How does a tire pressure gauge work and how is it used?

A tire pressure gauge, also known as a tire pressure meter or vehicle tire pressure gauge, is a special pressure gauge used to measure the pressure inside the tires of cars, trucks, bicycles, and other vehicles. To use a tire pressure gauge, align the measuring end with the tire valve and observe the reading to determine the tire pressure and whether to deflate or inflate. The unit is usually bar (megapascals). What is the appropriate tire pressure in bars? What is the working principle of a tire pressure gauge?