When you remove the filler cap on the tire the air is not released, this is because there is an airtight seal inside with a small pin keeping it in place. Inside the pressure gauge is a similar device also with a pin.
There are many different tire pressure gauges on the market today and the electronic ones are becoming increasingly popular. In this article we will take a look at the insides of a standard pencil pressure gauge. When you attach the pressure gauge to the tire, you form an airtight seal between the tire valve and the gauge, forming an airtight seal is important to obtain a true reading. The head of the pin inside the gauge presses the head of the pin inside the tire valve, this then releases pressurized air that is contained in the tire.
The insides of a pressure gauge are very simple in design. In addition to the seal and pin already mentioned (that are just inside the head of the gauge), there is a piston that is positioned at the beginning of the main chamber. A calibration rod with an outer compressed spring runs down the length of the chamber, and at the end there is a stopper with a hole allowing the calibration rod to exit through it.
When we attach a pressure gauge to the tire valve and the pins press together, air from the tire is exerted into the gauge. This sends the piston shooting down the chamber, forcing the compressed spring to contract and the calibration rod to move, and sending the end of the rod through the exit hole in the stopper. When we remove the gauge from the tire valve, the piston and spring return to their normal position, leaving the calibration rod in place to give us the psi reading.
Tire pressure gauges are designed to take a maximum psi of air. For example if the maximum is 50 psi, then half way down the chamber is 25 psi and so on.