FC-CYA-PH-metal stain relationship

[beleau]

I am on my second AA treatment. Stains came back after first time, I'm guessing because I did a 50% water change to lower CYA, (community well water) I also let the ph get to 7.5. I am curious though about the relationship with high FC and metal staining. I have read many times here that high ph combined with hi fc precipitates metal staining. My question is this; with a higher cya of about 70 (thus needing a higher fc level) am I fighting a losing battle with keeping the metal suspended? Like a said, I am on my second AA treatment now. Brought FC to zero, added 2 lbs of AA (38k pool), stains lifted perfectly. Then added 5 quarts jacks pink stuff. Let it circulate for 24 hours. Readings are;

FC = 0
TA = 70
PH = 7.0

I know that I need to bring up FC slowly, like 2ppm at a time I think so thats what I am planning. I am wondering if the higher Ph (7.5) on the first AA treatment is what brought the stains back. So now I am thinking about leaving the ph at 7.0 for now and slowly bringing FC up. Any thoughts?
Thanks

 

[JasonLion]

The most important step affecting the return of stains is to use enough good quality sequestrant and to replenish it regularly. With the correct dosage of sequestrant fairly high FC, PH, and CYA levels can be maintained without problems.

The next most important factor affecting stains is the PH. Higher PH levels are more likely to trigger staining. Or to put that another way, higher PH levels require higher sequestrant levels to prevent staining.

The effect of FC levels is a little more complex, and not nearly as important as PH. However, chlorine additions tend to directly affect the PH. We generally simplify this and talk about chlorine causing stains, but it is usually the short term increase in PH caused by the addition of chlorine that causes the stains, rather than any particular FC level. Bleach, liquid chlorine, and cal hypo all increase the PH when they are added, and then lower the PH back down as they are used up. So even though those forms of chlorine are net PH neutral, they will usually raise the PH right when you add them, and the PH will come back down later.

 

[beleau]

Interesting. So is my plan about bringing FC up about 2ppm at a time about right? (I am using liquid chlorine). Is there any problems with keeping my PH around 7.0?

 

[JasonLion]

We recommend not raising the FC level by more than 1 ppm per day after an AA treatment. You might still add more than 1 ppm of chlorine per day, just not all at the same time so that the level doesn't go up very quickly. Moving that slowly gives you time to react to even the slightest reappearance of staining.

 

[beleau]

Sounds good. "react" as in add more sequestrant?

 

[JasonLion]

Yes, generally lower the PH (which takes effect quickly) and add more sequestrant (which solves the problem but takes longer to have an effect).

 

[beleau]

is there any problem with keeping PH at 7.0? and if not about where should TA be? its at 70 now.

 

[JasonLion]

You don't want to keep the PH at 7.0 all the time because it tends to cause eye irritation. However, that is a fine place to set PH when you are doing an AA treatment or especially worried about stains. I wouldn't go below 7.2 if you plan to swim. Also keep in mind that the lower you try to keep the PH, the faster PH will tend to drift back up.

TA around 70 is fine.

 

[beleau]

great, thanks