SWG Ideal Levels Also Equals "Corrosive" CSI Values - Why?

[Redstar7t]

Hello All,

This post is kind of a second parter to this post I made: Black Algae or Something Else?
I've been dealing with very small brown stains on my plaster this year, with this year being the first year I've had a saltwater cell.
In the above post I originally thought maybe black algae, but ultimately decided it was not organic. I've ended up speaking to a Jack's Magic rep to help determine the stain, and I think both them and I are in agreement it is some metal (maybe iron) covered in scale, which is why ascorbic acid does not react to it.

Jack's Magic rep did not like the levels I have been keeping my pool at - specifically my pH and calcium levels. I had been aiming for 7.6-7.8 pH and have 400-500 ppm calcium hardness. Jack's says these are both high and that a pH around 7.3 and calcium 200-300 will help greatly with my possible scaling problem. So I relooked over the pool school's post on water balance for SWG pools, and the pool school pretty much agrees: Water Balance for SWGs.

Lower pH and lower (but not too low) calcium is good for SWG pools. But I feel like these ideal levels go against everything I've researched on something I've tried to be very careful with: balanced CSI. There's a reason why I had slightly higher pH and calcium levels, and it was to keep my CSI above -0.3.
These "ideal" levels:
pH 7.3
TA 70
CH 350
CYA 70
Borates 40
Salt 3300
And a theoretical water temp of 85 F
Give a CSI according to poolmath at -0.54. And 85 F is pretty hot, my pool is usually cooler than that.

So both pool school and Jacks Magic recommend very corrosive CSI levels, right at the edge of -0.6. What do you guys think/what are your recommendations? Am I worrying too much about corrosion? How should I correct my scaling?

Thanks

 

[mas985]

So both pool school and Jacks Magic recommend very corrosive CSI levels, right at the edge of -0.6. What do you guys think/what are your recommendations? Am I worrying too much about corrosion? How should I correct my scaling?

Actually a CSI of -0.6 is not very corrosive at all. You might want to read through this thread:

The CSI is Reliable for Plaster-based Pools

Using the Calcium Saturation Index (also known as the LSI in the pool industry) as a guide for maintaining proper pool water balance and to protect pool plaster, including quartz and pebble finishes, has become a mainstay in our industry for good reason. Several experiments have been conducted...

www.troublefreepool.com

Based upon this test data, the erosion rates for a CSI of -0.7 are actually quite low (~0.0016" per year) and IMHO, nothing to get overly concerned about.

The CSI is Reliable for Plaster-based Pools

So was the same setup (i.e. coupon size and water volume) used in both experiments? Yes. I look forward to your analysis. Maybe some day I will conduct a more specific comparison. Obviously a big improvement would be to use the same time period for the experiment.

www.troublefreepool.com

I generally target between -0.6 and -0.3 and my plaster is almost 20 years old. But with very hard fill water and low rain fall, PH drift higher is always an issue for me which is why I target a lower CSI.

 

[Redstar7t]

Actually a CSI of -0.6 is not very corrosive at all. You might want to read through this thread:

The CSI is Reliable for Plaster-based Pools

Using the Calcium Saturation Index (also known as the LSI in the pool industry) as a guide for maintaining proper pool water balance and to protect pool plaster, including quartz and pebble finishes, has become a mainstay in our industry for good reason. Several experiments have been conducted...

www.troublefreepool.com

Based upon this test data, the erosion rates for a CSI of -0.7 are actually quite low (~0.0016" per year) and IMHO, nothing to get overly concerned about.

The CSI is Reliable for Plaster-based Pools

So was the same setup (i.e. coupon size and water volume) used in both experiments? Yes. I look forward to your analysis. Maybe some day I will conduct a more specific comparison. Obviously a big improvement would be to use the same time period for the experiment.

www.troublefreepool.com

I generally target between -0.6 and -0.3 and my plaster is almost 20 years old. But with very hard fill water and low rain fall, PH drift higher is always an issue for me which is why I target a lower CSI.

That's very interesting and obviously there's real science behind that, it just goes against everything everyone says both elsewhere and on these forums. Over and over again, less than -0.6 (and most people say less than -0.3) is a problem. Where did that come from?

If not even -0.7 is a problem, then what causes plaster etching/erosion? Seems like it'd be difficult to get parameters to a place of real problem (on the negative side)

And I know you say you keep yours between -0.3 and -0.6, but the Jacks Magic rep is wanting to take me literally to -0.6 and beyond. Seems pretty drastic. I'd like to hear other peoples thoughts as well

 

[mas985]

That's very interesting and obviously there's real science behind that, it just goes against everything everyone says both elsewhere and on these forums. Over and over again, less than -0.6 (and most people say less than -0.3) is a problem. Where did that come from?

I believe it is just people just being overly cautious and passing on misinformation. Plus CSI is technically a scaling index and not a etching index although we tend to believe there is a corollary to etching as well.

@onBalance is one of regular contributors and has done some studies in the past:

onBalance – Pool Help

The TFP WiKi is very consistent on CSI although some members may not be:

CSI and LSI - Further Reading

www.troublefreepool.com

If not even -0.7 is a problem, then what causes plaster etching/erosion?

It is a logarithmic scale so it gets worse very quickly plus it can be a very uneven process. Although the mean surface etched away may be small, there could be areas with significantly more etching due to the non-uniformity of the plaster. This is the main reason we suggest keeping CSI above -0.6. Also, you have those that dose with acid using the slug method which will create pockets of very low CSI which can cause problems.

Seems like it'd be difficult to get parameters to a place of real problem (on the negative side)

Not with Tri-Chlor pucks as they are very acidic and can drive CSI well below -1.

And I know you say you keep yours between -0.3 and -0.6, but the Jacks Magic rep is wanting to take me literally to -0.6 and beyond. Seems pretty drastic. I'd like to hear other peoples thoughts as well

Keeping above -0.6 should be fine.

 

[onBalance]

I have read both threads of yours. The information you read about "calcite crystals and low calcium levels" is incorrect. I suggest ignoring it.

Also, my guess on the plaster stains is from something in the plaster itself. Perhaps some iron pieces was mixed into the plaster mix or pieces of iron falling into the pool, or from tie-wires that were used to tie the rebar together. Tie-wires can end up dissolving and ooze out of the plaster and becoming somewhat liquid and discolor the plaster. I would have to see better photos regarding that for a better guess.

The issue about negative CSI's and plaster has a time element to it. Also, the quality of the plaster plays is an important factor regarding durability. I don't like to suggest going below -0.5 or -0.6 unless there is a reasonable purpose in doing so for a short term. I don't see lowering the CSI is going to solve that problem. And unless there is a scaling problem, I would advise not to constantly maintain a low CSI for the long-term.
Hope this helps a little.

 

[Redstar7t]

@onBalance
If it were iron, wouldn't ascorbic acid react with it? Ascorbic acid does nothing
Also this plaster job is a couple years old now, I didn't see anything like this until this year, with this year being the first year I've used a SWG.

I'm gonna have to roll with what Jack's Magic is telling me unless there are any other ideas, because I'm out of ideas myself. I'm just going to keep pH and calcium lower than what I usually would, which puts me at -0.5 CSI (during warm weather)

 

[onBalance]

I don't know how many spots there are, but have you tried sanding with 80 grit wet&dry sandpaper?

 

[Redstar7t]

@onBalance
Maybe a dozen or so very small spots around the pool at any given time. I've tried wet&dry sandpaper, depending on the size of the spot/how raised off the plaster it is, sandpaper will get it off.
Pumice stone however removes it very easily. That's basically what I've been doing all summer, just occasionally diving around the pool and scraping them off with a pumice stone. Then they show up in (different and random) locations a few days to a week or so later

 

[onBalance]

That sounds like some metal fragments are somehow getting into your pool.
Ascorbic acid is not very strong (compared to pool acid and perhaps not strong to remove certain types of iron stains such as is found in lawn fertilizer that contains iron.
If so, IMO, maintaining your pool water at a negative CSI (or LSI) below -0.5 will not work and not good for your plaster in the long run.