I realize that this might be dependent on the inter-relationship of pH, calcium hardness level, and possibly the alkalinity ppm. Is there a chart that might explain these inter-relationships and where the cutoff point for orthophosphates might occur, in other words the upper limit before calcium phosphate scaling occurs? I am not at all concerned with high levels of orthophosphate as they relate to algae formation, and am in complete agreement with this forum that properly maintained chlorine, and CyA levels, adequately compensate for this.
The reason for asking the question is that I do not wish scale to return. In chemgeek's post here he mentions the possibility of this happening:
A few posts on this issue have alluded to the issue of high orthophosphates and possible staining but no definitive numbers are mentioned. They do mention very little orthophoshate is brought into the pool by wind borne sources. Additionally there are a number of internet articles describing this issue, but again no actual numbers are mentioned. Here are a few of these links:
A little background, I had drained the pool about 6 weeks ago, and converted back to liquid chlorine from saltwater. I managed to get rid of my calcium and iron staining through a combination of AA and HP, at an HP level 2.5 times as high as I had originally used in my prior post on the subject of HP. This was far more effective on scale removal.
Please Note Carefully: The HP procedure is extremely experimental and not recommended for "Newbies". It also will not help very much with those persons whose fill water contains iron.
The walls once drained looked nearly as white as the day the pool was installed some 17+ years ago. Once most of the pool was drained, on bypass, there was a huge amount of both light colored, near white, and yellowish-brown fine sandy material deposited at the bottom of the pool. It looked like a little beach, going back about 6 feet on two sides and 2 feet on the narrow side around the main drain, which I assume was the scale removed. Once the pool was refilled, my suction side cleaner removed most this material to the sand filter and was backwashed out, it took 3 backwashes, over two days; the pump and filter ran for 48 hours.
I added back Jack's Magic "Purple Stuff" at a level of 40 ppm. I now think this was a mistake, I should have added only 12 ppm, my reasoning being that most of my iron staining issues were a result of not getting rid of the underlying problem of scale. Scale, not iron, MIGHT, please note I wrote might, be better controlled with Jack's "Magenta Stuff". As this is an Acrylic Acid Copolymer, it seems from reading various sources, this is more effective at preventing scale. I have not yet added the "Magenta Stuff".
In 6 weeks the orthophosphate level has reached around 7,500 ppb, tested with two different kits. My municipal fill water has around 400 ppb. My assumption is that most of this additional 7,100 ppb came from Jack's "Purple Stuff". Since 6 weeks ago I have added no additional "Purple Stuff", and using Jack's test kit, the sequestrant level of the "Purple Stuff" still reads 40 ppm. Had I not raised the sequestrant level to 40 ppm, and only to 12 ppm, my orthophosphates might now only be 2,130 ppb plus 400 ppm from fill water, or about 2,500 ppb.
The pool currently has not the faintest hint of staining YET. And I would love to keep my pool scale and iron scale free in the future. Scale is even a bigger problem in Hawaii, according to Jack's tech person, than iron, but he could not explain why.
I believe my scale problem developed many years ago before I started to balance my pool according to this and one other forum's recommendation. Perhaps too little alkalinity (alkalinity can drop fast in our waters if not monitored), or too much calcium. And while I did the AA treatment, and it was successful at removing the colored stains, of what I presumed to be iron colored stains but they always came back. It is possible, please note this is pure conjecture, that the AA removed a veneer of calcium scale, and that iron was not a big a problem as I had always thought. I now believe that was because I never got rid of the underling problem of scale.
I even believe that I might no longer need to keep my water at a pH of 7.3, but at a more reasonable 7.5 or 7.6, cutting down on acid usage. Only time will tell if that works.
My present balance is:
TDS around 880
Salt around 720
Temp 78 - 80
Orthophosphate around 7,500 ppb
Municipal fill water Orthophosphate around 400 ppb
Sequestrant 40 ppm
Thank you to those that help.