
This is not applicable. You put them down and compare them to a table - they get darker over time. I've tried distilled, tap and bottled water - all the same and I know for sure that we have hard water where we live. Maybe I'm doing it wrong? It's been a while since I've taken any science classes.

80 Pieces Water Test Litmus Strips Litmus ph Paper Soil Acidity Meter Liquid pH Tester 1-14
10 Review

Efficiently Test Your Antifreeze And Battery Acid With A 3-In-1 Refractometer
18 Review

Digital PH Meter Tester - Tiaoyeer Water Quality Tester With ATC, 0.01PH High Accuracy, 0.00-14.00 Measurement Range, 0.01 Resolution For Drinking Water, Aquarium, Swimming Pools, Hydroponic
8 Review

TDS water quality tester - TDS water hardness meter (portable digital salt meter for water analysis)
9 Review