PH-009 pH Meter with 2 Pack of Calibration Mixture.  Measures 0 – 14 pH with .1 pH resolution.  Comes with protective case, calibration screwdriver pH 4 buffer and pH 6.86 buffer.

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From the product description, check product page for current description, price and availability:

● Measuring Range: 0-14.0 pH
● Resolution: 0.1 pH
● Accuracy: ±0.1 pH（20°C )
● Use Environment: RH 95%, 0-50℃（32～122℉ )
● Adjustment: trimmer potentiometer
● Battery: 3×1.5V LR button cell

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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

Digital PH Meter PH-009 Pocket Size Water Quality pH Tester for Household Drinking Water, Hydroponics, Aquariums, Swimming Pools – Extra Two Packs of PH Calibration Solution Mixture