Questions about Water Balance 3.5 weeks after fill

[lnorris]

I have just taken over maintaining the water from my builder and was wondering why following the suggest values in pool school shows "potentially corrosive" on the pool calculator. Water values using the TF100:

FC 4
PH 7.5
TA 90
CH 280
CYA 100 [builder said he recommends 100...]
Salt 3100
Borate ~15
Temp 70

CSI -.48

Is CSI not an issue as long as the other values are in range? I also noticed its fairly easy to go into the corrosive range when at the minimums of the ranges.

Also, since the builder put the CYA at 100, I was planning on letting it fall over time. Is leaving it at 100 going to be a significant issue?

The TA just jumped in the last few days. I will probably drop the PH closer to 7.2 as was suggested for the first month while the pebble is still curing. I'm not sure how necessary that is given the CSI.

Any advise would be appreciated. This website is very helpful... only wish I had found it sooner.

 

[Guest]

If you have new plaster it is very possible that your CH will rise over time so I would not lose any sleep over it right now. Once the plaster is cured I would keep the CH a bit higher. There is nothing wrong with a CYA of 100 if you have a lot of direct sun on the pool, which is possibly why the builder recommened it but I would not let the FC drop below 3 ppm in this case!
This pool school article might be helpful to you.
pool-school/water_balance_saltwater_generator

 

[lnorris]

I've definitely been closely following the pool school article you suggested, but my primary question was why the saturation index (CSI) seems to be on the corrosive side when following the recommended levels. The PB was suggesting keeping PH closer to 7.2, but I'm hesitant given that the CSI is already low. Should this be a concern?

I plan on sticking with the 100 CYA level for now. I'm interested to see how long it takes to come down from rain, splashout, etc.

 

[chem geek]

A target pH of 7.5 up to even 7.7 would be much more appropriate in a normal situation -- HOWEVER, with your new plaster (cement in PebbleTek curing) you want to keep the pH from getting too high which is probably what the PB is trying to prevent.

A significantly negative saturation index over an extended period of time would tend to dissolve calcium carbonate in plaster/gunite/grout (or the cement in PebbleTek). However, as waterbear noted, new plaster/cement will cure and tend to increase calcium hardness and also tend to make the pH rise, so keeping it lower helps prevent scaling (though if you are diligent and keep the pH from getting very high, then I don't see a problem with that).

If the PB continues to suggest keepting the pH at 7.2 in the future even after the first month or two when curing subsides, then I suspect the PB was trying to minimize scaling in the SWG cell by having more corrosive water, but the Borates will help a lot in reducing scale due to the Borates pH buffering capability (it significantly reduces the amount of pH rise near the cathode which is the plate that generates hydrogen gas bubbles). The higher target pH will also reduce the rate of pH rise in the pool.

It's possible the PB doesn't realize the more corrosive affect of higher salt levels (TDS, actually) and lower temperature. The salt reduces the saturation index by about 0.2 while the 70F compared to 80F reduces the saturation index by about 0.1 so combined lower the index by about 0.3.

Richard

 

[lnorris]

I find it interesting that even at the mid point for most of the recommended levels for a SWG using borates at 80degF, the CSI is still between (-.4) and (-.5). Are the recommendations intentionally left more corrosive to prevent scaling in the cell? I seem to be preoccupied with trying to get the CSI closer to 0/balanced and am wondering if that is not always the best objective....

 

[JasonLion]

There are a number of factors that suggest keeping CSI at least slightly negative. CSI will be higher at some locations inside the SWG cell and inside the heater. PH tends to go up over time, with fresh plaster TA and CH also rise over time. A lapse of attention to maintaining the PH can result in a sharp rise in PH, while sudden drops in PH are much less likely. All of the likely swings in CSI are upwards. Keeping CSI somewhat negative keeps the CSI from getting too high in any of those situations.

The recommendation to keep PH low is designed to minimize plaster dust, causing the plaster dust to dissolve instead of accumulating. A low PH recommendation is also a way to help novices, who don't understand what is likely to happen with fresh plaster, from allowing the PH to get way too high. The plaster dust portion of the PH recommendation could be translated as a CSI of perhaps -0.4 or -0.5.

If you are using borates you should probably raise you CH level by 50 to 80 to keep CSI reasonable. Likewise, when the water is cold it is best to keep the PH a little higher than you would with warm water.

 

[chem geek]

I find it interesting that even at the mid point for most of the recommended levels for a SWG using borates at 80degF, the CSI is still between (-.4) and (-.5). Are the recommendations intentionally left more corrosive to prevent scaling in the cell? I seem to be preoccupied with trying to get the CSI closer to 0/balanced and am wondering if that is not always the best objective....

The midpoint of the recommendations in Water Balance for SWGs for plaster is about -0.3 if one includes Borates (midpoint parameters are pH 7.65, TA 80, FC 4, CYA 75, CH 300, Salt 3000, Borates 50, Temp 80F on the cool side). I'm not sure where you are getting -0.4 to -0.5.

 

[JasonLion]

-0.4 to -0.5 is for new plaster startup, to minimize plaster dust during the first several weeks.

 

[lnorris]

I find it interesting that even at the mid point for most of the recommended levels for a SWG using borates at 80degF, the CSI is still between (-.4) and (-.5). Are the recommendations intentionally left more corrosive to prevent scaling in the cell? I seem to be preoccupied with trying to get the CSI closer to 0/balanced and am wondering if that is not always the best objective....

The midpoint of the recommendations in Water Balance for SWGs for plaster is about -0.3 if one includes Borates (midpoint parameters are pH 7.65, TA 80, FC 4, CYA 75, CH 300, Salt 3000, Borates 50, Temp 80F on the cool side). I'm not sure where you are getting -0.4 to -0.5.

I was using ph 7.6, TA 70, FC 4, CYA 75, CH 300, Salt 3200, Borates 50, Temp 80F = -.44 CSI as approx mid point. Thanks for all the feedback. It sounds like the negative CSI should not be an issue.

 

[Guest]

It's not unless it's really out of line for an extended period of time.