If you were to read my bio you would see that I am in the business. Sure I have a 18' above ground pool but that's not why I am here. I am here to say thank you. This is an awesome forum. I belong to just a couple others and this one ranks up there with the best in terms of patient veterans and accurate helpful information.
I've only been in the business for 2 years. The vast majority of what I have been taught is correct but based on my experiences I could tell there was more I wasn't being taught. This was only due to ignorance not intentional.
The real jewel of information has to do with the FC/CYA tables.
The second jewel has to do with phosphates. I don't need to go into it but I will say I GET IT!
This was absolutely perfect-
There is a relationship, but it is indirect. Phosphates are an essential nutrient for algae growth. So when phosphates are the limiting nutrient, higher levels will have algae grow faster thereby creating greater chlorine demand such that the SWG cannot keep up. However, this only happens when the active chlorine level isn't high enough to kill the algae faster than it can grow. If the FC is at least 5% of the CYA level in an SWG pool (7.5% in a manually dosed pool), then the chlorine kills green algae faster than it can grow regardless of nutrient level. This is because algae growth is ultimately limited by sunlight and temperature, taking 3-8 hours to double in population.
So you have a choice with the pools that you service. You can set an appropriate FC level for the CYA level and not worry about the phosphate level or you can use expensive phosphate removers and have a lower FC level. That's why I say that phosphate removers should be seen in the same vein as algaecides. They are supplemental products that are not necessary if one maintains the proper active chlorine level. Some municipal water districts have phosphates in their fill water so evaporation and refill will increase phosphate levels over time in that case so their removal is not always just a one-time treatment. The pool store in my local area services 2000 pools (a good portion of them SWG) with a target 4.5 ppm FC and they dilute the water when the CYA hits 100 ppm in the Trichlor pools. They aren't quite at a high enough FC for the CYA in the Trichlor pools so some of them get algae and they shock and use phosphate removers in those cases, but the SWG pools aren't a problem because their CYA level doesn't climb and their FC is high enough. As I wrote before, 4 ppm FC minimum with 80 ppm CYA works well for SWG pools.
Think of phosphates as fuels, such as wood, for a fire. The more wood in a fire, the more heat and fire there is. Think of chlorine as water that is dousing the fire. If you don't use enough water, then the fire can continue to burn albeit somewhat more slowly. If you use enough water on the wood, then a fire can't even get started because any spark or flame gets put out. The maximum size of the wood pile is limited by other factors so as long as you use at least the minimum amount of water on the wood for that maximum size of the wood pile, a fire cannot get started (i.e. algae growth cannot take off).
I will just conclude by saying one more time, THANK YOU!
P.S. SLAM that pool!