Instructions say add drops untill water turns blue, then multiply by 50 to get reading. first I tested the tap water again, water turned blue after 11 drops, Tap water equals 550. Then I tested the pool water again, After 30 drops I stopped adding the solution . The water never turned blue, which puts it at 1500 plus. I have not added water in two months because it fills with rain water after splash out. If rain water has no calcium then that should be better than my 500 tap. Thanks for the reply's guys .