What makes the difference?

[chem geek]

So, would a smaller cell have fewer plates or the same number but smaller plates?

Either approach works but as Mark pointed out it makes more sense to shrink the plate area which means shortening the length of the SWCG cell. If one removes plates then to retain the same current density the remaining plates stay where they are (because voltage increases so resistance must increase which means distance must increase so these cancel out and the remaining plates remain where they were in position). So by removing plates one hasn't made the overall cell physically smaller.

One of the determinants to cell life is current density. Technically, one could design a cell to last longer by lowering the current density and making the cell larger (greater plate area) to have the same output so manufacturers make a tradeoff in terms of cell size vs. cell life. Note that this is why the smaller cells have roughly the same life as the larger cells because their current density (current per plate area) is the same.

 

[mguzzy]

Ooh. great thread.. Some physics/chemistry behind how these thing-a-ma-bobs work. I should find the tech papers I dug up while researching my SWG 10 yrs or so ago.. but the internet was a different place back then and so was my computer.

Some good questions. Maybe there is some experience, but I think we PRESUME and perhaps rightfully so that if a cell runs less, it will live longer. Stands to reason, but I certainly have no proof or experience to back it up.

Yes, more plates in the cell increase chlorine gas output.

I don't have laboratory research to give you factual data. I do have a few decades of field experience. If you have a high output cell that runs less time or at a lower output, the cell lasts longer. That is my first hand experience.

...

This is exactly the philosophy that was quoted to me some 10 years ago when I converted my pool to an SWG. I put a T-Cell-15 (40K) on a ~20K pool and my cell lasted for almost 9yrs. Granted I am one data point with an anecdotal story. It would be nice if someone did a long term study to measure the changes in the plate depositions in a more quantifiable method for cells that were used like mine. And Compare it to someone that ran a 20K on a 20K pook and a 40K cell on a 40K pool. Does anyone know of such a study? I would be happy to donate my spent cell...

I have a decent background in science so that the reasoning seems sound, and seems to hold up from my experience with my cell. Do you pool guys see something similar in the field?

 

[chem geek]

When the cell is turned off it is not degrading (at least not at any rate that matters in practice). There is no calcium carbonate buildup because there is no pH rise because there is no hydrogen gas generation. There is no coating dissolution or substrate passivation because their is no electrolysis when the cell is off and there is no chemical attack from the pool water. The wear on the plate material comes from the chemical and physical processes and by-products of electrolysis, not from some metal plates simply sitting in the pool water.

For example, oxygen that is generated as a by-product (when chlorine gas is the desired product) can migrate to the titanium substrate and thicken the titanium oxide (TiO₂) passivation layer thereby increasing resistance which increases heat generation and temperature and can ultimately lead to cell failure by lowering its output (at the same applied voltage). This is why some plate coatings have an intermediate coating layer to inhibit such oxygen migration to the titanium substrate. Likewise, oxidation of the ruthenium oxide coating from RuO₂ to RuO₄ destabilizes and removes the coating lowering chlorine output, increasing oxygen output, and resulting in further oxygen oxidation attack especially to the now exposed substrate. Surface deposits on the coating effectively increase the current density of the remaining coating that leads to more wear, but polarity reversal effectively removes such surface deposits. Reversing polarity too frequently, however, results in faster degradation since in effect the current density becomes concentrated with gas generation at the outer coating surface thus stressing it until conditions shift to allow for gas generation over a thicker volume of coating. Also, until the acidic conditions build up from chlorine generation, the oxidation of the ruthenium coating is more likely.

RuO₂ + 2H₂O ---> RuO₄ + 4H⁺ + 4e^-
which is more likely under alkaline conditions during initial polarity reversal instead of
2Cl^- ---> Cl₂(g) + 2e^-
which is followed by
Cl₂ + H₂O ---> HOCl + H⁺ + Cl^-
that creates acidic conditions

It would seem to me that the polarity reversal should be done by first shutting off the cell giving time for conditions to become more pH neutral and then to more slowly ramp up the current (i.e. current limit the power supply) to give time for conditions to equilibrate and chemicals to migrate to favor chlorine production over oxidation of the ruthenium oxide coating.

This is why I started the thread Economics of Saltwater Chlorine Generators to show that the larger cells were much more economical because their prices did not increase linearly with their output and the lifetimes of the cells were similar.

 

[mas985]

I think the reason they probably don't remove plates for the smaller cells is that with a larger plate gap, in order to maintain efficiency, you need higher isolation at the plate edges to prevent current from bypassing the opposite plate . If you notice on the T-15, they use rather long plastic fins on the plate edges which helps to reduce this bypassing current. If you double the gap, you would need to increase the isolation fin length which would then make the cell total length longer. The easiest way to make a smaller cell is to just shorten the plates.

 

[JamesW]

The Aquapure 1400 has 13 plates. The aquapure 700 has 7 plates. They took out one half. The remaining half has the same plate spacing.

The smaller t cells have 13 smaller plates with the same spacing.

The ic 20 and ic 40 could use either strategy assuming that the 20 is half of the 40.

 

[mas985]

The Aquapure 1400 has 13 plates. The aquapure 700 has 7 plates. They took out one half. The remaining half has the same plate spacing.

They must also halve the voltage or the current would be twice as much?

The smaller t cells have 13 smaller plates with the same spacing.

So they are shorter plates or narrower?

 

[JamesW]

The voltage stays the same. It's about 26 volts from the center plate to the outer two plates (3 pins). The 700 has two pins one to the center plate and one to the outer plate. The current is half.

The smaller t cells have shorter plates.

 

[mas985]

I guess that makes more sense to do it that way for a 1/2 size Jandy unit. Hayward could have done the same since it is a similar setup but chose sizes other than 1/2 which pretty much forces them to reduce the plate size.

 

[JamesW]

It looks like AutoPilot also uses a 7 blade cell for at least one of their smaller cells. For Jandy and Autopilot, the 7 blade cells only have two electrodes, whereas the largest cells have thirteen blades (titanium plates) and three electrodes.

The center one connects to the center plate, the two outside electrodes connect to the outside plates and the 5 blades (on each side) between the center and outer blades are charged by the potential difference between the center and outer blades.

It seems that a manufacturer could use either strategy, or even both, to change the size of the cells.

For Jandy, Hayward and AutoPilot, it seems that the voltage is in the 24 to 30 volt DC range and they use either one set of seven blades or two sets of blades. Since the center works for both sets, the total with two sets is 13 blades, even though Jandy lists the 1400 as a 14 blade cell.

 

[mas985]

The best strategy depends on the cell size. They could use either strategy only if the smaller cell is exactly 1/2 the size of the bigger cell. Anything other than exactly 1/2 size and they would have to reduce the plate size like Hayward does or they could remove fewer plates but then they need to change voltage too so the voltage & current per plate remains the same.

So I think it makes more sense to remove 1/2 the cells (1 set of series cells) when making a 1/2 size cell and for other sized cells, to reduce the plate size. Which is exactly how each manufacture is doing it. Although Pool Pilot has the ability to change voltage as well within a specific cell size.

 

[DeadAquaRite]

This is exactly the philosophy that was quoted to me some 10 years ago when I converted my pool to an SWG. I put a T-Cell-15 (40K) on a ~20K pool and my cell lasted for almost 9yrs. Granted I am one data point with an anecdotal story.

Two data points. 40k cell in a 27k pool. Yielded almost exactly 9 years.

 

[Jaimslaw]

Two data points. 40k cell in a 27k pool. Yielded almost exactly 9 years.

So if I may, what is your cleaning regimen? I assume some MA, and if so, what is your dilution ratio? And do you drop it is a Bucket with the MA solution and wait for bubbling to stop? Also, how much scale build up do you find in the cell between what I assume are three month interval cleanings. Any other tips to get us into those long years of use?

 

[DeadAquaRite]

I have never cleaned that cell.

As for long life, it wasn't a plan of any und. I just don't run it any more than necessary. 8 hours summer, 4 hours winter, about 30-50% output as needed to keep FC in line. I use BBB (less borax) and shock with bleach.

As someone else mentioned. 40k cell in a 27k pool

 

[JayBauman]

If the use of MA to clean them leads to the deterioration of those plate coatings, you'd certainly think that using popsicle sticks would be less intrusive.

Maybe not, Woody. Over time, MA can chemically weaken the layer, and this more-or-less happens uniformly across the plates. But it's a necessary evil, since mineral build-up is a bigger problem.

Using your popsicle stick to scrape the plates can cause a different failure mechanism. By mechanically abrading the plates, you could scratch off the coating in a local area, which can cause all sorts of electrochemical "fun" in terms of localized current (and/or voltage) densities. These localized "hot spots" can then go on to cause premature cell failure.

I say pick your poison....but I'd never touch my plates with anything that can scratch their surfaces.

 

[Jaimslaw]

Maybe not, Woody. Over time, MA can chemically weaken the layer, and this more-or-less happens uniformly across the plates. But it's a necessary evil, since mineral build-up is a bigger problem. .

I've tried to devise all sorts of cleaning tools that would reach into those hard to get areas of the cell and also that would not have abraisive qualities. But alas, the mineral deposits that do accumulate are sometimes "glued" on tothe plates - ie, tough to "scrub" off - and that's when you can actually get those areas. I plan to cut back on my MA dilution from 3 to 1 (water to acid) to 4 or maybe 5 to 1. I also always try to get a strong blast of water but the angles and plates inside the cell really cut down on the force of the water stream and consequently, the effectiveness. My cell life (x2) has been in the 3 to 4 year range. Definitely would like to bump that up, since I too get the t-15 big boy (40k).

Hopefully, the acid injector system I just installed and am still tweaking will get optimal PH levels to get ito a "steady eddie" state. But can't do a whole lot about the extremely hard water that we have here in SoCal.

For most of us, it does seem to be a "pick your poison" dilemma. And agreed that staying away from anything that will scratch the surface is a must.

Cheers.

 

[JoyfulNoise]

If you add borates to your water you can cut down on the cell scaling immensely. It's one of the side benefits of borates, they cut the [EDIT] I incorrectly stated H2 production when it should be pH [END-EDIT] hydrogen gas production pH roughly in half.

Also, for acid cleaning, you absolutely want to use diluted acid. I never use less than 5:1 dilution. I prefer 9:1 as I don't get a lot of scale. You should always mix the dilution up before adding it to the cell and use a a cap on one side so that you don't have to immerse the whole thing. Just fill the cell up and let the solution clean for a few minute. If you don't have an end cap, a rubber handball works too.


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[chem geek]

If you add borates to your water you can cut down on the cell scaling immensely. It's one of the side benefits of borates, they cut the hydrogen gas production roughly in half.

It cuts the pH rise associated with the hydrogen gas production roughly in half. It does not change the hydrogen gas production itself. The same amount of hydrogen gas is produced as without borates. The borates are just a pH buffer that have the pH rise about half as much.

The following table shows how much the pH rise is cut down by using borates. The assumptions are for SWCG pools with TA 70 ppm, CYA 80 ppm, starting pH 7.7 and I show varying levels of hydrogen gas generation or distance from the plate (i.e. dilution rate) so varying amounts of pH rise (each line is double the generation of the previous line). You can see that the reduction in pH rise is more than half at higher pH. The pH buffering is quite strong.

Start pH . 0 ppm Borates . 50 ppm Borates
... 7.7 ............ 8.0 .................... 7.8
... 7.7 ............ 8.4 .................... 7.9
... 7.7 ............ 8.9 .................... 8.0
... 7.7 ............ 9.4 .................... 8.2
... 7.7 .......... 10.0 .................... 8.5

Since we usually see that without borates the CSI needs to be kept to no higher than -0.2 to prevent scaling in the SWCG cell and that we normally don't see regular scaling in non-SWCG pools until the CSI is +0.7 or higher (+1.0 or more, especially), I'm guessing that the pH near the hydrogen gas generation plate is normally in the 8.4 to 8.9 range so the use of borates brings down the CSI increase from +0.7 to +1.2 to only +0.2 to +0.3. So it may even be possible that when using borates the CSI need not be kept slightly negative (i.e. the -0.2 target isn't needed and +0.0 may be OK).

 

[Jaimslaw]

If you add borates to your water you can cut down on the cell scaling immensely. It's one of the side benefits of borates, they cut the hydrogen gas production roughly in half.

Also, for acid cleaning, you absolutely want to use diluted acid. I never use less than 5:1 dilution. I prefer 9:1 as I don't get a lot of scale. You should always mix the dilution up before adding it to the cell and use a a cap on one side so that you don't have to immerse the whole thing. Just fill the cell up and let the solution clean for a few minute. If you don't have an end cap, a rubber handball works too.

Looks like its time for me to get on thbe Borates train. Will be reading up on that, and make sure I have the right chem adding sequence going insofar as where I am at right now, ie., : just got to the 30 day mark following a replaster to getting my salt levels, etc. to optimal. Also just installed a Stenner acid injection system so am still in the tweak process there as well. But again, will have to be sure I have the right sequence insofar as what to balance out first (besides salt of course).

 

[JoyfulNoise]

It cuts the pH rise associated with the hydrogen gas production roughly in half. It does not change the hydrogen gas production itself. The same amount of hydrogen gas is produced as without borates. The borates are just a pH buffer that have the pH rise about half as much.

Thanks! I went back and edited my post with the correction.

- - - Updated - - -

Looks like its time for me to get on thbe Borates train. Will be reading up on that, and make sure I have the right chem adding sequence going insofar as where I am at right now, ie., : just got to the 30 day mark following a replaster to getting my salt levels, etc. to optimal. Also just installed a Stenner acid injection system so am still in the tweak process there as well. But again, will have to be sure I have the right sequence insofar as what to balance out first (besides salt of course).

It's truly optional and not required but it does have some nice side benefits, especially for SWG owners, if your willing to go through the expense of adding them.

 

[Jaimslaw]

- - - Updated - - -

It's truly optional and not required but it does have some nice side benefits, especially for SWG owners, if your willing to go through the expense of adding them.

I will definitely want to get on the learning curve for this. I have a dual spill over spa that I like to keep during pool pump hours - just wouldn't be the same without that nice splashy sound. But its aeration city to say the lease which has made dealing with the PH creep a career for me over the years. I was just getting into the TA lowering tweak. Will read up to see how that variable works into all of this.

Even though the Stenner acid injection will, hopefully, get the PH level into a good daily balance and on an auto pilot basis, I would still want to be able to reduce annual acid consumption if I can. I'll pull as many posts on this subject and review them to be sure I get it straight - tho can't say I'll ever reach the same level of understanding as you guys about all this.

When it comes time for my people to take over your planet, you guys will be spared.

Cheers.