Apera Instruments PHB-3 Economic Waterproof pH Pocket Tester. ±0.1 pH Accuracy. 0-14.0 pH Range. Auto calibration feature.  ATC, automatic temperature compensation, 0 to 50 deg C.  Auto power off to save batteries. Lower battery alarm.  Uses AAA batteries.  Includes calibration solution and carrying case.

I like that this is waterproof and that it uses (easy to find) AAA Batteries.  Search AmazonBasics Batteries

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

Apera Instruments PHB-3 Economic Waterproof pH Pocket Tester, ±0.1 pH Accuracy, 0-14.0 pH Range