DILISS Digital PH Meter / PH Tester.  Includes 3 packs pH buffer powder.

• Measuring pH Range: 0.0 – 14.0 pH. Resolution: 0.1 pH. Accuracy: 0.1 pH (20C), 0.2 pH. Operating Temperature: 0 – 50C (32 – 122F).
• PACKAGE CONTENTS: 1 × pH Meter , 1 × Carrying case, 1 × Calibration screwdriver, 3 × pH buffer powder (for calibration) ,1 × User manual

You can use a pH meter to check the pH of your water, mash, [test Star San effectiveness] and finished beer.

Update: Since this post was published the price, availability or promotion has changed.  Check the product page for current description, price and availability.  Also: Today’s Deals on Amazon

DILISS Digital PH Meter / PH Tester / Mini Water Quality Tester for Household Drinking Water, Hydroponics, Aquariums, Swimming Pools, 0.1PH Resolution – Extra PH Calibration Solution Mixture