Monopersulfate, or MPS, helps to maintain and increase the life of your sanitizer. MPS Spa Check™ 3in1 is the perfect solution for pool & spa owners who use MPS. Simply dip one test strip into your pool or spa for 5 seconds with a gentle back-and-forth motion. Remove the strip and shake once, briskly to remove excess water. Wait 15 seconds and match with the closest colors.