SWG scaling analysis

[nobody291]

I've noticed what I think is scaling starting to accumulate in my SWG (please let me know if it's not scaling ). I've attached a couple of photos. Does this seem like a little, a lot, or in between? My TA has drifted up over the last few months, so I'm starting to work on bringing it back down. My CH is at the low end, and my CSI would just be above 0 at a pH of 8 at the low winter temps, but that is changing as the water warms.

When a SWG starts to get too much scaling does its output gradually decline or drop off a cliff?

I'd appreciate any thoughts or suggestions!

Thanks, Eric

 

[ajw22]

My TA has drifted up over the last few months, so I'm starting to work on bringing it back down. My CH is at the low end, and my CSI would just be above 0 at a pH of 8 at the low winter temps, but that is changing as the water warms.

Give us actual numbers or have complete logs in PoolMath

TA - Further Reading

www.troublefreepool.com

Why Should TA be Lowered with a SWG?​

Usually a TA on the high end is fine if the pH is stable. However TA does need to be adjusted down with an SWG. The problem is, INSIDE the cell, the CSI is very different than in bulk pool water.

With high TA and aeration from the bubbles the SWG creates, you'll get faster pH rise inside the cell and so there's more potential for scaling. With current reversal, the cell tends to stay clean but you'll get more snowflakes out of the returns. Calcium hardness is only one part of the equation.

If the SWG cell pH rises much above 10 (and it can easily do that) and if there is sufficient levels of carbonate ions available (CO3--), then you will get calcium carbonate precipitation. The higher the TA and pH, the more carbonate anions are available.

Borates are particularly helpful preventing scale in a SWG because the pKa for boric acid / borate anion buffering is approximately 9. That essentially means you get the maximum buffering capacity at a pH of 9 and that tends to hold down the pH rise inside the cell. Keeping the pH below 10 in the SWG cell reduces the risk of calcium and magnesium scaling (insoluble magnesium hydroxide precipitates at a pH of 10.2 or so).

 

[JoyfulNoise]

It's hard to tell because the clear plastic cell looks so foggy ... is that normally how it looks? I can certainly see some build up at the edges of the plate which is normal given the way the current distributes itself during electrolysis. If you do clean the cell, please don't use muriatic acid right away. I would suggest you first try to blast as much of it away with the jet of water from your garden hose and then, if you can find a thin piece of stiff plastic, use that to clean between the blades with the hose water washing over it. Then, if you still feel like there is some haze or scale on the blades, use cleaning vinegar (acetic acid 6% ... you can find it in most hardware stores) and soak the blades for only as long as you see bubbles. If there's no bubbling after a few minutes, then there's no more scale and you can just remove the blades from the vinegar and rinse them clean with water. Muriatic acid, even diluted, is very corrosive to the catalytic coating on the blades.

 

[nobody291]

Give us actual numbers or have complete logs

My logs should be shared...my TA was 50 back in January and had risen to 100 on my last test. My CH consistently tests at 275 or 300.

is that normally how it looks? I can certainly see some build up at the edges of the plate which is normal given the way the current distributes itself during electrolysis.

I've attached a photo from September to compare and more scaling has obviously accumulated, but I can't tell if it extends beyond the edge. I did look more closely at the front and attached a pic and can see scaling only in the left-most channel.

If you do clean the cell

I don't plan to clean it yet. I'm more concerned about the need to more aggressively manage pH and TA. I haven't worried that much about them to this point, because I'm able to use softened water and keep my CH low.

 

[JoyfulNoise]

That’s pretty typical and normal scaling. The thing to understand about SWGs is that they ALL create calcium scale, it’s inevitable. That’s just how the physics and chemistry works when you have water that is saturated with carbonate alkalinity and calcium mineral hardness. If the water that passed through that cell all day long had no mineral hardness in it and no carbonate alkalinity, you’d never see the white stuff. The difference between what you have there and a cell that is completely clogged up with calcium is appropriate water management where the CSI is kept in the under-saturated regime. Then, the amount of scale formation is minimal as you show there.

Just try to keep a better eye on your TA and pH. It’s easy to ignore things in winter and, if you prefer to do that, then simply unplugged the SWG and chlorinate your pool with liquid chlorine until you’re ready to start swimming in earnest.

 

[nobody291]

Ok, thanks, I appreciate all of the input.

If scale was to continue accumulating, would the output of the SWG decrease in a linear fashion in proportion to the amount of the plates covered, or would the output decrease more suddenly as a certain threshold was crossed?

 

[JoyfulNoise]

Ok, thanks, I appreciate all of the input.

If scale was to continue accumulating, would the output of the SWG decrease in a linear fashion in proportion to the amount of the plates covered, or would the output decrease more suddenly as a certain threshold was crossed?

You tend to not see it in the chlorine output because the current will crowd towards more conductive areas of the plates and keep the output relatively constant. Where you see the effect show up is in the instantaneous voltage/current values but those are hidden by some vendors (Pentair). Hayward uses the cell efficiency to calculate salinity and so you can see the effect of scaled plates in the instant and average salinity values.

Just clean the plates when you notice scale. It’s doesn’t take much effort to do it and you can usually do it when you’re doing other maintenance like a filter clean.