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.