Press the power button to turn the AstroAI Tyre Pressure Gauge on. Remove the valve cap, then place your digital gauge on the valve stem and press down hard enough that the hissing sound disappears and the screen provides a reading. You can switch units between PSI, Bar, kPa, and KG/CM² by clicking the power button. Write down the reading and repeat this process for all four tires. After use, press and hold the button for 3 seconds to turn it off. The digital gauge will automatically turn off shortly after use.