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Thread: Mysteries of CSI and CH

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    Mysteries of CSI and CH

    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!
    Suley, West Hills, CA
    IG 23000 Gallons with IG Spa Gunite plaster, Pentair Intelliflo VS 3HP, Pentair Clean & Clear Plus 420 Cartridge filter, Teledyne Laars One series 400,000 BTU Nat Gas Heater, Hayward Max Flo 1HP Single Speed Spa pump, Noisy Spa blower, Dolphin Deluxe 4 robotic vacuum.

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    mas985's Avatar
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    Re: Mysteries of CSI and CH

    Higher CH results in higher CSI but the relationship is logrithmic.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Mysteries of CSI and CH

    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 Ksp. That's shorthand for solubility product constant.

    Now, there is also a Ksp 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?
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: Mysteries of CSI and CH

    Thanks!

    How does TA, CYA, Temp and pH effect CSI?
    Suley, West Hills, CA
    IG 23000 Gallons with IG Spa Gunite plaster, Pentair Intelliflo VS 3HP, Pentair Clean & Clear Plus 420 Cartridge filter, Teledyne Laars One series 400,000 BTU Nat Gas Heater, Hayward Max Flo 1HP Single Speed Spa pump, Noisy Spa blower, Dolphin Deluxe 4 robotic vacuum.

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    Re: Mysteries of CSI and CH

    Quote Originally Posted by Suley
    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.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: Mysteries of CSI and CH

    LOL!
    Quote Originally Posted by Richard320
    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, West Hills, CA
    IG 23000 Gallons with IG Spa Gunite plaster, Pentair Intelliflo VS 3HP, Pentair Clean & Clear Plus 420 Cartridge filter, Teledyne Laars One series 400,000 BTU Nat Gas Heater, Hayward Max Flo 1HP Single Speed Spa pump, Noisy Spa blower, Dolphin Deluxe 4 robotic vacuum.

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    Re: Mysteries of CSI and CH - AHA!

    AHA!

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

    Ahahahahaha!
    Suley, West Hills, CA
    IG 23000 Gallons with IG Spa Gunite plaster, Pentair Intelliflo VS 3HP, Pentair Clean & Clear Plus 420 Cartridge filter, Teledyne Laars One series 400,000 BTU Nat Gas Heater, Hayward Max Flo 1HP Single Speed Spa pump, Noisy Spa blower, Dolphin Deluxe 4 robotic vacuum.

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    Re: Mysteries of CSI and CH

    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.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Mysteries of CSI and CH

    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.
    Dave S.
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    Re: Mysteries of CSI and CH

    Quote Originally Posted by 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.
    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.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: Mysteries of CSI and CH

    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?
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Mysteries of CSI and CH

    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.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: Mysteries of CSI and CH

    Quote Originally Posted by mas985
    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.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: Mysteries of CSI and CH

    Quote Originally Posted by chem geek
    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...
    Suley, West Hills, CA
    IG 23000 Gallons with IG Spa Gunite plaster, Pentair Intelliflo VS 3HP, Pentair Clean & Clear Plus 420 Cartridge filter, Teledyne Laars One series 400,000 BTU Nat Gas Heater, Hayward Max Flo 1HP Single Speed Spa pump, Noisy Spa blower, Dolphin Deluxe 4 robotic vacuum.

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    Re: Mysteries of CSI and CH

    Quote Originally Posted by 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 Ksp. That's shorthand for solubility product constant.

    Now, there is also a Ksp 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!
    16k Gal fiberglass over plaster
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    Re: Mysteries of CSI and CH

    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 ---> Ksp (higher temperature lowers Ksp so increases saturation index)
    higher TDS lowers effective Calcium and Carbonate

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

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

    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.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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