Is an over-sized salt cell a wise investment for a pool owner?

[polyvue]

If we could expand this just slightly... There was some disagreement expressed in a recent thread as to whether an over-sized salt cell was a wise investment for a pool owner. Most of those posting seemed to have adopted the view that a cell's useful life is correlated to amps applied and the area of cell plating, but have discounted (or are unaware) of the notion that friction from turbulent water has any significant erosive action on the metal elements.

I've seen this controversy brought up more than once since I've been on the forum, so it doesn't seem as though it has been resolved to the satisfaction of all players -- if it's ever possible to get consensus on something like this.

Jason sent me a link to a technical discussion with Valera Orlingis that took place in 2007 that may be useful as a reference. I managed to get though about half of it last night...

swg-amps-volts-and-bipolar-cells-t2879.html

 

[JasonLion]

Re: Is an over-sized salt cell a wise investment for a pool owne

The cell plates in a SWG are coated with special catalysts that help the cell produce chlorine instead of oxygen. As the cell is used, those catalysts break down. Eventually the cell plates crumble away and need to be replaced. The rate at which the plates break down is determined by the number of amp hours they are run, which corresponds to the total amount of chlorine they have produced. The exact rate/lifetime depends on the specific plate coating used and how well it is applied, but will generally be more or less the same for all plates produced by a single manufacturer.

In almost all cases, a larger cell has more cell plate area than a smaller cell from the same manufacturer. In effect a cell that is twice as large is made up of the plates from two smaller cells all stuffed into the same container. I will continue with the assumption that the "large" cell is twice as large as the "small" cell, which is true when comparing the Pentair IC20 to the IC40. In a given pool the small cell will need to be energized for twice as long as the large cell to produce the same amount of chlorine. Since the small cell is turned on for twice as long, it will wear out it's plates in half the time.

There can be exceptions to this general principal. If you smash the cell with a sledge hammer it will be used up instantly, regardless of what size it was to start with. Cell failures that have nothing to do with the plates wearing out do happen, but they are relatively rare, perhaps a couple of percent at most. Thus the expected life time of the larger cell won't be exactly double, it will be more like 195% as long. On the other hand the larger cell doesn't cost anywhere near twice as much. The IC20 costs around $500, while the IC40 costs around $600. Spend about 20% more and the cell lasts about 95% longer, a dramatic price savings in the long run.

There are limits to this principal. Once you get larger than a certain size you need to switch to commercial cells to find larger cells, and they often cost substantially more. Also, once the SWG is over about three times larger than is required you start to lose resolution in the percentage setting, which lowers efficiency. But for most people most of the time, getting a cell the next size larger is a great investment.

 

[spishex]

Re: Is an over-sized salt cell a wise investment for a pool owne

I think that bigger cell size is definitely a wise recommendation. For all the reasons that have been laid out above, and because it eliminates the risk of accidentally winding up with a cell that is too small which would cause serious headaches.

 

[polyvue]

Re: Is an over-sized salt cell a wise investment for a pool owne

Its not my intention to take a position on this issue, only to understand it better.

[...]Since the small cell is turned on for twice as long, it will wear out it's plates in half the time.

The logic is impeccable... it must be true, all other things equal. But where's the empirical data that supports it? [If it's in the latter half of the 2007 thread referenced earlier I haven't yet finished reading it.]

Cell failures that have nothing to do with the plates wearing out do happen, but they are relatively rare, perhaps a couple of percent at most. Thus the expected life time of the larger cell won't be exactly double, it will be more like 195% as long.

So, I think you're not lending creedence to the idea that water erodes the plate material (independent of operating amperage, charge, chemistry etc). I'd best let Scott give his own reasons for this, so I'll be quiet now and follow the developments.

 

[JasonLion]

Re: Is an over-sized salt cell a wise investment for a pool owne

You are not going to find any empirical data, it is all proprietary. But you can find many telling statements by various people in the industry. Take a look at this Pentair manual and search for "usage hours". The cell is designed to be turned on for 10,000 hours. Likewise, if you search for commercial salt cell warranties, many of them specify that the warranty is good for 8,000 hours of cell turned on time. Likewise, in the topic I referred you to, there is a bit of a disagreement about watt hours vs amp hours, but either way it amounts to the same thing, cell life is based on the number of hours the cell is actually turned on.

 

[spishex]

Re: Is an over-sized salt cell a wise investment for a pool owne

Cell failures that have nothing to do with the plates wearing out do happen, but they are relatively rare, perhaps a couple of percent at most. Thus the expected life time of the larger cell won't be exactly double, it will be more like 195% as long.

So, I think you're not lending creedence to the idea that water erodes the plate material (independent of operating amperage, charge, chemistry etc). I'd best let Scott give his own reasons for this, so I'll be quiet now and follow the developments.

The biggest factor is cost. If there's a downside to getting a bigger cell it's the investment. As Jason said, you could have an IC40 standalone on a 5,000 gallon pool and not have the ability to control production as well as you need to, but that's not a real common issue.

So if price is the issue, take a look at his example of IC20 vs. IC40 cells: 479.95 and 549.95 respectively at PoolSupplyWorld. That's $70, or about 15% more, for the bigger cell. So even if there was wear caused by something other than power consumption it would have to be VERY significant to make the extra $70 not worth it.

Cutting into the lifespan that much comes down to serious water balance issues, and I wouldn't make recommendations based on an assumption of poor chemistry unless someone came in and said off the bat "I have never and will never test my water. What kind of SWG is best for me?"

 

[Strannik]

Re: Is an over-sized salt cell a wise investment for a pool owne

Its not my intention to take a position on this issue, only to understand it better.

[...]Since the small cell is turned on for twice as long, it will wear out it's plates in half the time.

The logic is impeccable... it must be true, all other things equal. But where's the empirical data that supports it? [If it's in the latter half of the 2007 thread referenced earlier I haven't yet finished reading it.]

i think this page has the info you are after:

http://www.tdconsulting.com.au/anode.php

 

[PoolGuyNJ]

Re: Is an over-sized salt cell a wise investment for a pool owne

What is being said is contrary to what I was taught, that it's not necessarily how hard you drive the unit but is the combination of the factors of power, water friction, chemistry and temperature changes that effect cell life.

The temp concern is not a rapid change type, such a condition is not normally encountered. With the exception of water, AFAIK, all matter expands and contracts at specific rates and amounts as temps rise and fall, respectively. The difference when two different materials are bonded, so there is direct contact, will stress that bond.

In the case of, say, a bimetalic strip as used in a thermostat, the strip will bend a measurable amount due to their being ductile. In the case of a cell blade, one of the two materials, the metal or the coating, and I don't know which, one will shrink more than the other. This will stress the bond. It has to. Ruthenium Oxide, according to my having Googled it for some time, last night and this morning is in a crystalline form when used for cell manufacturing. If the makers of cell blade use a ceramic matrix to bond it to the blade and it has some kind of elasticity, I can see it surviving. Neither ceramics nor crystals are my areas of expertise but I have never heard of either being ductile or elastic.

I do know that crystals have aligned molecules or atoms, depending on whether they are elemental or molecular. Many ceramics are not, such as most glass but many are. Exert pressure on either and they break along fracture lines. Some ceramics are partly crystalline.

My understanding is the coating is to prevent corrosion, not some catalytic process. The electrical current that flows between each set of plates, if no salt was in the water, would simply produce Oxygen and Hydrogen. Since we want chlorine, we add salt. The same charge splits the salt molecules as well as water molecules. The use of the coating is not to protect the titanium blade from the chlorine being produced, but rather from the caustic byproducts that cause our pH to rise. This may one of the more significant factors in a salt cell's life and by spreading the generation over a larger surface, may result in a longer continuous use life. That being said, since many close their pools due to cold weather, the temp concerns, I think are still valid.

I am not a scientist. I am a pool guy. I am not qualified in stating definitively, with credentials, reports, research, etc... usually disclaimer stuff. I have been taught and I have looked for information, just as any one of us can do. I know I have customers that don't want the cell brought in at closing and those that do. I know I have replaced the cells of those haven't brought them in, both cells my then employer sold as well as others. I know I have yet to have to replace any of the cells I have been bringing in for years. The number of them is not large, maybe 6 or 7 so, IMHO, that doesn't make for a definitive sample, yet. If the cells for those customers last another few years, they will be approaching the 10K hours of use. That makes me go hmmm.

Scott

 

[lbridges]

Re: Is an over-sized salt cell a wise investment for a pool owne

A white paper addressing the ruthenium oxide/titanium electrodes can be found at www.waterstarinc.com/files/Resources/White_paper.pdf

It has a bit of history as well as a discussion of 7 different failure modes for the electrodes. It ends with a sort of general overview of what sort of parameters an OEM would consider in a design set.

My apology if this is old stuff - haven't seen it referenced here before.

 

[JasonLion]

Re: Is an over-sized salt cell a wise investment for a pool owne

lee32903, thanks! That is interesting reading.

 

[spishex]

Re: Is an over-sized salt cell a wise investment for a pool owne

Wow. That paper is great. So to sum up and discuss the things that can prematurely age a cell:

1. Generation of oxygen instead of chlorine, which is possible in highly alkaline environments or in the absence of chlorides.
2. Erosion due to the rapid generation of gas on the surface of the cell.
3. Formation of inorganic compounds. They can be removed with an acid wash but the deposits take bits of catalytic coating with them.
4. Organics that are oxidized in the cell can adhere to the plate and "blind" the cell (cool term).
5. Titanium oxide can form at the coating-substrate interface due to generation of oxygen.
6. Presence of more aggressive halogens (fluoride/bromide ions) generates the more those gasses and dissolves the titanium.
7. Current Reversal, though convenient, "significantly reduces" the life of the cell vs. manually acid washing due to hydrogen generation.

 

[spishex]

Re: Is an over-sized salt cell a wise investment for a pool owne

#1 is mostly answered by measuring conductivity as all cells do, but I wonder what "highly alkaline" means?

I wonder if #2 caused the erosion confusion? Seems that high water flow would actually reduce this issue, same with 3 & 4. #4 is also interesting due to the discussions we've had about oxidation/superchlorination inside the cell minimizing the need to shock in SWG equipped pools.

I'm not science-y enough to understand how #5 differs from #1 except that it occurs on under the coating rather than on it.

#6 would seem to preclude the use of sodium bromide in pools with SWG's, but Autopilot mentions using sodium bromide in their manual? They use Titanium, don't they? I wonder what effects they've seen on lifespan from doing so are.

#7 is very interesting. Are there any major residential manufacturers still making non self-cleaning models?

 

[Strannik]

Re: Is an over-sized salt cell a wise investment for a pool owne

#7 is very interesting. Are there any major residential manufacturers still making non self-cleaning models?

Autochlor does

However they manufacture different types of material based on intended application, as per the link i gave above.

By the way something that just occurred to me.

Autochlor AC(single polarity) cells have cathode made out of bare titanium. In practice, cathode usually outlives anode. Autochlor provides a service for people to have their cells repaired by only replacing the anode on the AC cell, which comes out cheaper than buying a new cell. Usually cathode can live through 1 or 2 anode changes (7-15 years). The main reason for cathode failure is usually the fact that the plates start to bend for some reason and after a while it's just impossible to fit them back into the cell.

This directly contradicts Scott's statement about coating being needed to protect against corrosion.

I'll have a read through that white paper and give my thoughts on the rest of points a bit later.

 

[Strannik]

Re: Is an over-sized salt cell a wise investment for a pool owne

Wow. That paper is great. So to sum up and discuss the things that can prematurely age a cell:

1. Generation of oxygen instead of chlorine, which is possible in highly alkaline environments or in the absence of chlorides.
2. Erosion due to the rapid generation of gas on the surface of the cell.
3. Formation of inorganic compounds. They can be removed with an acid wash but the deposits take bits of catalytic coating with them.
4. Organics that are oxidized in the cell can adhere to the plate and "blind" the cell (cool term).
5. Titanium oxide can form at the coating-substrate interface due to generation of oxygen.
6. Presence of more aggressive halogens (fluoride/bromide ions) generates the more those gasses and dissolves the titanium.
7. Current Reversal, though convenient, "significantly reduces" the life of the cell vs. manually acid washing due to hydrogen generation.

Very interesting paper. But it seems to be heavily biased towards membrane cells

Here is how I see it:

1. Only an issue for membrane cells, as SWG cells usually have high water flow which doesn't allow surface to become highly alkaline (especially with polarity being reversed). Assuming pH within correct range.
2. As above.
3. Not sure how much of in issue it would be in SWG cells, but it would be mitigated by reverse polarity, I would imagine.
4. As above + acid cleaning
5. This one talks about the film between titanium and coating. It's true that if you have this film on a surface coating will not bond. In extreme cases it will just flake off during manufacturing process. Seen that a lot in early stages of AIS anode development But this would only be a problem experienced during production of an anode due to workers not following the technology precisely and quality control should pick it up.
6. No idea, however coating should prevent ions from getting to substrate (titanium plate). So i would imagine it will only be an issue when parts of the coating already fell off or on the edges.
7. More of a problem with Irridium oxide anodes, hence why they can't be used for reverse polarity applications. With Ruthenium oxide anodes you just need a stronger coating.

 

[chem geek]

Re: Is an over-sized salt cell a wise investment for a pool owne

The cathode where hydrogen gas is produced will be alkaline/basic (higher in pH) though as was pointed out the flow of water helps to minimize this. Nevertheless, this is the primary cause of scaling at that plate. The use of 50 ppm Borates helps to reduce this problem since it is an additional pH buffer particularly suited to buffer against a rise in pH cutting such a rise perhaps in half. If some of the reduction in cell life is due to the high pH at the cathode, then the Borates should help extend such cell life.

The auto-reversal would cut down the time that the plate is exposed to high pH in half (since it is only a cathode half of the time). It also helps to remove any scale that may have built up. Nevertheless, I've seen written elsewhere that auto-reversal reduces cell life though I'm not sure of the cause. Maybe if there is no auto-reversal that different kinds of special coatings can be used that are designed for the different kinds of chemical environment at the anode vs. cathode, but that's just speculation on my part. An auto-reversing cell would have to use the same coatings for anode and cathode so be resistant to the low pH high chlorine level at the anode and the high pH high hydrogen gas level at the cathode.

 

[Strannik]

Re: Is an over-sized salt cell a wise investment for a pool owne

as i said above, in practice cathode made out of bare titanium is capable of lasting 10+ years in non-reverse polarity cells.

the reversal does reduce the lifespan of material

 

[lbridges]

Re: Is an over-sized salt cell a wise investment for a pool owne

as i said above, in practice cathode made out of bare titanium is capable of lasting 10+ years in non-reverse polarity cells.
..

Perhaps you would choose not to say for proprietary reasons, but it would seem to me that a layer of essentially non-conductive Titanium oxide would form, inhibiting any current flow, and hence cell production.

Am I missing something, or is my impression of oxide formation and conductivity out of whack?

 

[Strannik]

Re: Is an over-sized salt cell a wise investment for a pool owne

well in my opinion the problem with oxide film is not that it prevents current flow, but that it prevents a firm bond between titanium surface and the coating at the time coating is applied. when you have bare titanium plate it doesn't seem to be an issue (and to be honest i don't know how conductive that film is).

it's kinda like when you paint your house, if you have dirt on the wall the paint is likely to fall off very soon

 

[cliff_s]

Re: Is an over-sized salt cell a wise investment for a pool owne

The original question was, would a larger cell last longer. A larger cell
would have smaller current density per cm2. I think the real question is
what is the failure mode of the cells.

The basic function of the cell is to be a current conductor between the
cell current and the water(+salt to provide conductivity). The byproduct
of current through a liquid is that the conductive metals by their nature
will migrate(plate). These tend to accumulate on either anode depending
on the polarity and type of metal. By reversing the current the anodes
and cathodes swap places, thus reversing the plating process and clean
the anodes or cathodes.

Most metals will oxidize and their oxides in most cases are what we call insulators,
or the reduction of current flow. Since the amount of chlorine that is generated
is a direct function of current flow any reduction in current flow will reduce the
chlorine output. Cell failure is usually the reduction in the amount of current that will
flow within the limits of the power supply. The material a plate is made of and any
oxide resisting coating that is applied determines the resistance to current flow.
This coating is only there to resist the accumulation of oxides which reduce the current flow.

Swimming pools do not contain just water and salt, they contain minerals of various sorts.
The worst one is Calcium. Calcium is a metal and it is conductive, thus it will migrate
from a solution to a plate, depending on polarity. Calcium by nature is a rather hard chemical
and makes a deposit that is difficult to remove. It will dissolve in acid, but not completely.
There are other metals, copper, zinc and some of the organic ones that will collect on the plates.
Some of the these reversing the current will not completely remove them. They will collect on the
plates a impede the current flow.

Then there are the corrosive ones, chlorine, salt and others. Any corrosive liquid(or gas) will
try to oxidize the plates, thus the need for non-corrosive metals for the plates and added coatings
to help minimize this oxidation.

For maximum cell life its enemies must be reduced. Water that contains no added minerals
and is pH neutral will be the best. The only problem is that this condition doesn't exist.
In various parts of the country the water pH will vary greatly(can be corrected), but not without
the addition of chemicals which some are conductive. Others contain high levels of minerals from
usually well water(calcium) which will clog the cell. Then require cleaning with acid and that will
reduce cell life as it removes some of the coating.

With all this said, what is the answer. Well, it might just a tossup between added cost and
longevity. Although the cell life will be more dependent on what is in the pool water rather than
the size of the cell.

Cliff s

 

[chem geek]

Re: Is an over-sized salt cell a wise investment for a pool owne

Whether the reason for failure of an electrolytic plate is due to plating of metals and their oxides or to electrolytic dissolving of the plate surface or of protective coatings, the rate at which these processes occur is a function of the current density at the plate surface. A larger plate area producing the same total amount of chlorine has a lower current density which means a lower quantity of problems per unit area on the plate. A larger plate (cell) therefore will last longer so if the price does not rise proportionately with area (and it doesn't), a larger cell is more economical in the long-run.