Lots of flakes from SWG and falling CYA

[GeoQuack]

Not sure if this post should be here or in the advanced topics section. My apologies if this is the wrong place. Mods please move.

Over the last year or so my SWG is making more and more flakes of scale and I cannot work out why. My CSI has always between -0.2 and -0.3 until recently where I've been lowering my pH more to try and remove HCO3 and have a lower CSI to prevent the precipitation in the SWG. Also my CYA is falling. I thought it was supposed to stay in solution and the problem was removing it if you're not following the TFP and adding a chlorine product with CYA. My problem is needing to replace it regularly. My salt and Ca are stable so it's not due to water loss and dilution. CYA was 80 on December 1 and is now 20, a loss of 60ppm over approximately 90 days. This makes me think CYA must be a component in the precipitation. My SWG is still making chlorine although is needing about a 20% higher setting or runtime. There's a link below showing what my robot picked up with about 5 days between runs.

I'm at a loss. I don't want to spend a heap of money on a new cell (or whole SWG unit) to find it's a chemistry problem.

2 new photos · Friday, Mar 8, 2024

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[mgtfp]

How old is the cell? Have you been cleaning it with muriatic acid? The scaling sounds like worn out plate coating.

Can't imagine a way how CYA would precipitate out of the water unless there was melamine in the water, but I don't really see how that would be possible.

Still using Clear Choice Labs for testing? New batch of reagents? Old reagents (even though the CYA reagent should be pretty much indistructable)?

 

[mgtfp]

And you seem to let CSI get up to 0.3, I registered that on first reading as -0.3

To prevent scaling in the cell, you should keep CSI negative, between -0.3 and 0.

Try to maintain TA a bit lower, which should stabilise pH around 7.8, this will allow a more stable CSI with less swings.

 

[Rancho Cost-a-Lotta]

For me, regardless of what my CSI is, if TA is allowed to increase above 80 or 90 ppm with warmer water temps, my SWG will begin to blow flakes all over the pool. As @mgtfp suggested, try to keep your TA at 70 ppm or below. Adjust CSI based on the lower TA.

 

[GeoQuack]

How old is the cell? Have you been cleaning it with muriatic acid? The scaling sounds like worn out plate coating.

4 or 5 years. It's a generic replacement of the original. I've been cleaning it with double strength vinegar. Cleaning does not seem to change anything with the flakes.

Still using Clear Choice Labs for testing? New batch of reagents? Old reagents (even though the CYA reagent should be pretty much indistructable)?

Reagents new in Oct 2023. Yes, still using clear choice labs.

And you seem to let CSI get up to 0.3, I registered that on first reading as -0.3

Your original reading was the intent if not the content of my message. I've edited the original post. Until recently I've been keeping the CSI between -0.2 and -0.3. Now a bit more negative.

Any ideas on the falling CYA?

 

[Jimrahbe]

GQ,

If you lower your TA to 60, the SWCG flake problem will go away.

Not sure about your CYA.. Is your salt level also going down?? Do you have to add water often?

My CYA used to be rock solid, but for the past couple of years is seems not to 'last' as long.. This year I plan to buy a different brand (More expensive) and see if that makes any difference.

Thanks,

Jim R.

 

[GeoQuack]

GQ,

If you lower your TA to 60, the SWCG flake problem will go away.

Not sure about your CYA.. Is your salt level also going down?? Do you have to add water often?

My CYA used to be rock solid, but for the past couple of years is seems not to 'last' as long.. This year I plan to buy a different brand (More expensive) and see if that makes any difference.

Thanks,

Jim R.

Thank you.

No, I don't add water often and salt and calcium levels are stable. I don't think it's dilution.

 

[mgtfp]

Good that you didn't clean with muriatic acid, that usually makes it worse as it removes some of the the plate coating.

If it just recently got worse without changes in the water chemistry, then I'd guess on worn out plates.

I'd try to maintain TA a bit lower and see if that helps. Otherwise it might be a new cell. Could well be that the generics are a bit stingy on the plate coating.

No idea what's going on with the CYA. 60ppm loss over 3 months is definitely not normal. Have all the tests over this period been made with the same batch of reagent?

 

[GeoQuack]

I'm trying to get the TA down and I'll report back. Kids splashing are quite an effective way of aerating a pool it seems.

 

[Rancho Cost-a-Lotta]

Kids slashing are quite an effective way of aerating a pool it seems.

Cannonbaaaaaalllllll!

If you have an attached spa, allow it to overflow into the pool. Set returns to spa and suction from pool.

Have you seen this article?

Aeration - Further Reading

www.troublefreepool.com

 

[GeoQuack]

Cannonbaaaaaalllllll!

If you have an attached spa, allow it to overflow into the pool. Set returns to spa and suction from pool.

Have you seen this article?

Aeration - Further Reading

www.troublefreepool.com

Thanks for the link. I didn't find it when reading through the pool school stuff again before posting. I've ordered a large aquarium bubbler. We'll see how we go.

 

[GeoQuack]

Aquarium bubbler worked, but it was slow. I got concerned exposing the rest of the pool to a significantly negative CSI for longer than necessary. It would also be counterproductive to the goal of lowering HCO3- if any CaCO3 were leached from the pool surface as it would increase HCO3 again.

So, I rigged up a shop vac with its tubing tied to the pool pole. I removed the bin and bag, etc to reduce the pressure gradient it had to work against. In about 4 hours I did 3 cycles from 7.0 to 7.6 and got the TA to 70. The bubbler can do the last 10. See pic of the aeration below. I had the shop vac unit tied so that it couldn't fall in the pool and tested the RCD on the circuit it was plugged into before starting.


My results are now:

FC 11.5
CC 0
pH 7.6
TA 70
CH 450
CYA 80
Salt 4400
Temp 30C (86F) - I know it's a bathtub, but my kids stay in it for hours at this temp.
CSI -0.23

I turned the SWG back on. I didn't have time to clean it, but despite this, no flakes! It also seems to be making more chlorine, as my FC is increasing and I need to turn it down.

Thank you to @Rancho Cost-a-Lotta, @mgtfp and @Jimrahbe.

It must be that when the water is heated and pH increases, the CSI increases more when the TA is high. I'm interested in an explanation from any chemistry nuts.

Thanks again.

 

[mgtfp]

It must be that when the water is heated and pH increases, the CSI increases more when the TA is high. I'm interested in an explanation from any chemistry nuts.

Not sure if I understand what effect you are describing there. Can you give a bit more detailed description of what you want an explanation for?

 

[GeoQuack]

CSI describes the propensity of calcium carbonate to move from aqueous to solid (+) or solid to aqueous (-). The CSI in the pool itself is similar now to before, however, I have no flakes forming in the SWG. Therefore reducing the TA must reduce the (+) change in CSI that occurs in the SWG when H+ is taken out of the water at the negative electrode to form H2.

 

[mgtfp]

Ah, OK. Just sounded a bit more complicated what you first described.

The main part of Total Alkalinity (TA) is Carbonate Alkalinity (CA). The main buffer system in a pool is the the carbonate buffer system by adding baking soda. You end up essentially with carbonic acid (which turns mainly into dissolved carbon dioxide at pool pH) and bicarbonate and carbonate ions. Those last two have buffering capability as they can accept protons. All these carbonate species (the term "carbonate" used more loosely here for the whole group of different molecules) are connected via pH-dependent equilibrium reactions. At pool pH you have mostly bicarbonate ion as a buffer, hardly any carbonate ion.

There are other contributions to TA, mainly from CYA and Borates (when used).

The CSI formula will then subtract the (also pH-dependant) Alkalinity contributions from CYA (and Borates), and calculate how much of the resulting Carbonate Alkalinity is made up of actual carbonate ions (CO₃^2-).

The CSI is then essentially the logarithm of the product of the calcium ion (Ca²⁺) and carbonate ion concentrations (more precisely the activities, which brings in the effect of other charged dissolved ions, predominantly from added salt in a salt pool that shield the ions in question), divided by the Calcium Carbonate solubility product.

In short: The higher the TA, the more carbonate ions, the higher the probability that a calcium ion will find a reaction partner to form calcium carbonate.

 

[Divin Dave]

Aquarium bubbler worked, but it was slow. I got concerned exposing the rest of the pool to a significantly negative CSI for longer than necessary. It would also be counterproductive to the goal of lowering HCO3- if any CaCO3 were leached from the pool surface as it would increase HCO3 again.

So, I rigged up a shop vac with its tubing tied to the pool pole. I removed the bin and bag, etc to reduce the pressure gradient it had to work against. In about 4 hours I did 3 cycles from 7.0 to 7.6 and got the TA to 70. The bubbler can do the last 10. See pic of the aeration below. I had the shop vac unit tied so that it couldn't fall in the pool and tested the RCD on the circuit it was plugged into before starting.

View attachment 559253
My results are now:

FC 11.5
CC 0
pH 7.6
TA 70
CH 450
CYA 80
Salt 4400
Temp 30C (86F) - I know it's a bathtub, but my kids stay in it for hours at this temp.
CSI -0.23

I turned the SWG back on. I didn't have time to clean it, but despite this, no flakes! It also seems to be making more chlorine, as my FC is increasing and I need to turn it down.

Thank you to @Rancho Cost-a-Lotta, @mgtfp and @Jimrahbe.

It must be that when the water is heated and pH increases, the CSI increases more when the TA is high. I'm interested in an explanation from any chemistry nuts.

Thanks again.

 

[Divin Dave]

I'm no chemistry nut, and certainly do not claim to be an expert in the subject. Far from it.
But I can tell ya, CSI fascinates me because it's so finicky in what effects it and by how much.

If you have never played around with the original Pool Math, you can use it to see what the effects of x, y, and z are, and maybe sort of get a grip on what happens to CSI as your water parameters change. I have found it to be very helpful ............

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.

www.troublefreepool.com

 

[GeoQuack]

In short: The higher the TA, the more carbonate ions, the higher the probability that a calcium ion will find a reaction partner to form calcium carbonate.

But if I lower the pH to achieve the same CSI I push the equilibrium reaction away from CO₃^2-. Making the probably of Ca²⁺ finding CO₃^2- the same.

My only thought is that at a lower pH we are closer to the pKa of carbonic acid, and therefore more buffering to bicarbonate would occur in the SWG than at a higher pH, lower TA and the same CSI.

If you have never played around with the original Pool Math

I have it pinned in the browser on my phone.

 

[mgtfp]

But if I lower the pH to achieve the same CSI I push the equilibrium reaction away from CO32-. Making the probably of Ca2+ finding CO32- the same.

Yes, you can achieve the same CSI at higher TA by lowering pH. But due to CO2 outgassing pH will always want to push up and you get into that neverending pH battle to maintain low enough pH. Much easier to maintain TA a bit lower and allow pH to settle a bit higher.

My only thought is that at a lower pH we are closer to the pKa of carbonic acid, and therefore more buffering to bicarbonate would occur in the SWG than at a higher pH, lower TA and the same CSI.

In the cell, near the cathode where hydroxide is being produced, pH is much higher anyway, which is why the cell scales up first. Polarity reversal helps to keep that in check to a degree.

Most efficient way is to add a secondary borate buffer system with a pKa that is much better suited to buffer the localised pH-rise within the cell.

 

[GeoQuack]

Most efficient way is to add a secondary borate buffer system with a pKa that is much better suited to buffer the localised pH-rise within the cell.

I have done this previously, however the place where I bought a big bag of boric acid from no longer supply it. I see from your sig that you're based in Melbourne. Can you suggest where to get boric acid?