CSI with plaster pool and SWG (summer/winter variations)

[babanovac]

Hi everyone. I am a long-time lurker who has greatly benefited from the plethora of pool knowledge shared in the forum. But I am now somewhat swamped (not with algae, though I have some of that, but with CSI concerns).

I've done my due diligence, and technically speaking, the issue I'm asking about is addressed in the "Deep End" here: http://www.troublefreepool.com/saltwater-chlorine-generator-swg-recommended-levels-t41198.html. But I'm asking for some tailored suggestions to deal with this specific situation.

I have some experience dealing with my own swimming pool in the past. But, I've recently taken over care of my parents' plaster pool that shows signs of deterioration from what I assume (based on my parents' account of the last few years of pool chemistry) is years of corrosive CSI.

The stats currently are:
FC=2.0
CC=0.5
pH=7.4
TA=70-80 (my repeated result; pool store anomaly said 110)
CYA=30
CH=200
Salt=3500
Pool Volume=80-90,000 litres (21-24,000 gallons) [hard to calculate volume geometrically, but it took 7kg/15.5lbs of calcium chloride to get CH from 130 to 200, which suggests 24,000 gallons)

Other details:
* Upon opening earlier in the spring, I immediately added enough calcium chloride dehydrate (Cal Boost) to raise the CH to 200 ppm and waited to get more of a feel for the pool.
* From what I remember myself from previous years, with TA on the higher side (100-110), pH was constantly drifting upward (which is to be expected with a SWG).
* I started this year with lower TA (70-80, depending on how you interpret the last drop of reagent , and pH has been rock solid at 7.4.
* Because of climate, the pool goes from 24-30 degrees Celsius (75-85 F) during the swimming season (May to October) to freezing temperatures in the dead of winter. Obviously, CSI varies significantly with water temperature.
* Over the winter, about 25-30% of the water is drained (to below the jets)

The question: How do I deal with water chemistry to ensure no further (likely) corrosion based on CSI, throughout the year (both summer and winter conditions)?

Concerns to go with the above question:
* I seem to remember reading that CSI is less of a concern at lower water temperatures (like during the winter), but I note that the pool is closed for approx. 7 months a year (and CSI during that time, based on above stats, would vary from -1.05 in freezing water to -0.73 in water at 18 C/64 F before opening pool); both values are quite corrosive.
* I am loath to raise TA, as that will cause pH to keep rising with SWG use, and then I will have to deal with it on a regular basis.
* I assume some of the recurrent algae problems (I presume mustard) is that even with the usual routine (i.e. shock at mustard algae level, ensure no overnight FC loss etc.) some algae are always left behind, because brushing does not get to all the nooks and crannies of the pool, whose plaster is fairly rough by now throughout, with occasional pitting
* So do I just raise pH to maybe 7.6, get CH to 300 (which marginally takes care of CSI during the summer), and completely ignore winter (and shoulder season temperatures, when the pool is closed)?
* At what CH/CSI levels do I need to worry about SWG cell?
* Do I always raise CH at the beginning of the season (fill water has low CH and is needed to replace 25-30% of pool volume every spring), like I do with the salt lost to partial draining on pool closing?
* Any more economical way to raise CH, other than Cal Boost (cheapest available here in Toronto is $25 for 15 lbs pail)? I guess that only becomes relevant if you recommend I raise CH significantly.

Ok, I'm done ranting . Sorry for swamping you with such a lengthy first post.

Bogdan

 

[JasonLion]

The PH goes up on it's own during the winter. As long as you don't hold the PH low, you shouldn't have any significant CSI shift over the winter.

Most people need to add CH each season.

You are keeping the PH lower than I recommend. Ideally PH should be around 7.7. Also, CH should be around 300 to 350.

 

[chem geek]

If the plaster corrosion were due to low CSI, then you should see the CH and TA rise along with the pH as calcium carbonate would dissolve into the pool water and exposed calcium hydroxide might as well. If you had a lot of water dilution, then that could hide the CH rise. As Jason notes, you usually deal with preventing too low a CSI in a salt pool by targeting a higher pH since that also has less of a pH rise from that point and you also compensate with a higher CH.

If I start with your numbers and calculate the Calcium Carbonate Precipitation Potential, this calculates how much calcium carbonate precipitates when over-saturated or dissolves when under-saturated. If the plaster were dissolving, the pH would rise from 7.4 to 7.95, the TA would rise from 75 to 85, the CH would rise from 200 to 210 and this represents nearly 2 pounds of calcium carbonate at which point it would stop since the higher pH, TA and CH self-extinguish the process. That's a volume of around 20 cubic inches which is for example 1 millimeter thickness over 3.6 square feet (in practice, if it's pitting, then the area would be much larger). So this isn't very much dissolving of plaster before it extinguishes.

In other words, one must continue to add acid to lower the pH in order for the dissolving to continue. That makes it unlikely for this sort of thing to happen over the winter if you aren't monitoring and adjusting pool water chemistry over that time.