Salt test strips (Acucheck) tend to be accurate within +/- 300ppm...it could be the SWG unit, but if CYA is declining, then salt is too. Like salt, CYA does not evaporate. It can vanish (another topic...can convert to ammonia) in the offseason, but if the pool is properly maintained during the swim season CYA comes down with dillution, as does salt.