Salt accumulation or loss?

[Maestro]

I'm curious about this statement in the Pool School article Water Balance for SWGs: "Starting out slightly higher (200-400 ppm) than the ideal level is recommended, since the salt level will tend to fall over time."

According to PoolMath (web version), if I run my SWG at 100% for 24 hours a day (using a T-15 cell) I'll generate generate 1.47 lbs of chlorine gas per day which will add 6.6 ppm of salt to my 22K gallon pool. Lets say I cut that in half and add 3.3 ppm of salt per day. If I'm adding a little bit of salt every day, why does Pool School say my salt level will decrease?

Salt does not evaporate with water, so I shouldn't be losing any when my pool level goes down in the summer and I refill, although I guess I would lose a tiny bit when I backwash and rinse. (I never vacuum to waste.)

(I'm assuming that PoolMath calculates chlorine gas in ounces by weight, not liquid ounces, and that the salt number is ppm, not ounces. JasonLion, is that the way it works?)

 

[AimeeH]

I'm curious about this statement in the Pool School article Water Balance for SWGs: "Starting out slightly higher (200-400 ppm) than the ideal level is recommended, since the salt level will tend to fall over time."

According to PoolMath (web version), if I run my SWG at 100% for 24 hours a day (using a T-15 cell) I'll generate generate 1.47 lbs of chlorine gas per day which will add 6.6 ppm of salt to my 22K gallon pool. Lets say I cut that in half and add 3.3 ppm of salt per day. If I'm adding a little bit of salt every day, why does Pool School say my salt level will decrease?

Salt does not evaporate with water, so I shouldn't be losing any when my pool level goes down in the summer and I refill, although I guess I would lose a tiny bit when I backwash and rinse. (I never vacuum to waste.)

(I'm assuming that PoolMath calculates chlorine gas in ounces by weight, not liquid ounces, and that the salt number is ppm, not ounces. JasonLion, is that the way it works?)

A SWG does not produce salt. It uses the salt in the water to produce hypochlorous acid to chlorinate the pool.

And the notion that the salt would go down over time is due to splash out and rain causing backwash.

 

[jblizzle]

The effects of adding chlorine calculation is assuming you are literally bubbling chlorine gas into the pool which some pool maintenance companies do. The swg is just splitting the existing salt into chlorine and sodium when the chlorine does its work it recombines with the sodium to return to salt. Using the equivalent of chlorine gas just allows the calculation for how much FC an swg can produce.

 

[Jimrahbe]

M,

I initially misread your original post, and thought the same as Aimee... But after rereading it, I understand your point about the chlorine gas adding a small amount of salt. I'm not sure if that is true when the chlorine gas is generated by a SWCG or if it is only true when using bottled chlorine gas.

Either way, I have three saltwater pools and all three of them loose two or three bags worth of salt each year. For reference, two of the pools have DE filters, but I never backwash and only clean the filters twice a year when they are disassembled and cleaned.

My only comment is that there is "theory" and there is real life...

Maybe one of our chemical experts will chime in on this one..

Thanks,

Jim R.

 

[Maestro]

OK, now I understand where my thinking went working on this. I know that my SWG produces chlorine gas, but I didn't realize that PoolMath was only talking about adding it from an outside source. Of course it makes sense that the SWG separates NaCl into Na and Cl, it doesn't add any extra NaCl in the process.

?...when the chlorine does its work it recombines with the sodium to return to salt.

But, having no chemistry background at all, I've never understood how the chlorine doesn't get used up in the process. What exactly does it do when it fights algae in the water, and why is it all available to recombine with the sodium when it's all over? Sounds like magic to me.

 

[jblizzle]

Honestly, seems like magic to me too But i am sure there is a thread or 2 in the deep end that explain it.

 

[cfherrman]

Matter cannot be "used up", just converted to a different chemical. Thank of hydrogen and oxygen. If you burn hydrogen, it breathes oxygen and the heat changes it to water, add electricity to water and it splits back to hydrogen gas and oxygen gas. I would not be able to explain it correctly but the oxidizing process changes clorine as a chemical reaction. It in one of chemgeeks post here.

 

[AUSpool]

...

But, having no chemistry background at all, I've never understood how the chlorine doesn't get used up in the process. What exactly does it do when it fights algae in the water, and why is it all available to recombine with the sodium when it's all over? Sounds like magic to me.

I’ve often wondered about that too and on several occasions have searched the forums without success. I’ve flicked through some old text books, micro biology, chem, enviro chem, water quality etc., and they avoid any reference to specific detail. One book suggested that hypochlorous acid can readily pass though the cell membrane, once inside the cell it reeks havoc with many cellular functions.

HOCl is a strong oxidising agent, the Cl molecule wants to get an electron and keep it all to itself at the expense of the hydrogen and oxygen molecules. For just the chlorine reaction the the chlorine molecule grabs an electron and becomes a quite stable chloride ion which is released to the bulk water when the cell falls apart.

Cl + 1e —> Cl-

Going beyond just the one molecule and reaction, the cell would be flooded with HOCl molecules. HOCl has many reaction pathways including the production of chlorinated molecules but one that dominates is grabbing an electron from the cells electron transport system and overwhelming the cells respiration system resulting in the release of chlorine ions to the bulk water when the cell dies and falls apart. The complete story is a lot more complicated but I think I'm happy with the short version for now.