Salt water generators are closed systems... what?

[TexEdmond]

72% of facts are made up

Also, 4 out of 5 dentists agree that the other guy is an idiot.

 

[TexEdmond]

Many people who convert to SWGs make the mistake of thinking their water has zero chloride ion in it and calculate their salt need based on that when, in fact, they are almost 1/3rd of the way to a salt water pool. Every gallon of 10% LC added to 10,000 gallons of pool water adds 16ppm salt which comes from both the excess salt water in liquid chlorine from the manufacturing process as well as the breakdown of hypochlorite to chloride ion.

I have been working towards adding a SWG for a month or so now. I thought I was being real slick tossing my extra Cal-Hypo in the pool because it gave me a few extra PPM salt and calcium every time. But then I saw the salt addition for just glugging bleach and I realized the silliness of my ignorant cheapness...

All that wasted time brushing Cal Hypo on the pool floor got me like:

 

[Homebrewale]

Cannot a small amount of chlorine leave the pool in the form of chloramines that off-gas into the air? No enough to result in adding more salt but there is a minute loss of chlorine.

 

[phonedave]

As @JamesW stated, the chloride constantly builds up because you are constantly adding it to the water. It is not at all unusual to find that manually chlorinated pools (in areas without a lot of rain dilution and overflow) have a salt content up around 1000ppm. Many people who convert to SWGs make the mistake of thinking their water has zero chloride ion in it and calculate their salt need based on that when, in fact, they are almost 1/3rd of the way to a salt water pool. Every gallon of 10% LC added to 10,000 gallons of pool water adds 16ppm salt which comes from both the excess salt water in liquid chlorine from the manufacturing process as well as the breakdown of hypochlorite to chloride ion.

I think that is the part that many people miss.

In overly overly simplistic terms.

You add chlorine (bleach) to a pool, and when it does its sanitization job, it turns to salt

You then have two choices.
A) Leave that salt in the pool and just add more bleach
B) Use energy (electricity) to turn that salt back into bleach

And to preemptively answer the follow up question: Why does my salt concentration have to be so much higher in a SWCG pool? Because the reaction in the cell is not 100%. If you could somehow convert all of the salt in your pool back into chlorine, then you would not need a higher concentration. But for a variety of reasons, the cell is not converting every single ion that flows through it.

 

[Plettschner]

I'd like to add to this... What happens to the depleted materials of the SWCG?

 

[JoyfulNoise]

Cannot a small amount of chlorine leave the pool in the form of chloramines that off-gas into the air? No enough to result in adding more salt but there is a minute loss of chlorine.

You answered your own question. Most chloramines don't leave the water but are further oxidized by chlorine and UV light to form nitrogen gas, nitrates, water, hydrogen ion and chloride ion. So the vast majority of the chloride you add to the pool in the form of salt never leaves the pool through any kind of sanitation reaction.

 

[JoyfulNoise]

And to preemptively answer the follow up question: Why does my salt concentration have to be so much higher in a SWCG pool? Because the reaction in the cell is not 100%. If you could somehow convert all of the salt in your pool back into chlorine, then you would not need a higher concentration. But for a variety of reasons, the cell is not converting every single ion that flows through it.

The efficiency isn't what matters to chloride ion concentration. It's actually simpler than that - the resistance of the electrochemical cell is inversely proportional to the chloride concentration. If the chloride concentration is too low, then the cell resistance will be too high. This will limit the current flow which is what determines how much chlorine is generated. And if the resistance of the cell is too large, the voltage needed to make current flow will be so high that you will parasitically produce more oxygen and hydrogen gas (water electrolysis) than chlorine gas. So ultimately the required chloride ion concentration vis-a-vis salt additions is dominated by the required electrical conductivity of the solution.

 

[JoyfulNoise]

I'd like to add to this... What happens to the depleted materials of the SWCG?

They form mostly insoluble oxides that eventually get filtered out. Once the titanium plates are exposed due to loss of catalyst coating, the cell dies pretty quickly.

 

[Sodium Chloride]

Interesting in that some of the answers give me more questions. I was under the impression that the salt in a pool was a solution, meaning evenly distributed throughout its volume. If you have 10k gallons and a salt reading of 3600 ppm; if you drained 5k gallons, wouldn't you then have still have salt at 3600 ppm in 5k gallons?

If optimal salt level for your SWG is 3600 will it produce more Cl at 3800? Will it produce less at 3400?

The rabbit hole certainly gets deeper with the more knowledge you seek.

 

[Abnaxis]

Interesting in that some of the answers give me more questions. I was under the impression that the salt in a pool was a solution, meaning evenly distributed throughout its volume. If you have 10k gallons and a salt reading of 3600 ppm; if you drained 5k gallons, wouldn't you then have still have salt at 3600 ppm in 5k gallons?

If optimal salt level for your SWG is 3600 will it produce more Cl at 3800? Will it produce less at 3400?

The rabbit hole certainly gets deeper with the more knowledge you seek.

If it's anything like the electrolysis cells I worked on in college (electrolyzing zinc not chlorine but still basically the same process), the SWCG tries to maintain a fixed electrical current to generate a fixed amount of chlorine (electrical current is directly proportional to chlorine gas generated), and varies the voltage across the cell to account for changes in temperature and salt concentration.

In other words, having more/less ppm or lower/higher water temperature (which makes a huge difference IME with other materials) doesn't change the amount of chlorine generated, but rather how much power the SWCG draws making it, because the voltage has to increase to make up for the increased "resistance" (slight oversimplification) of the water to maintain the same current.

At least, trying to maintain a fixed electrical current how I would design it so I assume that's how it's done . It's a relatively simple circuit to implement.

I'm not sure where you got the impression that the salt in the water isn't in solution?

 

[JamesW]

the SWCG tries to maintain a fixed electrical current to generate a fixed amount of chlorine (electrical current is directly proportional to chlorine gas generated), and varies the voltage across the cell to account for changes in temperature and salt concentration.

Some work like that and some do not.

The Aquarite puts out a fixed voltage, but the Intellichlor varies the voltage in an effort to maintain a fixed output.

The aquarite current definitely changes based on salinity or water temperature, but it does shut down at 8 amps on the high side or 2,400 ppm on the low side.

 

[Abnaxis]

Some work like that and some do not.

The Aquarite puts out a fixed voltage, but the Intellichlor varies the voltage in an effort to maintain a fixed output.

The aquarite current definitely changes based on salinity or water temperature, but it does shut down at 8 amps on the high side or 2,400 ppm on the low side.

Mental note: do not buy an Aquarite. Using a fixed voltage just sounds like a terrible pain in the rear for the end user.

 

[JamesW]

Mental note: do not buy an Aquarite. Using a fixed voltage just sounds like a terrible pain in the rear for the end user.

Not really, the amount that any cell puts out has to be adjusted somewhat periodically to tune it to the conditions.

 

[Abnaxis]

Not really, the amount that any cell puts out has to be adjusted somewhat periodically to tune it to the conditions.

It really shouldn't, if it runs off a fixed current? That current is literally the electrons detaching from the chloride to make chlorine, that's the whole point of making a fixed current circuit.

Meanwhile, if the fixed voltage units aren't a lot smarter than you're making them out to be, their output is going to change with water temperature and chemistry--i mean, those things don't change very fast but it's still a potentially large swing in chlorine output from one day to the next.

 

[Orion7319]

It really shouldn't, if it runs off a fixed current? That current is literally the electrons detaching from the chloride to make chlorine, that's the whole point of making a fixed current circuit.

Meanwhile, if the fixed voltage units aren't a lot smarter than you're making them out to be, their output is going to change with water temperature and chemistry--i mean, those things don't change very fast but it's still a potentially large swing in chlorine output from one day to the next.

Not really. I have an aquarite, I’m not running around adjusting it all the time. You crank it up a bit when the water gets warmer because you are simply loosing more chlorine when it’s warmer. The pool math app is pretty spot on in calculating how much chlorine it will add. I lost around 100ppm of salt last night from a storm and having to drain several inches of water out. It’s still maintaining the same level of chlorine as it was the day before. It’s very simple to use.

Edit: Also the CYA will prevent it from crashing. On cloudy days it might make a bit more. I also learned a short time ago that running it 24/7 keeps it even more stable. If you can keep aeration to a minimum, let your TA drop down to where it wants to be, keep your CYA levels consistent you really don’t have to add much of anything at all and your chemicals will just kind of stabilize (admittedly I have a vinyl pool and I don’t have to worry about chemicals as much as people with plaster pools do). I have only gone though half a gallon of MA this season so far. Sometimes I “shock” the pool when something gross happens, add CYA after a few major rain storms, that’s about it.

 

[mas985]

Actually, you want higher production with an increase in water temps because the breakdown of FC is faster at higher temperature. So having a fixed current/output is not necessarily an advantage. I have had my SWG for over 15 years and have never considered fix voltage as a disadvantage in any way.

 

[JamesW]

For most people, water temperature goes up with sun exposure, so as there is more sunlight, you need more chlorine to compensate.

 

[JoyfulNoise]

Cell voltage is bounded, you can’t apply whatever voltage you want to fix a specific current. Cell voltage is what controls the electrolysis reactions and if the voltage gets too high you will generate parasitic chemical species like oxygen gas and chlorates. If the voltage is too low, you won’t generate any chlorine. These power supplies have to operate in a bounded voltage-current domain or else they won’t do what you want them to. Whether you pick current or voltage as the control point is somewhat arbitrary. Temperature also affects what reactant are formed - too low and you’ll get more oxygen than chlorine; too high and you’ll start to form chlorates (although not usually at pool/spa temps). This is one reason why cells cutoff below a certain temp.

 

[Sodium Chloride]

If it's anything like the electrolysis cells I worked on in college (electrolyzing zinc not chlorine but still basically the same process), the SWCG tries to maintain a fixed electrical current to generate a fixed amount of chlorine (electrical current is directly proportional to chlorine gas generated), and varies the voltage across the cell to account for changes in temperature and salt concentration.

In other words, having more/less ppm or lower/higher water temperature (which makes a huge difference IME with other materials) doesn't change the amount of chlorine generated, but rather how much power the SWCG draws making it, because the voltage has to increase to make up for the increased "resistance" (slight oversimplification) of the water to maintain the same current.

At least, trying to maintain a fixed electrical current how I would design it so I assume that's how it's done . It's a relatively simple circuit to implement.

I'm not sure where you got the impression that the salt in the water isn't in solution?

I'm not sure where you got the impression that the salt in the water isn't in solution?

In previous posts it was mentioned that splash out will lower salt levels. I know evaporation will increase salt levels and dilution will reduce levels. I wouldn't think splash out would change anything.

 

[Sodium Chloride]

Cell voltage is bounded, you can’t apply whatever voltage you want to fix a specific current. Cell voltage is what controls the electrolysis reactions and if the voltage gets too high you will generate parasitic chemical species like oxygen gas and chlorates. If the voltage is too low, you won’t generate any chlorine. These power supplies have to operate in a bounded voltage-current domain or else they won’t do what you want them to. Whether you pick current or voltage as the control point is somewhat arbitrary. Temperature also affects what reactant are formed - too low and you’ll get more oxygen than chlorine; too high and you’ll start to form chlorates (although not usually at pool/spa temps). This is one reason why cells cutoff below a certain temp.

I see you are in Tucson, AZ and I'm in Bradenton , Fl. Both are very warm locations. Are there any advantages in having a slightly higher salt level to lower the amount of voltage needed by the cell in warm locations (my water temp under screen can hit 90)? I guess the real question I'm asking is if there is anyway to fine tune the work done by the SWG to get its maximum life or are the differences so minuscule that it doesn't matter