Newbie Fighting Green Aglae Cc 5.2, Tc 0

[sethz]

Greetings, I am new pool owner. Long story short, the pool was neglected by previous owner last fall, they did hire people to close the pool, but my guess is they didn't vacuum it nor check chemistry.

For some reason, I just opened the pool 3 days ago (yeah, not too much time left, hehe), the water is green and muddy, bottom is covered by leaves. The opening guys throw a bottle of algaecide and two bags of shocks. Then I keep the filter running, and vacuum/brush, and backwash/add DE, again and again.

Today the pool looks much better, still little greenish though. I went to pool shop for water test, here it is:

FC 0

CC 5.2

pH 7.2

TA 70

CH 130

CYA 0

I have read through the pool school last 2 days. If I understand correctly, CC are the dead soldiers fought algae or other organic. FC 0 mean I don't have any live soldier available. So I am losing the battle against algae, right? I poured 5 lb cal-hypo 65% 4:00PM today, according to poolcalculator, it will raise FC to 20 for a shock. (next time I will try household bleach).

Also there is no cyanuric acid at all in the pool. So I plan to put 5lb stabilizer tomorrow (according to poolcalculator, it will raise CYA to 30).

I am a total newbie. All my knowledge came from reading pool school and Tamminen's book last 48 hours. Please feel free to advice/comment. Thank you

[quantumchromodynamics]

Please do not use any more calcium hypochlorite. If you use calcium hypochlorite while your pH and alkalinity are low, you could get black staining where the cal-hypo hits the floor.

Hold off with the cyanuric acid until you get your water clean and clear.

Use bleach to keep your free chlorine at 15 ppm until all of the combined chlorine is gone, and the pool is clean and clear.

Get the Taylor K-2006 test kit. http://www.taylortechnologies.com/products...&KitID=2230

[chem geek]

Did the pool have CYA in it when you closed it? If so, then bacteria may have converted the CYA into ammonia in which case you may need to add a lot of chlorine. With a good test kit that doesn't bleach out at high chlorine levels, you can keep adding chlorine every half-hour or so until it starts to hold. Or you can do a bucket test to see how much chlorine it will take before it starts to register -- 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC.

Soon after you start to hold chlorine in your pool, then you can add a small amount of CYA to protect it from sunlight (even 10-20 ppm would be OK to start out). I wouldn't wait until the pool is clear before adding CYA -- too much will get broken down by sunlight during the day -- but right now when you aren't getting a chlorine reading just use chlorinating liquid or 6% unscented bleach and wait on the CYA until later on when the chlorine registers shortly after adding it.

Remember that chlorine is a consumable so you need to keep adding it if it gets used up quickly -- but you also need a proper test kit to ensure you aren't bleaching out the chlorine test. If you can't get the Taylor K-2006 quickly, then at least get a cheap OTO (shades of yellow) chlorine test that will at least tell you if you've got chlorine in the pool, but get that Taylor K-2006 as soon as you can -- it's at a good online price here or you can get the TF100 from tftestkits.net here with the latter kit having 36% more volume of reagents so is less expensive per test.

[quantumchromodynamics]

The reason that I advised the O.P to wait to add the cyanuric acid is that I was concerned that the bacteria would just consume it and convert it into more ammonia. In addition, the chlorine is more effective at lower cyanuric acid levels. Although, you could just increase the chlorine level. Once the bacteria or biofilm is under control, then the OP should begin adding the cyanuric acid.

The OP should add baking soda to raise the pH and total alkalinity. They should also aerate to raise pH and get some dissolved oxygen into the water. The bacteria are anaerobic and don't do as well with dissolved oxygen in the water.

The higher pH will also help form monochloramine instead of dichloramine and nitrogen trichloride. The monochloramine is an effective algae and bacteria killer. Yellow-Out is based on creating monochloramine and their directions specify raising the pH.

I would also recommend testing for ammonia to see what the levels are.

Sodium percarbonate or sodium perborate would be effective in this case to raise pH and oxidize combined chlorines although the OP would probably not be able to get any fast enough to help unless they could find it locally (ProTeam® System Support). I am not recommending this approach; I am just including it as an option for reference.

In this case, I think that the chlorine option is the best way to go.

[chem geek]

Got it. For sure, until any ammonia or other byproducts are gone and an FC starts to hold it doesn't make sense to add any CYA which is hopefully the point I was making in my post. However, after FC starts to hold I think the protection of chlorine from sunlight by CYA becomes important unless one is staying with the pool adding chlorine every hour or so. In direct noontime sun, half of the FC can drop in around a half-hour. In any event, if CYA is used, it shouldn't be very much since it does reduce the chlorine effectiveness.

Bottom line, hitting this pool hard with chlorine will clear it up faster, one way or the other, and will require persistence until the FC starts to hold. The slowest part of the clearing process is after the algae is dead and the water is cloudy where the full oxidation or filtration of the algae takes days and during that process one is usually not by the pool so having CYA preventing chlorine loss from sunlight would let one not have to stay by the pool, though it may slow down how long it takes to clear somewhat.

We don't really know if there is ammonia in the pool so I didn't want to count on monochloramine being formed to help kill the algae. You don't really have to raise the pH for this effect. It's only at far lower pH where the formation of dichloramine becomes substantial enough to worry about (I have a breakpoint chlorination model you can play with here). The addition of chlorinating liquid or bleach will raise the pH anyway. I'm not sure I see the point in raising the TA. As for oxygen in the water, that's interesting because it is anaerobic bacteria that utilize CYA and convert it to ammonia, but I don't know how effective aeration will be to slow them down -- it probably wouldn't hurt.

[sethz]

Thank you, quantum and chem geek. I will keep your advices in mind for the comming days adventure.

A follow up, this morning the pool water LOOKs crystal clear. I bring the water sample to the pool shop again:

FC 0

CC 0

pH 6.9

TA 50

CH 225

CYA 0

Phosphorus 200

So even I putted 5 lb cal-hypo 65% yesterday 4pm, now all chlorine is gone.

This time I pour 368 Oz bleach, 4lb borax, 4lb baking soda at 4pm. (will do a bucket test after the post)

I went to local Leslie's, they don't even stock the rebranded Taylor K-2006 kit that everybody recommended. Will order online.

BTW, pool shop guy mentioned the 200 ppm Phosphorus is high, and that's why pool won't hold chloride (I doubt this). He recommended expensive phos-X, I recalled pool school's comment about phosphorus, I said I will pass.

[chem geek]

BTW, pool shop guy mentioned the 200 ppm Phosphorus is high, and that's why pool won't hold chloride (I doubt this). He recommended expensive phos-X, I recalled pool school's comment about phosphorus, I said I will pass.

Good move, since he told you baloney. My pool has 2000-3000 ppb phosphates and my fill water has 300-500 ppb phosphates in it and the pool holds chlorine just fine (see here. Phosphates are just a nutrient for algae (as are nitrates and carbonates) but algae can be killed and stopped in its growth using chlorine alone. You don't have visible algae in your pool and at this point with the amount of chlorine you are adding it does sound like you've got ammonia that came from bacteria converting CYA into ammonia. As I mentioned before, you could get an inexpensive ammonia test kit from a pet/fish/aquarium store or just do the bucket test to see how much more chlorine it will take. And yes, be sure to get the good test kit, either the Taylor K-2006 or the TF100 as linked to earlier.

[quantumchromodynamics]

Be careful about adding too many chemicals at the same time. It takes time for the chemical to mix into the water and to be accepted into the water. Most people only allow a minimal time for chemicals to mix before they add the next chemical; this can cause cloudiness and other problems. Always allow a minimum of two hours between chemical additions. For pH increaser and any calcium-containing product, allow a minimum of 12 hours. For Alkalinity increaser and any calcium-containing product, allow a minimum of 8 hours.

I recommend that you add enough baking soda to increase your total alkalinity to 100 and you should maintain chlorine at 15 until it starts to hold. Ignore phosphorous/phosphates. Make sure that you dissolve the baking soda in a bucket of water before adding it to the pool so that it gets mixed in quickly.

What equipment do you have?

What is your method for chlorine?

Do you have a salt-water chlorine generator?

Can you test your fill water and post the results?

How did your calcium go from 130 to 225?

I think that in future cases like this, where there is no free chlorine, high combined chlorine, suspected ammonia and suspected cyanuric acid reducing bacteria, that hydrogen peroxide 27%, sodium percarbonate or sodium perborate would be a good choice. They will oxidize the combined chlorines, ammonia, bacteria and algae. They will also oxygenate the water, which will create inhospitable conditions for anaerobic bacteria.

27.5 Hydrogen peroxide is readily available as Baquacil shock (Not the sanitizer).

http://www.chemistrystore.com/cart.cgi?gro...amp;child=88756

[sethz]

What equipment do you have?

The nice senior couple left everything they have. Hayward EC65 DE filter, Hayward super pump 1HP. Vacuum, robot, submersible pump, loop-loc cover.

What is your method for chlorine?

Household bleach. Used cal-hypo for the first shock, but be convinced and will use bleach since now.

Do you have a salt-water chlorine generator?

No, I don't. Actually I learned such thing as SWG exist yesterday night.

Can you test your fill water and post the results?

Good idea, never thought about that. Will do it over weekend.

How did your calcium go from 130 to 225?

The only thing between two test was 5lb cal-hypo 65% (plus some backwash/DE refill). So I guess that's source.?

I will do a water test tomorrow then continue adding baking soda to achieve 100 TA.

About maintaining 15 FC shock level, when will I know it's enough? like 1ppm drop overnight?

I will keep in mind about the option of hydrogen peroxide for future. learned a lot these two days.

[sethz]

As I mentioned before, you could get an inexpensive ammonia test kit from a pet/fish/aquarium store or just do the bucket test to see how much more chlorine it will take. And yes, be sure to get the good test kit, either the Taylor K-2006 or the TF100 as linked to earlier.

Thank you for the encouragement, chem geek. I just ordered K-2006 online.

I have problem comparing the OTO or DPD color shade chart, titration test will be the way for me.

I will do another water test tomorrow, I will keep you guys posted. It's the fifth day since I opened the pool, lots of energy and dollars. But seeing the pool becoming the way I dreamed well worth the efforts.

[quantumchromodynamics]

The hydrogen peroxide, sodium perborate or sodium percarbonate would only be for use under certain very limited conditions. They remove all chlorine, including free and combined, and they should not be used on a regular basis.

Now that you are getting the correct test kit and are learning the proper way to maintain your pool, this should not happen again. Although, it did happen to Richard (Chem Geek), and he is one of the smartest people I know. It just goes to show that anyone can make a mistake and let their pool get away from them.

5 pounds of cal-hypo would not cause a 100-ppm increase in your calcium hardness. Most likely, it is just a testing error. Your new K-2006 will give you good reliable results. Be sure to read the booklet that comes with the kit, it is full of good water chemistry information.

With zero cyanuric acid, I think you can keep your chlorine at 10 ppm. Keep it there until all combined chlorine is gone and your overnight test has less than 1 ppm loss. Add cyanuric acid once most of your chlorine is free chlorine and not combined chlorine.

You should keep your Saturation Index at 0.0 to 0.3 to protect your plaster. Go to www.thepoolcalculator.com to calculate your S.I.

If your S.I is negative, your plaster will be dissolved and ruined. Not rapidly for -0.1 to -0.2, but steadily. The rate of dissolution increases exponentially as the S.I goes more negative. If your S.I is excessively high, you will get calcium carbonate scaling.

How does your plaster look now?

[sethz]

Hi Guys. I did another test in pool shop this noon:

FC 0.2

CC 0

pH 6.9

TA 70

CH 283

CYA 0

Now I have doubts on their test. Using the same sample, I did both OTO/DPD test before running to the pool shop. Both tests are around 3, definitely not zero (mentioned before, I have difficulty judging color shades, so roughly). Bucket test also suggest the water is responding with chlorine, so I assume ammonia is gone.

Soon I will find out after K-2006 kit's arrival.

CH increased again, 130 -> 225 -> 283 in 3 days, I didn't put any calcium yesterday, According to poolcalculator my CSI is -0.9, so the water eats plaster that fast? or maybe test error again.

The plaster looks ok (well, I am only able to see the pool bottom for recent 2 days), with discoloration somewhere (blue color plaster surface), but I don't know it's from years of usage, or last year's negligence, or these couple days.

After the tests, I poured two gallon %6 bleach, targetting FC to 10.

Will use Borax or Baking soda later to address pH/TA, which one first?

[quantumchromodynamics]

I think you should add baking soda right away to get your pH and alkalinity up. They are too low.

[sethz]

To report back, my Taylor K-2006 kit arrived today.

FC 1

CC 0

pH 7.6

TA 90

CH 150

CYA not tested (added last weekend, expected 25~30)

The water looks clear, I guess now it's back to normal maintenance status. Thank you all for the help.

btw, I also tested my tap water (from private well):

FC 0

CC 0

pH 6.9

TA 10

CH 30

[quantumchromodynamics]

Make sure to keep your saturation index positive to avoid dissolving your plaster. Go to http://www.thepoolcalculator.com to calculate your SI and for info on balancing.

[sethz]

Make sure to keep your saturation index positive to avoid dissolving your plaster. Go to http://www.thepoolcalculator.com to calculate your SI and for info on balancing.

The current CSI is -0.05. I think I will increase CH bit. Thank you. I learned a lot these days.

[chem geek]

A CSI of -0.05 is absolutely nothing to worry about. Test error is far higher than that and a pH fluctuation of 0.1 will completely wipe out that very slightly negative CSI. Usually there isn't concern unless it's out of range by 0.3 or more, though the more picky among us will try to get within 0.1 or 0.2, but it's not necessary. Tests with plaster have shown that the process of degradation is very slow even when the index is -0.7 to -1.0 though over years that would become a problem. Likewise with scaling which is usually not seen until the index gets to +0.7 to +1.0 (scaling in SWG cells occurs sooner, but that's because one of the plates is at high pH; scaling in heaters is also sooner due to the higher temperatures).

[quantumchromodynamics]

Unless someone has a salt-water chlorine generator, I recommend that plaster pools maintain a positive Saturation Index at all times.

The OP notes low reading for pH, Total Alkalinity and Calcium for their fill water.

I recommend a pH of 7.5 to 7.6, a Total Alkalinity that keeps the pH stable at 7.5 to 7.6 and a calcium level that gives a Saturation Index of 0.1 to 0.3.

My personal experience with plaster shows that a negative SI will damage plaster.

[quantumchromodynamics]

...a pH fluctuation of 0.1 will completely wipe out that very slightly negative CSI.

pH can fluctuate up or down; a positive CSI gives some buffer to the CSI to keep it from going negative. You have a plaster pool; where do you keep your CSI? Have you had to replaster your pool yet?

[chem geek]

The pool industry has used an index that has tended to report too high a saturation index for SWG pools because it didn't account for TDS properly. Using more recent numbers for equilibrium constants and temperature dependency, the CSI I use is very close to the Taylor watergram. Because we use a gas heater in our non-SWG pool quite a bit in the spring and fall, I have the index slightly negative, at around -0.1, though it's been -0.2 at some times in the past (and only lower than that briefly if trying to lower the TA -- perhaps once or twice). The pool is 7 years old and shows no sign of wear -- surfaces are still smooth. I hope to go 20+ years or more before even thinking about replastering. The only issues with my pool's plaster are some small metal (iron) stains from when I used Trichlor in the first two seasons when the floating feeder parked itself near stainless steel bars in the pool and rusted the two closest mounts.

So how negative in your experience does the saturation index have to be before you see plaster pitting or other issues? What index did you use? Traditional Langelier Saturation Index (LSI)? See this blog post for some interesting info on quality of plaster construction vs. aggressive water.

[quantumchromodynamics]

Less than 0.0 will cause plaster to dissolve. Since the units are in units of pH, the effect increases exponentially as the index gets more negative.

For non-SWG pools, I would use the Taylor Watergram. For SWG pools, I would use the formula given by the SWG manufacturer (like Autopilot) that accounted for TDS. I use the carbonate alkalinity by subtracting 1/3 of the cyanuric acid level from the total alkalinity.

http://www.autopilot.com/images/pdfs/digital_manual.pdf

If your heater has good water flow, the water temperature inside the heater should not be much more than five to ten degrees above the pool water temperature.

At ten degrees, there is less than a 0.1 rise in the Saturation Index. I never have scaling in heaters when keeping a positive Saturation Index. You should keep your index greater than or equal to 0.0.

A SWG is the only reason to go slightly negative, but it is a compromise. For a SWG, I would start out at 0.0 and only go slightly negative if necessary.

[quantumchromodynamics]

Here is a rough estimate of plaster life vs. S.I

...SI.....pH......Years

+0.1 = 7.6 = 31.486

+0.0 = 7.5 = 25.000

-0.1 = 7.4 = 19.857

-0.2 = 7.3 = 15.773

-0.3 = 7.2 = 12.531

-0.4 = 7.1 = 09.952

-0.5 = 7.0 = 07.906

-0.6 = 6.9 = 06.280

-0.7 = 6.8 = 04.988

-0.8 = 6.7 = 03.962

-0.9 = 6.6 = 03.147

-1.0 = 6.5 = 02.500

Formula: Life of plaster in years = 25/10^negative SI.

Example: 25/10^0.5 = 7.906

So, as you can see, you will still get almost 20 years with a -0.1 S.I, but you are losing about 5 years as compared to keeping a 0.0 SI and about 11 years as compared to keeping a +0.1 SI.

I don't think that the formula is useful for S.I values above +0.10 or below -1.00.

[chem geek]

Where did the data for your table come from? There is a difference between thermodynamics which is what the saturation index is based on and determines whether a chemical reaction is possible vs. chemical reaction rates including dissolving rates. If there is a chemical species such as hydrogen ion that acts as a catalyst, then lower pH could speed up dissolving of calcium carbonate, but if it is just low calcium hardness (CH) levels then the forward reaction rate of dissolving is constant -- it is the reverse rate that from solution back to plaster that varies with the amount of saturation in the water and that is logarithmic. When I am referring to CSI, I am talking mostly about still keeping the pH near 7.5, but having different CH or TA levels. The pH should not be too low for multiple reasons including eye comfort and preventing metal corrosion (which is catalyzed by lower pH).

If one is actually having plaster dissolved, then one should find their CH rising over time, after accounting for water dilution which lowers CH (if the fill water is low in CH) and evaporation/refill which would increase CH from that in the fill water. Because we use a pool cover, we have minimal evaporation and with an oversized cartridge filter cleaned only once per season we have minimal dilution. The CH has been very stable and if anything slightly dropping over a season, probably due to the small amount of splash-out. So I'm not worried at all and I question the numbers you show. The CH was only rising during the first season and perhaps a very small amount the second, most likely due to plaster curing.

Even dissolving 1.3 pounds of calcium carbonate in my 16,000 gallon pool would raise the Calcium Hardness (CH) by 10 ppm and would raise the pH from 7.5 to 7.9 at a TA of 100 ppm (and no Borates). I haven't seen anything like this during any 7-month swim season (over the winter, I dilute using winter rains and re-adjust the chemistry but since reaction rates are temperature dependent anything happening during the winter with water at 45-55F would be far slower).

[quantumchromodynamics]

1.3 pounds of calcium carbonate contains 0.52 pounds of calcium. The carbonate can outgas by way of carbon dioxide. Therefore, you can only count the 0.52 pounds of calcium for sure. That equates to a rise of 3.9 ppm.

1.3 pounds of sodium carbonate will raise the pH by about 0.28. Since there is roughly the same amount of carbonate in 1.3 pounds of sodium carbonate (anhydrous) as there is in 1.3 pounds calcium carbonate, I would think that the pH rise would be about the same (0.28 vs. 0.4). Do you ever add acid?

Furthermore, the most susceptible element in plaster is calcium hydroxide, not calcium carbonate. Perhaps your pool builder used a pozzolan, which converts some of the calcium hydroxide to calcium silicate hydrate, which is much less susceptible to chemical attack.

Is your aggregate Calcium Carbonate, Calcium Magnesium Carbonate (Dolomite), or Quartz? Calcium Magnesium Carbonate is less susceptible to chemical attack, and half of the compound is Magnesium, which does not show up on a calcium test. Quartz is also much less susceptible to chemical attack.

The data in the chart are based on my experience, and refer to plain calcium carbonate (marble) aggregate without any special additives, such as pozzolons. Plaster life refers to my opinion of an acceptable plaster surface vs. an unacceptable one. It does not mean that the plaster is completely dissolved down to bare gunite.

It does not take much plaster loss to create an unacceptable surface. Once plaster becomes etched and pitted the maintenance increases significantly. The plaster requires more brushing, which accelerates plaster loss.

I'm not sure why you think that unsaturated water won't dissolve plaster to any significant degree. Even saturated water can dissolve calcium compounds. Calcium is more likely to dissolve than the reverse, especially in a pool where the water is constantly in motion.

Why even take the risk if there is no benefit to a negative CSI? Is there something about maintaining a positive CSI that you think is disadvantageous?

Provide numbers for the chart that you think are more accurate.

[chem geek]

Calcium Hardness (CH) in pools/spas is measured in units of ppm CaCO₃, that is as calcium carbonate equivalent, not as calcium ions themselves. The pool plaster that is dissolving is calcium carbonate and what results in the water is calcium carbonate and the units of measurement are calcium carbonate. As for outgassing, that would only increase the pH and would remove carbon dioxide and not calcium -- calcium will not ougtas (i.e. I know of no calcium compound formed in the pool that would outgas). Though the CH is measured in calcium carbonate units, the test itself measures only calcium. I know this is confusing, but is the way these things are done.

I do add a cup or two of acid every month or two, but that is expected from a combination of the excess lye in the 12.5% chlorinating liquid I use (theoretically requires a minimum 1/3 cup of acid per month) plus some carbon dioxide outgassing when the pool is open and used 1-2 hours every day (more on weekends) with the pool's 100-110 ppm TA plus any hypochorous acid outgassing (if 10% of the chlorine outgasses as hypochlorous acid, then this creates a demand for about 1 cup of acid per month). The TA tends to slowly rise over a full season in spite of the acid addition (which when countering a strong base should have the TA be stable) most likely due to evaporation and refill since the fill water has 80 ppm TA. These are small changes (usually 10-20 ppm) over 7 months because the pool is mostly covered. The fill water has CH of around 50 ppm so evaporation and refill would have a relatively negligible effect on the pool's CH. I haven't noticed any change in acid usage as a function of saturation index though it hasn't varied very much.

Where do you get that the most susceptible element in pool plaster is calcium hydroxide? There is no calcium hydroxide initially pool plaster. Are you referring to calcium oxide that is bound to the silicates as with 3CaO•2SiO₂•4H₂O? The curing of plaster does release calcium hydroxide, but this is not a structural component of pool plaster itself after it is cured, though if some remains during the curing and does not migrate to the water then yes, it would be a weak structural link in plaster -- is that what you are referring to? That's what the use of pozzolan is supposed to minimize. The curing of pool plaster is mostly the following and the hydroxide portion comes from the water used in hydration:

2(Ca₃O•SiO₄) + 7H₂O --> 3CaO•SiO₂•4H₂O + 3Ca(OH)₂ + heat

Now unbound calcium oxide is very much thermodynamically favored to produce calcium carbonate as with the following:

CaO + CO₂ ---> Ca²⁺ + CO₃^2- + heat

However, the tight binding to the silicates essentially prevents the above from occurring. Note that saturation with calcium carbonate would slow the above reaction down (i.e. increase the reverse reaction) and that it implies that a higher CH and lower TA would be better since that would reduce the amount of carbon dioxide on the left-hand-side while maintaining saturation on the right-hand-side. As you said, normal pool plaster contains limestone (calcium carbonate) or dolomite (calcium magnesium carbonate). I am not talking about the underlying gunite, but the pool surface itself. This limestone or dolomite is the aggregate used with Portland cement to make a "white" plaster pool exceptionally bright. By the way, marble is also calcium carbonate. Quartz is silicon dioxide so if the aggregate was only quartz or other minerals and not limestone/marble based, then such surfaces would in theory last longer as you said.

The reason that plaster tends to pit rather than just thinly dissolve is that the calcium silicates are far more stable than the calcium carbonate. As for what is used in my pool surface, I don't know whether it is calcium carbonate or calcium magnesium carbonate (I was never told and I never asked). I am fairly certain it is not quartz or similar special material. It could have pozzolan (to reduce leftover calcium hydroxide in cured pool plaster); I don't know.

I don't think that unsaturated water won't dissolve calcium carbonate. It's the rate that is the issue; that is, how long it takes. As you point out, even at saturation there is a slow reforming of the surface that over time can make it uneven. A saturation index of -0.3 would have the calcium carbonate product be half its amount at saturation. So in theory whatever the limiting rate of dissolving would be if there were no calcium nor carbonate in the water then that time would be roughly doubled. Without low pH catalyzing the dissolving of plaster, it's not clear that having plaster in water with virtually no calcium carbonate would produce a pitted surface that quickly. Your table showed lower pH along with the more negative saturation index; the question is what happens with a lower saturation index but a reasonable pH near 7.5.

According to this link more dissolved carbon dioxide in the water leads to faster dissolving of limestone which at first didn't make sense but I suspect it is due to the lower pH if the increased carbon dioxide was due to pressure (i.e. making the water more carbonated than normally found from air). My hunch is that the reaction rate is strongly influenced by the pH since that is something consistently reported with respect to limestone as well as marble. So if the saturation index is low because of low pH, then that is probably far worse in terms of pool plaster life than having it low but at a pH of 7.5. It may also be the case that a higher CH and lower TA would be better, but again this is just speculation on my part based on the aforementioned reactions. If there is calcium hydroxide remaining in pool plaster, then this is more affected by pH than anything else and a higher CH and lower TA would be better as well.

Anyway, it would clearly be conservative to have the saturation index very close to zero. The downside is that the risk of scaling in gas heaters and saltwater chlorine generator cells is increased. This can be at least partly mitigated by use of 50 ppm Borates in the water which will provide additional pH buffering (as well as being a mild algicide). I don't have any suggested table revisions since I don't have experience with this beyond what is reported on these forums by people such as yourself. Do you have any estimate of the expected CH rise over time from dissolving plaster due to a low saturation index where one would notice a plaster surface being unacceptable? I think your table would be very reasonable if low pH were the reason for the low saturation index, but I'm not so sure you'd find the same results if the pH were near 7.5 and the low saturation index were due to low CH or TA. What are your thoughts on that?

Richard