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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. Â
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. Â In an outdoor pool, the sun will diminish the available chlorine rather quickly, thus the need for CYA (aka stabilizer). Â 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. Â 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. Â In a pool, splashout, backwashing, etc help keep CYA levels down because you constantly add water. Â In a spa, periodic water changes solve the problem. Â 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. Â There is no perfect sanitizer and there is no perfect chlorine. Â