SWG chlorine output and salt level

[New2Me]

Split from salt-testing-t10788.html by TFP Moderator into it's own thread for clarity



[Thread Hi-Jack ]

I don't want to steal your thread, but thought that the smart people might be able to answer these similar salt questions too.

Will a SWG produce more chlorine at a given output setting with a higher salt concentration? Is it better to be at the higher salt levels, or is that less productive?(shorter cell life? higher electrical consumption?)
Thanks!

[ End Hi-Jack ]

 

[Guest]

Re: Salt Testing

[Thread Hi-Jack ]

I don't want to steal your thread, but thought that the smart people might be able to answer these similar salt questions too.

Will a SWG produce more chlorine at a given output setting with a higher salt concentration? Is it better to be at the higher salt levels, or is that less productive?(shorter cell life? higher electrical consumption?)
Thanks!

[ End Hi-Jack ]

Salt level has no effect on chlorine output but it does effect cell life. Low salt levels can shorten cell life since the conductivity of the water is lower requiring higher current to produce chlorine. The higher current can shorten cell life.
IMHO, it's best never to let the salt level fall lower than the manufacturer's recommended level (usually 3000-3500 in the US but some are as high as 5000 ppm). Running 100-200 ppm higher can give you some room during rainy season (if you have one) to keep the salt from falling too low when the water level in the pool goes up from rains (or is drained out by the autofill and drain system.

Next time, Please start your own thread instead of hijacking another's as it can get confusing!

 

[chem geek]

I think that what happens depends on whether the SWG electronics are regulating voltage or current. At the same voltage, the greater conductivity of higher salt levels will result in higher current and faster chlorine production per time. Depending on whether the electronics are measuring current flow, this may automatically reduce the cell's on-time. If it doesn't, then you would end up doing that manually yourself (i.e. turning down the on-time) since you want a steady Free Chlorine (FC) level. The net result is that the product of current times time is the same so cell life should be roughly the same (but read below).

If the salt levels were very high, then the much higher currents might cause problems elsewhere in the system if not designed for such currents. Though they are for a shorter period of time, higher current through resistance means more heat generated and that can lead to higher temperatures, perhaps in the electronics, wires, etc. that might not be dissipated as effectively and could cause problems. I suspect you'd have to get the salt level pretty high before you see such effects.

If the salt level is too low, then you may not be able to produce enough chlorine even if the on-time is 24 hours. If the salt cell is measuring current and attempting to overcome the lower production by increasing voltage, then that might cause other issues. Specifically, if the salt level is too low, then more oxygen gas can be produced compared to chlorine gas so in effect the cell becomes less efficient at generating chlorine and that would certainly reduce cell life which is mostly a function of the product of current times time.

Richard

 

[mas985]

I had the same question last year and decided to find out for myself with a series of tests in my spa. The spa was an easy choice since it was pretty easy to change the salt level without significantly changing the salt level in the pool. Here is what I found out for my SWG which is an AquaLogic PS8 series cell and controller.

First, my unit is what would be considered a constant voltage bipolar SWG. Second, the tests confirmed that higher salt levels resulted in higher amperage as well as higher chlorine production which is exactly what electrolytic theory and chemgeek would predict. Here are the test results for each salt level and water temperature:

Test: Baseline
PH: 7.4
Water Temp: 64 deg
Salt: 2600 ppm (actually below the recommended level)
Volts: 26.44
Amps: 4.72
CL: 1.2 ppm start and 3.6 ppm after 18 min
CL Production = 21 (grams/hr) = .008 (grams/hr/ppm(salt))

Test: Increased water temperature
PH: 7.4
Water Temp: 81 deg
Salt: 2500 ppm
Volts: 25.8
Amps: 5.48
CL: 2 ppm start and 4.4 ppm after 18 min
CL Production = 21 (grams/hr) = .008 (grams/hr/ppm(salt)) (note no change for temperature)

Test: Increased salt level
PH: 7.4
Water Temp: 66 deg
Salt: 3400 ppm
Volts: 25.8
Amps: 5.79
CL: 1.8 ppm start and 5 ppm after 18 min
CL Production = 28 (grams/hr) = .008 (grams/hr/ppm(salt))

Chlorine production increases with salt level but not water temperature as theory would predict.

I agree with Chemgeek that cell life should not be dependent on salt level as long as you adjust the SWG for the same chlorine production. Given that most SWG’s are rated in amp-hours for cell life (which makes sense since this is what controls the electrolytic reaction), how long a cell lasts will be dependent on cell run time and salt level. This also determines chlorine production so in reality a SWG cell life should be determined primarily by how much chlorine is being produced per unit of time. Higher salt levels should require shorter run times and lower salt levels should require longer run times for the same chlorine production. Therefore the amp-hours should be the same and thus both conditions should have the same cell life. Am I missing anything here?

Also, there are some SWGs which have the ability to control both the voltage and current such that the chlorine production will remain constant with salt level but mine obviously didn't.

 

[Guest]

Mark,
I find that very intersting in light of the fact that conductivity not only changes with salinity but also with temperature. Perhaps one of the SWG manuafacturers will chime in here to explain why the vast majority of SWGs have a low temp shutoff to protect the cell.

 

[mas985]

I would like to know as well. From what I understand, electrolytic processes are suppose to be more efficient at higher solution temperatures. I didn't test over a very large range so perhaps the effect was masked by measurement accuracy. Most of the tests at higher temps did show a very slight increase in CL production but less than 5%. If I get a chance, I will try this again at 50 degrees and 100 degrees. That should show a difference if one actually exists.

Also, if chlorine production and efficiency drops significantly below 50 degrees the unit may shut off to save cell life. It may not really be needed anyway because of low algae growth.

I have at least 60 days of below 50 degree water temp each year and I don't bother adding any extra chlorine when the cell shuts off and haven't had much of a problem. Sometimes when I test the water, there is little to no chlorine left during this period but I let it go just to see what happens. So it would seem that at least for my pool, below 50 degrees, the SWG isn't really need much anyway.

 

[chem geek]

The conductivity temperature compensation parameter "alpha" for sodium chloride (at 10%, which is way more than the 0.3% in pools) is 2.14 which is multiplied by the temperature difference in Celsius (see this link). This is about 2% per ºC increase in conductivity. For 10ºF, this should be an increase of around 11%. For a 50ºF increase, this should be an increase of 56% so should be clearly seen.

Richard

 

[mas985]

The conductivity temperature compensation parameter "alpha" for sodium chloride (at 10%, which is way more than the 0.3% in pools) is 2.14 which is multiplied by the temperature difference in Celsius (see this link). This is about 2% per ºC increase in conductivity. For 10ºF, this should be an increase of around 11%. For a 50ºF increase, this should be an increase of 56% so should be clearly seen.

Richard

Am I missing something or in order to get a 2% change per ºC in the conductivity, doesn't alpha need to be .0214? The formula is: Gt = Gtcal{1 + α(t-tcal)}. 2.14 would be a 214% change per ºC.

0.0214 would be close the calculated alpha of 0.016892 for the measurements I made.

According to this article, it is .0214.

 

[chem geek]

You're right. Their formula is wrong and needed to divide the alpha value from their table by 100 (basically, the table was a %, not a direct alpha number as indicated in their formula). Their "2% per ºC" in the text was about right and what I've read elsewhere.

 

[New2Me]

I'm sorry! I knew better than to hijack, but couldn't resist. Thank you for starting a new thread.

It looks like I was correct in my thinking that others are wondering what the target salt level should be for optimum performance. I had read in other threads that the rest of the world runs at higher levels(up to ~6000ppm), and concluded that was better for chlorine production and cell life. Here in the litigious U.S. I figured that lower salt levels=lower electrical current+shorter cell life, which =lower electrocution(lawsuit) hazard+higher cell usage(sales=profit).

I plan to operate at the highest salt level my controls will allow.

 

[mas985]

Actually, as we concluded above, as long as your salt level is within the specified range, there is no difference to the cell life with higher or lower salt levels. With a higher salt level, the unit will need to be operated for a shorter period of time but at a higher amp level and visa versa for a lower salt level. The Amp-Hours should be the same to get to the same chlorine level so the life of the cell should be about the same.