Mysteries of CSI and CH

[Suley]

Hello all!

Can someone please explain the relation of CH and CSI? I understand that CH is the calcium hardness in water but the pool calculator results for CSI confuse me when i play with the CH level.

Any help would be appreciated!

 

[mas985]

Higher CH results in higher CSI but the relationship is logrithmic.

 

[Richard320]

Simple as I can explain it: Water has a saturation point. There is a finite quantity of whatever that will dissolve in it. We did an experiment once in high school chemistry with a beaker of water and a pile of sugar. Dump in some sugar, stir. Repeat. It gets syruppy and at some point, the water is saturated, any more sugar just sits on the bottom in a pile, undissolved. Then by heating, a lot of that pile dissolves. Because temperature affects the K_sp. That's shorthand for solubility product constant.

Now, there is also a K_sp for Calcium. You can dissolve so much into the water and at some point, it won't absorb any more. Tinkering with pH and temperature may get it to dissolve more. Likewise, tinkering the other way will cause a clear solution to start precipitating calcium. Cloudy at first, and then it will settle to the bottom of the beaker.

Move to the real world - your pool. Depending on temperature, pH, TA, and a few other variables, Calcium will stay in solution (dissolved) or precipitate out - start crystallizing on the walls, in the plumbing, the ladder, whatever. We call it scaling. The CSI predicts which way it will go.

Positive CSI risks scaling. Negative CSI means it will start leaching Calcium out of the plaster walls, etching them and weakening it. Keeping CSI +/- 0.6 means that neither scenario is likely. I can say from personal experience that when CSI gets around -.6, it does dissolve Calcium. Fast enough to see a difference in a week. I've been slowly descaling my pool that way for roughly the past year.

You will see as you play with the numbers on Pool Calculator that the Calcium is not the biggest factor for scaling - pH is. TA can also have a large effect. The difference to CSI between 800 and 850 CH is minimal. However, there will come a time, and I have been there, where CH is so high that the only way to keep it in solution is to have pH out of the comfort zone. Meaning, the pool looks great, but you don't want anyone swimming in it! And that's when it's time to drain some water and refill it.

Does that help any?

 

[Suley]

Thanks!

How does TA, CYA, Temp and pH effect CSI?

 

[Richard320]

Thanks!

How does TA, CYA, Temp and pH effect CSI?

Go play with Pool Calculator and see for yourself!! Plug your numbers in as current and target, then start changing pH and the rest one at a time and see what happens to CSI. It can be enlightening.

 

[Suley]

LOL!

Go play with Pool Calculator and see for yourself!!

Sometimes answers to questions are so simple! Why didnt i think of that!

Thanks Richard! that explains a lot!

 

[Suley]

Re: Mysteries of CSI and CH - AHA!

AHA!

Everything increases CSI except Salt, Borate and CYA decrease it!

Ahahahahaha!

 

[mas985]

Salt should also decrease CSI albeit it takes a lot of salt to move CSI. But the reason that Borates and CYA decrease CSI is because TA should include those two things and so when they are backed out and make the alkalinity smaller, the CSI gets smaller. When Borates or CYA increase, TA also increases.

[EDIT] oops - should have said salt decreases instead of increases CSI.

 

[duraleigh]

Understand that if you keep your parameters within the guidelines suggested here on TFP (and why wouldn't you want to? ), then csi is largely irrelevant and will not be an issue in your pool.

Just in case anyone on my intellectual level stumbles across this thread, I would suggest you ignore it.

A principle of BBB is simplicity........I don't feel like we're KISS'ing it when we fret over csi. :lol:

 

[Richard320]

Understand that if you keep your parameters within the guidelines suggested here on TFP (and why wouldn't you want to? ), then csi is largely irrelevant and will not be an issue in your pool.

Some of us have hard enough water that CH of 200-400 means biennial drains, that's why!

Around here, I don't think I've even seen Calcium increaser in the pool stores. If CH is too low after the initial fill, just wait a couple weeks....it will be fine.

 

[duraleigh]

Some of us have hard enough water that CH of 200-400 means biennial drains, that's why!

Much higher CH is manageable (as it builds, in your case) if you stay on the lower end of pH and TA.

It's not clear to me how you would use it as a useful tool. Couldn't you simply replace water when CH exceeded some predetermined point?

 

[JasonLion]

If you have very high CH levels, CSI is a useful tool for figuring out how far you need to adjust PH and TA to compensate.

 

[chem geek]

Salt should also increase CSI albeit it takes a lot of salt to move CSI.

It's actually the other way around. Salt, or any other TDS if it is charged, decreases the saturation index. The reason is that it increases the ionic strength in the water and more charged ions in the water effectively shield other ions including calcium ion and carbonate ion. So this makes them act as if they are at lower effective concentration (known as "activity" in chemistry) so they are further away from saturation as they otherwise would be.

 

[Suley]

Salt, or any other TDS if it is charged, decreases the saturation index.

Thats what the pool calculator also said...inversely proportional to salt levels...

 

[siliconvalleymatt]

Simple as I can explain it: Water has a saturation point. There is a finite quantity of whatever that will dissolve in it. We did an experiment once in high school chemistry with a beaker of water and a pile of sugar. Dump in some sugar, stir. Repeat. It gets syruppy and at some point, the water is saturated, any more sugar just sits on the bottom in a pile, undissolved. Then by heating, a lot of that pile dissolves. Because temperature affects the K_sp. That's shorthand for solubility product constant.

Now, there is also a K_sp for Calcium. You can dissolve so much into the water and at some point, it won't absorb any more. Tinkering with pH and temperature may get it to dissolve more. Likewise, tinkering the other way will cause a clear solution to start precipitating calcium. Cloudy at first, and then it will settle to the bottom of the beaker.

Move to the real world - your pool. Depending on temperature, pH, TA, and a few other variables, Calcium will stay in solution (dissolved) or precipitate out - start crystallizing on the walls, in the plumbing, the ladder, whatever. We call it scaling. The CSI predicts which way it will go.

Positive CSI risks scaling. Negative CSI means it will start leaching Calcium out of the plaster walls, etching them and weakening it. Keeping CSI +/- 0.6 means that neither scenario is likely. I can say from personal experience that when CSI gets around -.6, it does dissolve Calcium. Fast enough to see a difference in a week. I've been slowly descaling my pool that way for roughly the past year.

You will see as you play with the numbers on Pool Calculator that the Calcium is not the biggest factor for scaling - pH is. TA can also have a large effect. The difference to CSI between 800 and 850 CH is minimal. However, there will come a time, and I have been there, where CH is so high that the only way to keep it in solution is to have pH out of the comfort zone. Meaning, the pool looks great, but you don't want anyone swimming in it! And that's when it's time to drain some water and refill it.

Does that help any?

Excellent explanation!!!! Being new here and getting my head (finally) around pool chemistry I was using my Taylor "watergarm" tool to the same thing. The Pool calculator is much better of course. Nicely done!

 

[chem geek]

One minor point of clarification, it's not calcium that gets saturated, but calcium carbonate, and is scale if over-saturated and is what dissolves from plaster if under-saturated. This is why pH and TA have an effect, because they determine the amount of carbonate in the water. So the CH determines the calcium, the pH and TA determine the carbonate, and the product of calcium and carbonate determines the saturation level. The temperature affects the point at which saturation occurs and the TDS also has an influence via ionic strength essentially making the calcium and carbonate levels behave as if they are lower than measured.

CH ---> Calcium
pH, TA ---> Carbonate
Temperature ---> K_sp (higher temperature lowers K_sp so increases saturation index)
higher TDS lowers effective Calcium and Carbonate

Saturation is when [Calcium] * [Carbonate] = K_sp and the saturation index is defined as:

CSI = log₁₀( [Calcium] * [Carbonate] / K_sp )

Technically, it is carbonate alkalinity, not total alkalinity that determines the carbonate level which is why the accurate formulas for CSI or LSI adjust TA for other components such as CYA or borates.