Scale

[nocaster]

Two seasons ago I was fastidious at keeping my water balanced. I used bleach, aerated to lower my TA, kept my pH in the correct range, and had fabulous water thanks to all the knowledge shared on this forum.

Last season I had a infant and was extremely busy with other things. I didn't have time and neglected my water balance. I used trichlor tabs because I didn't have time to obsess over my water chemistry. At the end of the season my water began to precipitate calcium at an alarming rate. If I would leave a vacuum pole in the water for a day or two scale would form on it. Scale formed over every surface of the pool and yellow algae took up residence in the scale. It was unsightly to say the least.

So, I just finished acid washing my pool and I'm filling it as I type. I am wondering what conditions cause calcium to precipitate out of the water. My fill water comes from a well and it is very high in CH and TA. My pH will easily go over 8 if I don't add acid on a continuous basis. I suspect my neglect last year caused my pH to get way to high and created the conditions for precipitation of the calcium. Are there any articles or other posts on here that go into more details on this?

 

[PaulR]

The Pool Calculator will figure out the Calcium Saturation Index (CSI) for you. If you just play around with the numbers you can see what makes the CSI go up and down, and how much. Temperature has a significant effect, for example. A high CSI will make scaling more likely, so you want to keep control of the factors that will help keep it down.
--paulr

 

[chem geek]

My guess is that with evaporation and refill, the CH and TA climbed due to those values being high in the fill water. As the TA rose, the pH tended to climb as well, though one would think that with use of Trichlor pucks/tabs that the pH wouldn't get so high and that the TA wouldn't climb as quickly either. At any rate, if CH, TA and/or pH get high, then scale can develop.

 

[nocaster]

The Pool Calculator will figure out the Calcium Saturation Index (CSI) for you. If you just play around with the numbers you can see what makes the CSI go up and down, and how much. Temperature has a significant effect, for example. A high CSI will make scaling more likely, so you want to keep control of the factors that will help keep it down.
--paulr

Thanks for the link. I put some numbers in that I think were pretty close to what I saw last year and noticed the CSI would go over .6. If it goes over this threshold am I getting into conditions that are favorable for scale?

I found some of my old posts and my well water has a CH of 530 and TA of 480. Just those numbers and a perfect pH put the CSI at .86. If the pH gets to 8 the CSI is 1.35! I'm going to keep an eye on this because acid washing the pool was not much fun.

 

[chem geek]

Anything above zero is technically a scale possibility, but in practice you usually don't see scale until around 0.7 or so -- so the 0.6 is certainly very close to that risky range. That's why we say not to go above around 0.3.