mark
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    Re: a potential aid to calcium scale?

    Yes, but that is kind of the point. The magnesium scale combining with the calcium scale makes the scale softer and easier to remove.

    Magnesium hardness seems to be measured as ppm CaCO?. Everything I've seen says that it is determined by subtracting CH from TH, which means it will share the same unit they use.
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    Re: a potential aid to calcium scale?

    The solubility product for magnesium carbonate is over 1000 times higher than that of calcium carbonate so for practical purposes magnesium carbonate scale won't form. So any effect that magnesium has is likely to be that of forming small quantities of magnesium defects in calcium carbonate scale. However, it's more likely that in the high pH environment near the cathode, that magnesium hydroxide forms and it is indeed relatively soft and probably interferes with the formation of harder calcium carbonate scale.
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    Re: a potential aid to calcium scale?

    My primary interest in this topic is not so much SWG scale (although that is certainly interesting) but rather if magnesium will have the same affect on the pool surfaces.

    We all are familiar with the headaches of calcium scale on pool walls and I am interested if this "softening" of calcium scale would occur on pool walls
    Dave S.
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    Re: a potential aid to calcium scale?

    My educated guess is that the small amounts of magnesium discussed here (i.e. 20 ppm) will have virtually no effect on preventing scaling of plaster surfaces in high CH water. The reason is that the CH is usually high because total hardness is high in the fill water and in that situation there is almost always a lot of magnesium as well. So to prevent the calcium scale by having it be softer, I suspect the magnesium levels would have to be quite high.
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