Review on FOMOOUR pH Meter - High Accuracy Water Quality Tester - 0.01 pH Measurement Range - Ideal for Water, Brewing, Food, Soil, Sauce, and Laboratory Testing - Includes TDS Meter and pH Pen by Ryan Sandell

The calibration procedure is described in the instructions accompanying the device. The videos here on the Revain page for this product and on Youtube show a different calibration procedure. Both calibration methods require the calibration button to be held down for 5 seconds until the display flashes and reads 6.86 in the first calibration solution. The tester I have does not do this. Is always. I tried the calibration button for 5 seconds, 3 seconds, 10 seconds, 30 seconds. I tried pressing the calibration button more than once. It never goes into calibration mode. I took out the batteries, waited a few minutes, put the batteries back in and tried again. Same result. It measures calibrated solutions with a pH of 0.5, so it measures pH, but my calibrated results vary by as much as 0.2 from test to test. With good calibration solutions, this device has at best an accuracy of +/-0.5 for pH and +/-0.2 for short-term reproducibility. I was hoping for better than +/- 0.7 pH.