Scaling on cell...why?

[JimK]

Since 2004 I've never had an issue with scaling on the cell, an Aqua Rite T-15 cell (we're on our second cell). Last fall when I closed, I notice quite a bit of hard white deposits on the cell. I cleaned it out using water pressure from the hose nozzle and some scraping with a wooden toothpick. I didn't want to use acid as I've read it will reduce the life of the cell.

There's a few changes I made last season based on advice here and wonder if any may caused the scaling.

1. Based on advice here, I stopped adding acid every time the pH reached 7.6 - 7.7. I was told as long as it didn't go above 7.8, then it was fine. Since my pool seemed to stabilize at about 7.7, this meant I rarely had to add acid last season.

2. I use to add a maintenance dose of Jack's Magic Purple Stuff each week because of a metal staining issue in 2006 or 2007. The product states it also helps protect the cell from scaling. Last season I did the bucket test for metals and it came up negative, so I was told here I could probably stop using the Purple Stuff. I was hesitant to quit all together (really don't want to deal with metal staining again..what a costly pain in the neck!), so I just added half the normal dose each week (no metal staining occurred).

3. I use to shock on a regular schedule, every two weeks, with cal hypo. I was told here that I only needed to shock if CC got above .5. Since I make an effort to maintain proper CL levels per Ben's best guess chart, I rarely had to shock last season. Btw, even shocking every two weeks with cal hypo, my calcium level didn't get above 260 (typically it runs from 220 to 250).

For reference, here are the levels I maintain throughout the season.

FC 5
CC < .5 (usually 0)
pH 7.6 - 7.7
TA 80 - 100
CYA 70 - 80
Borates 50 - 60
Salt 2800 - 3200 (as recommended by Aqua Rite)
Calcium typically 220 - 250, but I expect this will drop this season since I haven't added calcium since 2011 and I stopped shocking on a schedule with cal hypo. This season I plan to just use plain bleach.

I also ad 4oz of polyquat each week as insurance against algae (I may try without this season).

Any ideas if any of the above changes contributed to the scaling?

Thanks.

[JimK]

Anyone?

[mas985]

#1 is likely your problem. SWGs can easily scale if the CSI (calcium saturation index) gets too high. You are better off with a slightly negative CSI to keep the scale off. You can use the poolcalculator.com to figure out your current CSI.

Also as cells age, then tend to accumulate more scale. I would avoid scraping the scale since that could also scrape the cell coatings. You are much better off using acid to clean cell then any mechanical method.

[JimK]

#1 is likely your problem. SWGs can easily scale if the CSI (calcium saturation index) gets too high. You are better off with a slightly negative CSI to keep the scale off. You can use the poolcalculator.com to figure out your current CSI.

Also as cells age, then tend to accumulate more scale. I would avoid scraping the scale since that could also scrape the cell coatings. You are much better off using acid to clean cell then any mechanical method.

Thank you for the information and suggestion on how to clean the cell.

Are you saying that allowing pH to get up to 7.7 is an issue even though its still considered in the normal range? Should I go back to adding acid whenever pH reaches 7.6?

I played around with the pool calculator using a variety of levels in each field my pool might see throughout the season and found the CSI varied between -0.74 and -0.02 (it always came out in the negative).

[mas985]

The "correct" PH depends on other factors like TA and CH which is why I suggested the calculator. However, having a negative CSI does not guarantee a scale free cell, only reduces the likelihood. You could try lowering the PH a little and see if that helps (i.e. keep CSI between -1 and -0.5) which should not be a problem since you have a vinyl pool. Also, it would be a good idea to make sure you are actually getting scale and not something else. Scale is indicated by fizzing when immersed in a acid solution.

[JimK]

The "correct" PH depends on other factors like TA and CH which is why I suggested the calculator. However, having a negative CSI does not guarantee a scale free cell, only reduces the likelihood. You could try lowering the PH a little and see if that helps (i.e. keep CSI between -1 and -0.5) which should not be a problem since you have a vinyl pool. Also, it would be a good idea to make sure you are actually getting scale and not something else. Scale is indicated by fizzing when immersed in a acid solution.

Thanks for the advice on where to keep CSI. Having that range to shoot for is helpful. Right now I'm at -.77. Using the calculator it's interesting to see how much effect temperature has on CSI. Something else I think should help is I expect my CH level to drop since I do not plan to use cal hypo this season, just plain bleach when I need to shock.

Regarding the use of Jacks Magic Purple Stuff, do you think it is helpful in preventing scaling?

If I see more "scaling", I'll take a sample and put it in some acid solution to see if it fizzes like you mentioned.

[JimK]

....I would avoid scraping the scale since that could also scrape the cell coatings. You are much better off using acid to clean cell then any mechanical method.

I was reviewing the owners manual and noticed this;

If deposits are seen, use a high pressure garden hose and try to flush the scale off. If this is not successful, use a plastic or wood tool (do not use metal as this will scratch the coating off the plates) and scrape deposits off of plates.....Mild Acid Washing: Use only in severe cases where flushing and scraping will not remove the majority of deposits....

So it seems I followed correct procedure for cleaning the cell (the scaling wasn't what I would call severe).