Poit PH-02 P-type PH Meter Tester.  .005 accuracy, .01 resolution, 0-14 pH range.  ATC (automatic temperature correction).  One touch calibration button.  Comes with meter, 3 x 1.5 LR44 batteries, 3 x pH buffer powders.

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

Poit PH-02 P-type PH Meter Tester, 0.05 Accuracy, ATC Function, Auto Calibration