CSI Help

[daveooph]

I've looked and read through a lot of the beggining material on the forum but can't find anything on the particulars of the CSI scale except that .6 is not good....ie don't go too far negative or too far positive.

If you are on the more negative side what does that mean verse being balanced more on the positive side? Are you more subject to scaling if your CSI is positive? Is your plaster more likely to have problem if you are too negative?

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[chem geek]

The forum officially takes the position that as long as you remain within the Recommended Levels you don't need to worry about the CSI. This is in spite of the fact that if one were at the lowest end of those recommendations for pH, TA, CH and the highest for CYA that the CSI for SWG (3000 ppm salt) plaster pools would be at -0.5 or thereabouts depending on temperature and -0.6 if borates are used.

As for the meaning of the CSI, at 0 the water is saturated with calcium carbonate so it will neither form scale nor dissolve plaster. When the CSI is positive, it can form scale but in practice we don't see this occurring in pools until at least +0.7 except in saltwater chlorine generators at their hydrogen gas generation plate. In hot spas scaling may be seen at around +0.3. When the CSI is negative, then it can dissolve the calcium carbonate in plaster and grout surfaces. It is most critical to avoid negative CSI with young plaster since it is most susceptible to degradation when it is new. After it is hardened, it takes longer to dissolve. Plaster coupons were seen to release calcium at a CSI of -0.6 to -0.7 or so, but there is debate as to how negative is how bad when one is looking at trying to preserve plaster for many years, even a decade or more. A low pH is most detrimental to plaster and it may accelerate the dissolving of it for any negative CSI.

Technically speaking, a CSI of -0.3 has half the amount of calcium or carbonate or their product of concentrations compared to being saturated. -0.6 has one-fourth the amount of saturation. -0.9 has one-eighth the amount. -1.0 has one-tenth the amount. So even though these sound like small negative numbers, they are a logarithmic scale so even -0.3 is much bigger than it sounds -- again, half the amount of saturation. Similarly, +0.3 has twice the amount of calcium or carbonate or their product of concentrations compared to being saturated.

 

[daveooph]

The forum officially takes the position that as long as you remain within the Recommended Levels you don't need to worry about the CSI. This is in spite of the fact that if one were at the lowest end of those recommendations for pH, TA, CH and the highest for CYA that the CSI for SWG (3000 ppm salt) plaster pools would be at -0.5 or thereabouts depending on temperature and -0.6 if borates are used.

As for the meaning of the CSI, at 0 the water is saturated with calcium carbonate so it will neither form scale nor dissolve plaster. When the CSI is positive, it can form scale but in practice we don't see this occurring in pools until at least +0.7 except in saltwater chlorine generators at their hydrogen gas generation plate. In hot spas scaling may be seen at around +0.3. When the CSI is negative, then it can dissolve the calcium carbonate in plaster and grout surfaces. It is most critical to avoid negative CSI with young plaster since it is most susceptible to degradation when it is new. After it is hardened, it takes longer to dissolve. Plaster coupons were seen to release calcium at a CSI of -0.6 to -0.7 or so, but there is debate as to how negative is how bad when one is looking at trying to preserve plaster for many years, even a decade or more. A low pH is most detrimental to plaster and it may accelerate the dissolving of it for any negative CSI.

Technically speaking, a CSI of -0.3 has half the amount of calcium or carbonate or their product of concentrations compared to being saturated. -0.6 has one-fourth the amount of saturation. -0.9 has one-eighth the amount. -1.0 has one-tenth the amount. So even though these sound like small negative numbers, they are a logarithmic scale so even -0.3 is much bigger than it sounds -- again, half the amount of saturation.

Thanks for the reply. Building now and hope to be be done by end of August. For new plaster then I assume I should aim to be on the positive side?

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[JoyfulNoise]

If you want a very technical discussions with lots of chemical equations and charts - see this post

CSI is just a measure of how saturated your water is with calcium carbonate. It predicts which way the calcium carbonate will be favored to go (dissolve into solution or scale out of solution) but DOES NOT indicate a rate or if the reaction will initiate. There a lots of factors which determine where your CSI would be best for any given pool. For my pool with very high CH, plaster surfaces, aeration and a SWCG, it's best for me to keep my CSI slightly negative. For a pool with a freshly replastered surface, you want to the CSI to be quite positive and close to +0.6. So where your particular CSI should be depends on your pool and pool water.

 

[chem geek]

Thanks for the reply. Building now and hope to be be done by end of August. For new plaster then I assume I should aim to be on the positive side?

Your pool builder may give you specific requirements during startup and for the first month after the plaster job is completed. They may void the warranty if you do not follow their recommendations so you should consider that even if their advice is not necessarily sound. The best type of plaster startup that produces the least amount of plaster dust so keeps most of the calcium in the plaster itself is described in the A Bicarb Start-up guide for TFP members, but your builder may be doing an acid startup or a traditional startup. The bicarb startup and the immediate maintenance period in the week or two afterwards have a CSI in the water of at least 0.3 and below 0.8, but you can read the specifics about TA, CH and pH in the link.

The basic idea of the bicarbonate startup is to have over-saturation of calcium carbonate in the water to ensure that newly formed calcium carbonate in the plaster surface does not dissolve AND to have sufficient bicarbonate in the water so that calcium hydroxide created during the curing of plaster will form calcium carbonate in-place in the plaster itself rather than dissolving in water. An ideal bicarbonate startup sees only a rise in pH with no change in CH and no plater dust in the pool water. Acid is added to maintain the pH between 7.6 and 8.0 and this also slowly lowers the TA over time towards TFP levels by the end of the month.