Split off of this topic. JasonLion

For a plaster pool, you need to avoid a negative CSI, or your plaster will be damaged. Go to http://www.poolcalculator.com/ to see your CSI. I see that you list an "Auto Chlorinator" as part of your equipment, but you don't list any salt level. What kind of auto chlorinator do you have? If it is a salt water chlorine generator, what is your salt level? If you are using a Trichlor feeder, I would recommend that you raise your alkalinity to at least 120 ppm by adding 12 lb. of baking soda. If you are using a salt water chlorine generator, you need to be careful to not scale up the cell with a too high CSI.

By all means avoid a significantly negative CSI, but a slightly negative CSI is actually the best place to be.

If there is a SWCG, then I would agree to a slightly negative CSI. Otherwise, I would stay neutral to positive.

I have never seen a report of plaster pitting when CSI was higher than -0.7. Sources differ on exactly how low CSI/LSI can go before there is any risk, but I have never seen anyone suggest that -0.3 or higher was an issue.

CSI is far more likely to increase than it is to decrease. Nearly all of the variability in PH is towards increased PH, other than trichlor users PH does not tend to go down. High TA levels and aeration will tend to drive PH up towards 8.4. Increased PH will increase CSI. Pool heaters tend to run 10 to 30 degrees warmer inside the heat exchange coil, which can increase the CSI by up to about 0.25. Inside of a SWG there are regions of locally high PH, which will increase the CSI significantly in those areas. Thus it is good to maintain the pool at a slightly negative CSI, to provide additional head room on possible CSI increases.

Count me as someone who does suggest that a negative CSI will damage plaster, especially older plaster.



The original poster notes their use of Trichlor in an Auto Chlorinator; although they also note that they "plan to switch to the BBB method of chlorination." For BBB, they would expect some rise in pH, and would need to plan accordingly.

It is my opinion that plaster pools and pools that have copper heat exchangers should maintain a neutral to positive CSI unless there is a reason to have a slightly negative CSI as with a SWCG. For a heater with an expected temperature rise of 20 degrees F, a neutral 0 CSI should prevent scaling assuming a 0.20 rise in the CSI.

I think that people should be working with an acceptable targeted CSI range rather than trying to maintain a specific CSI due to the variability of all of the factors like pH, alkalinity, temperature and calcium that can and will fluctuate over any given time.

And the easiest way to do so is to get the water balance paramaters into accepted range and then control pH, since changes in pH will create the most drastic changes in CSI. pH is also a very easy factor to keep under control.
This is why I say if your water is within accepted ranges pH is the main factor that will determine whether it is scaling or aggressive. This makes pool maintenance much simpler for those without a good grasp of the chemistry involved.

This conversation just went over my head. I just bought this house a little over 2 months ago. Although I have learned a great deal about how to care for my pool, I still have a long way to go.

WHen your CSI goes one way or the other too far, how do you bring it back to neutral?

CSI is controlled by adjusting the PH TA and CH levels. Some natural variation in all of those levels is to be expected. As long as you keep your numbers in the reccommended ranges you don't normally need to worry about CSI.

For rapid adjustments to CSI you adjust the PH. For longer term you adjust TA and CH.

Jason, thanks for the quick response.

When I put my targets in the pool calculator, I get a CSI of 0.19. This should be well within acceptable ranges, correct? I saw that lowering my target pH would also lower my CSI. Here are my targets.

FC - 6
pH - 7.5
TA - 100
CH - 300
CYA - 40

Your numbers are fine where they are.

I take care of many concrete/plaster pools and I can actually see the plaster dissolve when the CSI is negative. When I brush or just rub my hand across the plaster, the plaster comes off as a dust. However, when the CSI is increased to positive, I notice a dramatic difference. Virtually no dust comes off at all. I am very confidant that a negative CSI dissolves plaster. I am not saying that a -0.1 is going to ruin the plaster overnight. I would estimate that for every 0.1 below the 0, the plaster will lose 4 years. Properly maintained, plaster can, and will, last for over 30 years with good overall performance.

Part of the problem may be that the saturation index used by the pool industry is wrong in both its temperature dependence and especially in its TDS dependence. However, even with a TDS of 3200 for a 3000 ppm salt level, the traditional index is only 0.1 higher than the correct index that I use based on CODATA and CRC thermodynamic tables and that matches the Taylor watergram. On the other hand, other thermodynamic data used by Wojtowicz has the saturation index come out about 0.15 higher than my calculations so if anything mine is more conservative against plaster corrosion (but may scale sooner than expected).

A (theoretically correct) saturation index of 0 is fully saturated with calcium carbonate. A saturation index of -0.3 means that the product of calcium and carbonate concentrations is half its saturation amount so one way this can occur is for each of calcium and carbonate to be around 70% of its saturation amount.

Do the pools you take care of use Trichlor? If so, then the pH will usually drop until someone adds pH Up or Borax so the saturation index will swing. Also, what formula do you use for calculating the saturation index? Do you notice any difference between non-SWG pools and SWG pools at the same index or do you manage them to different index targets?

The dissolving of pool plaster will tend to make the pH rise until the index gets close to 0 so if one finds that a higher CH stabilizes the pH with all else constant, then this would tend to indicate that the water was too unsaturated, but I suspect this is much too slow to be noticeable and that other factors that affect pH would overwhelm this effect most of the time.

Richard

1) The aggregate in the plaster that I am dealing with is marble, which is a form of calcium carbonate. So, the CSI is particularly relevant. Perhaps other aggregates as not as susceptible to dissolution as is marble.

2) For a non SWCG plaster pool I use the Taylor Watergram and keep the Saturation Index at 0.0 to +0.3.

3) For a SWCG plaster pool I use the formula given by Autopilot. I keep it as close to zero (+/- 0.1) as possible and carefully monitor the cell for scaling. If scaling occurs, I lower the SI. Careful monitoring of SI and cell performance is critical. If the SI has to be kept too low, I will consider removing the SWCG. Partly based on how I think the plaster is doing.

4) I like to use a combination of different chlorine types depending on the chemical adjustment that are needed. For example, a trichlor feeder can be added to a SWCG pool to add cyanuric and lower pH. Liquid chlorine is good to use to shock a SWCG pool to reduce the overall use of the cell and get better shock results. Dichlor can also be used if cyanuric is needed.

On a non-SWCG pool, Trichlor and liquid chlorine can be used in conjunction, adjusting the percentages of each based on chemical adjustments needed. On a trichlor and bleach pool I can avoid ever having to adjust pH with acid or pH increaser by adjusting the percent of each chemical and adding baking soda as needed. I can keep the pH in a very tight 7.4 to 7.6 range without too much trouble.

5) There are several more things you can do to protect the cell of a SWCG. Add a pressure gauge after the filter and before the cell to help identify scaling conditions. Keep a blank cell substitute to replace the cell anytime you need to remove it for any reason. Make it from a piece of 1.5 inch PVC, two female adaptors, and two quick disconnect unions that exactly match the cell.

Balancing the needs of the cell to avoid scaling and the plaster to avoid dissolving are tricky, but they can be managed through careful monitoring of all relevant factors.

If you are able to maintain the saturation index +-0.1 then that sounds like a reasonable approach. However that can be lots of extra work. Nearly everyone will have their saturation index vary over a much wider range (regardless of their intentions). In practice SI will vary either through unusual situations or lapses in attention unless the pool owner is especially diligent.

If you can't guaranty holding the saturation index in that narrow range then aiming for a slightly positive SI is running a noticeable risk of getting to scaling conditions, which means difficult and often expensive to repair scaling problems. Aiming for a slightly negative SI avoids that problem. Whatever problems there may be from slightly negative SI, they are not detectable in any obvious way over periods of up to a decade, as extensive experience has shown.

I don't believe that the plaster lifetime is reduced, nor does any reputable source suggest that that might be so, though I can't rule out small effects which would be difficult to detect over reasonable time periods. Still, even if the plaster lifetime is somewhat reduced, that seems like a good tradeoff since it avoids thousands of dollars in acid washing (solving scaling problems) over the lifetime of the pool.