Saltwater Chlorine Generator (SWG) Recommended Levels

[chem geek]

In a recent post discussing the Calcite Saturation Index (CSI), I brought up the inconsistency between the recommended levels for non-SWG pools vs. SWG pools. As noted in that post, the following are the CSI ranges with the current recommendations (at 80ºF and assuming 525 ppm TDS for non-SWG and 3150 ppm TDS for 3000 ppm salt with SWG):

Bleach (non-SWG) plaster pool has a CSI from -0.23 to +0.35 with a mid-point of +0.07
SWG plaster pool has a CSI from -0.69 to -0.04 with a mid-point of -0.34 (minor corrections from earlier post)

Originally, we wanted to target a somewhat negative CSI for SWG pools to avoid scaling in the salt cell, but since that time we made other changes including recommending a lower target TA level, the use of a higher level of CYA, and the use of 50 ppm Borates (the above doesn't use 50 ppm Borates which would make the CSI about 0.08 lower). The lower TA and higher CYA make the CSI lower. Also, the higher TDS (salt level) makes the CSI about 0.2 lower as well. It would be much more consistent for us to change the pH recommended range to be 7.6 to 7.8 since the pH tends to rise in SWG pools anyway and higher pH tends to be more stable and for us to change the CH recommended range to 350-450 in which case we would have the following CSI range:

SWG plaster pool has a CSI from -0.46 to +0.06 with a mid-point of -0.18 (minor corrections from earlier post)

This still has a slightly negative mid-point, but the range is far more acceptable and allows us to say to people "just follow the Recommended Levels" and not have to worry about the CSI. To be consistent, the fiberglass range for CH can be 320-420 while vinyl can be 50-400 though these are less important.

It really makes absolutely no sense for us to have the SWG recommended levels that we do since the TA is lower, the CYA is higher, the TDS (salt) is higher, but there is no corresponding balancing of the CSI from either pH nor CH in the current recommended levels.

 

[JasonLion]

The creation of the recommended levels was a complex process, which did lead to some inconsistencies. Still, some part of the difference in CSI ranges between SWG and non SWG pools was intentional. Calcium scaling inside the SWG cell becomes an issue at much lower CSI levels than scaling elsewhere in the pool becomes likely. The goal at the time was to aim non-SWG pools for a CSI of around zero and SWG pools for a CSI of around -0.3.

Changing the PH range from 7.5-7.8 to 7.6-7.8 for SWG pools makes a lot of sense to me. I am somewhat less comfortable with raising the CH recommendations as much as you suggest, unless they are known to be using borates.

Some of the issues that come up for me around this include: where someones levels are likely to settle is not uniformly distributed across the recommended levels. It is far more likely that the PH will be up around 7.7 to 7.8 than that it will be below 7.7. It is also likely that PH will go up to 8.0 or higher on occasion, and far less likely that the PH will go down below 7.5. Thus, the concept of looking at the entire range of possible CSI values for any possible combination of levels is not really fair assessment of the recommendations. You also need to take into account the levels people are most likely to spend long periods of time at, and the probable costs of the occasional extreme excursions outside the recommended ranges.

Another issue is that problems with calcium scaling inside the SWG cell seem to occur with some frequency even when CSI is a low as zero. Keeping CSI around -0.3 seems to reliably prevent cell scaling for just about everyone. Thus I generally favor slightly lower CSI targets for SWG pools than the ones you propose.

 

[Lershac]

Having gone down a similar road, but in other subjects, I will (uninvited) toss in my .02 here.

While the science and numerical calculation are great for trying to understand the interactions and action/reaction relationships... I have had it forcibly pointed out to me, the fact that in empirical science we are attempting to MODEL the real world (which implies simplification and damping out of "noise" from our calculations so that we can actually calculate), for the purpose of understanding the real world better than we do. We then take that information back to the real world and try it out, see where we erred in our model, correct for the error and begin the cycle again. Somewhere down the line, we usually develop a "real world factor" to account for unknown variables and factor that into our model.

Our model is inaccurate is the message here, and it behooves us to make adjustments to our targets when the real world fails to conform to our lab conditions.

I say all this after just having suffered from this flaw myself to the tune of a few tens of thousands of dollars of equipment... "that really should have worked fine!" but nature decided to not conform to my model and we get to start again from scratch.

On a much smaller scale, this is much the same thing. The lab and calcs give us a starting point that we need to adjust according to how the real world treats us, because after all, we create models of the real world, not real worlds from our simplified models.

 

[Brentr]

Can you tell me if the pool calculator will reflect these changes? http://www.poolcalculator.com/

 

[JasonLion]

Brentr, the Pool Calulator has not been getting updated much recently. Once we agree on new recommendations we can hope that they will update their numbers, but it is not likely to be right away.

 

[Aquatica]

chem makes a great lot of sense ..which I have been working with the last few weeks. In cold weather keeping you pH higher at 7.8 is good. Borates are bad as they decline csi. better to adjust TA down to around 60. no lower. doesn't work at 50 for us. so pH between 7.6 - 7.8 but you have to lower TA to a point where csi doesn't roller coaster and climb too high to create scale in the salt cell. I'm installing see-through cells in all my pools so I can monitor better and see what csi is perfect for swg's. I think -0.2 to -0.1 is good but we will see.

CH is better high but not too high (500 is too high). I like at least 350. maybe 450 might be good as well so maybe 350 - 450.

also water temp makes huge difference in csi. if a client is running a heater in the winter at 94F it makes a huge difference in csi. we need to find some levels that can stabilize csi somewhat no matter the temp.

 

[onBalance]

I think Chemgeek's recommendation is valid. I can understand the desire to reduce scaling in the salt unit, but the plaster surface is the issue. Why encourage a negative index number?

Another thing to consider is that some on this website suggest following the National Plasterers Council's water chemistry recommendations. The NPC has stated that no negative index is allowed at any time. While that is unreasonable and unwarranted, still, why promote a negative CSI? We shouldn't put pool owners into a defensive position and jepordize pool plaster warranty issues.

 

[JasonLion]

Because the damage from cell scaling is obvious and often costs real money to repair or remediate, while the costs of just slightly negative CSI are minimal to the point that no one can agree on exactly what they are or if they actually exist. Likewise, just slightly negative CSI greatly reduces the chances of accidental travel into significantly high CSI and potentially very expensive calcium scaling on the pool surface.

I am not trying to belittle warranty issues, just to point out that it is far from a straightforward decision.

 

[Aquatica]

if the pH is kept stable then travel to + is not an issue. I have several pools now with TA of 60-70 and the pH hardly moves at all. One of the pools one week the pH is 7.6 with a TA of 65, next week it is 7.7

I'm using a CH of 350-450 now for my swg pools. also installing see-through cells to monitor if any scale builds in the cells.

here's an example. the lower water temp makes the c.s.i. lower than I would like. one of our pools from this weeks testing..


C.S.I. = -0.34 (Potential to become corrosive to plaster)

Free Chlorine = 5.5

Chloramines = 0

pH = 7.8

Total Alkalinity = 60

Calcium Hardness = 340

Stabilizer = 80

Salt = 3200

Temp = 71F

 

[Lershac]

Not to mention... on those warranties? Good luck collecting on most of them. They will fight you tooth and nail for the most part. Just a sales tool.

 

[chem geek]

Note that what I am recommending still has a slightly negative CSI and is more negative than the non-SWCG range, just not as negative as the current SWCG recommendation.