A little help with C.S.I. please

Eva · Aug 22, 2015

Hello,

I read the section on CSI in Pool School and used Pool Math, but I don't know how the result is calculated, or how to interpret it.

I'm in Tucson. The water here is alkaline and hard as a rock as far as dissolved solids. Calcium scaling along the water line is evident in most of my friends' pools, and kept me from getting the cobalt blue tile I really wanted.

So here I am with my new pool and I want to figure out how to avoid the problem. My pool guy says they charge $800 to blast it off somehow and it comes back in a few months -- not my idea of fun.

So, here's where we stand today:

Temp = 92
CH = 400
TA = 100
pH = 7.2, but that's just because I really lowered it so I could go out of town for a few days and it wouldn't creep above 8. Normally it's at 7.8 or at best 7.6.

Pool Math says C.S.I. = -0.4

Which seems okay but I'm not sure I want to, or can, keep my pH at 7.2.

What's my best strategy? Surface is pebblefina.

Many thanks.

chem geek · Aug 22, 2015

What's your CYA level? That's part of what is needed to calculate the CSI since internally it calculates the adjusted TA removing the effects from CYA and Borates (if used).

So pH directly adds to CSI so at a pH of 7.8 your CSI would be -0.4 + (7.8 - 7.2) = +0.2 which is not very high. [EDIT] This is wrong -- see later post that puts the numbers into PoolMath to get -0.05, not -0.4, so at 7.8 the CSI is +0.53).

So it seems in your case you've got more splashing up and evaporation leaving calcium than you have actually scaling. What would probably help is to target a lower TA level which should also help against the pH rise not only in your pool but should limit how much it rises from splash up to the tile and that may help reduce the amount of calcium carbonate left on that tile.

Of course, your high TA fill water isn't going to make that easy since you'll have to regularly add acid to keep the TA down from evaporation and refill. Using a pool cover would reduce that effect -- for your hot area a white or reflective cover would be best since you aren't interested in heating the pool water (I presume).

Eva · Aug 23, 2015

Hello chem geek. Thanks. My pool is under a roof and also has a vinyl cover which is closed all the time unless I'm swimming. My CYA is 30. I'm not using any berates. What is the difference between scaling and calcium deposited by evaporation? I guess I don't know what scaling is.

I am interested in heating the pool actually in the winter. Right now it's 92 and will probably increase another degree or two this summer before it starts to fall.

Eva

Divin Dave · Aug 23, 2015

Hi Eva,
Calcium scaling is calcium precipitating out of the water and forming scale over lots of your plaster surface underwater. This happens when the CH, TA and pH are way out of balance.

Here are some pics of what it looks like
Bing Images

chem geek · Aug 23, 2015

So I just put your numbers into PoolMath (using 0 for Salt and Borates; salt will be automatically calculated at the minimum based on TA and CYA and CH in that case) and I get the CSI as -0.05, NOT -0.4. How did you get -0.4? So in this situation when your pH gets to 7.8 the CSI rises to +0.53 so it's not surprising you are seeing more calcium carbonate above the water line.

Since you have new plaster, it is not a bad thing to have the CSI be on the higher side to help ensure that calcium hydroxide formed when the plaster cures is converted to calcium carbonate in the plaster itself. If this is happening as it should, then when you add acid to lower the pH the TA should slowly drop as well. 25 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) should drop the TA by 10 ppm. So you could just add acid to maintain the pH at around 7.5 (I wouldn't drop it lower than that for now) and the TA should drop over time. If it doesn't, let us know. If it does, then let it drop to around 70 ppm and then notice how fast the pH is rising and let us know.

If the rate of pH rise drops a lot so you aren't adding much acid, then if your TA is still above 70 ppm you can lower your TA faster by aerating the water. At a TA of 70 ppm and pH target of 7.5 your CSI will be near neutral and you should have less calcium carbonate form above the waterline.

Eva · Aug 27, 2015

Hi,

Well what's a little order of magnitude error among friends? Pool math is giving me -0.04 CSI, sorry.

So is -0.04 too low to make sure the calcium hydroxide is being converted to calcium carbonate in my plaster? What's the best way to protect the curing plaster and simultaneously prevent calcium carbonate appearing along the water line? To be clear that's not an issue now, but I see it on many many pools in Tucson and it is difficult and expensive to get rid of.

If I keep the pH at 7.4 and the TA falls (how much? And is the presumed fall within the level of detection of the TF-100?) then I should have to add less acid to keep the pH at 7.4?

Once the TA reaches 70 ppm, is it okay if it continues to fall?

Not sure why this seems so difficult to grasp -- I feel like I don't have all the basic understanding I need. Is there a more in depth discussion of TA and CSI than the short paragraphs in pool school?

Many thanks!

chem geek · Aug 27, 2015

No, -0.04 is not too low at all. First of all, your plaster is likely done with the bulk of its curing since it doesn't take long, certainly if it's past the first month it's mostly done. Second, is that any calcium hydroxide that makes it to the surface is going to dissolve a little which raises the pH at which point the rest will form calcium carbonate so being near zero is good enough. It's only in the start-up phase where in a Bicarb Startup where one has the CSI be positive, but after the initial month one gets the CSI closer to zero.

As for calcium carbonate scale above the water line, that can still happen because evaporation concentrates the chemicals and carbon dioxide outgassing can raise the pH, but if your CSI is near zero or slightly negative then you minimize this problem -- the problem would be far worse at a more positive CSI.

Yes, as the TA gets lower you should have less of a pH rise from carbon dioxide outgassing, but if you target a higher pH then that will also help. Of course, if you are still getting pH rise from the plaster, then the lower TA and higher pH won't change that -- these parameters only affect lessening carbon dioxide outgassing. This table shows how over-carbonated pool water is with respect to air at various pH and TA levels. You don't calculate anything from it -- it's just a teaching guide to give you a sense for how higher TA and lower pH is more over-carbonated so outgases carbon dioxide faster so the pH rises more quickly and requires more acid addition to maintain the pH.

You shouldn't need to worry about all of this right now since your pool seems to be in reasonable shape. The Pool School does not go into detail about avoiding calcium carbonate deposits above the water line. Instead, the Recommended Levels are so people can look up in a simple table and for most pools it's OK and this is intentional because putting numbers into a calculator to spit out a CSI number to manage is believed to be well beyond what most people can handle or at least that they would be scared away if first coming to this forum and needing to do that. It's only when there are special circumstances like more pH rise from carbon dioxide outgassing, say because of more aeration of the water (spillover, waterfalls, fountains, lots of splashing activity, etc.) that one goes to more advanced methods of lowering the TA and targeting a higher pH and if necessary adjusting the CH higher so that the CSI is still in good shape (i.e. if you veer away from the Recommended Levels too much, then it becomes more important to check the CSI).