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Your weekly shock dose probably adds as much CYA as your daily dose over the long term, so using MPS could cut your CYA in half. I have switched to MPS because I don't have to wait to use my spa, it is a complete shock and idiot proof (if you use too little chlorine to shock...you haven't shocked) and last, it adds less to CYA.
Why Not,Here is some info for you on CYAhttp://www.ppoa.org/pdfs/PrP_Cyanurics%20-%20Benefactor%20or%20Bomb.pdfhttp://www.dsf.health.state.pa.us/health/cwp/view.asp?a=180&q=234664CapMorgan
To put in very simple terms, Cyanuric Acid (CYA) is like a series of little umbrellas that protect chlorine from the sun. In an outdoor pool, the sun will diminish the available chlorine rather quickly, thus the need for CYA (aka stabilizer). While chlorine will disappear from the water, CYA stays, something like TDS or calcium...the only way to dillute or eliminate is to change the water. Now that we know that these little umbrellas don't disappear, when there are too many of them, the chlorine becomes overprotected and has a more difficult time doing its job of sanitizing. In a pool, splashout, backwashing, etc help keep CYA levels down because you constantly add water. In a spa, periodic water changes solve the problem. You will find many articles regarding safe levels of CYA, but most public authorities recognize that 100 ppm should be max for a pool or spa.In the small volume of water of a spa, all sanitizing methods have their good points and bad. There is no perfect sanitizer and there is no perfect chlorine.