The parts of a typical pressure gauge look like this:
There are three simple steps involved in measuring a tire's pressure with a pressure gauge:
- Get in a steady position to apply the pressure gauge to the valve stem.
- Apply the gauge, forming a good seal between the gauge and the stem and releasing air from the tire into the gauge. Note how the pin inside the gauge presses against the valve pin inside the valve stem to release air from the tire.
- Read the pressure from the gauge.
Inside the tube that makes up the body of the pressure gauge, there is a small, tight-sealing piston much like the piston inside a bicycle pump. The inside of the tube is polished smooth. The piston is made of soft rubber so it seals nicely against the tube, and the inside of the tube is lubricated with a light oil to improve the seal. In the picture below, you can see that the piston is at one end of the tube and the stop is at the other. A spring runs the length of the tube between the piston and the stop, and this compressed spring pushes the piston toward the left-hand side of the tube.
The funny spherical thing on the left end of the gauge is hollow. The opening in the sphere is designed to engage a tire's valve stem. If you look in the opening, you will be able to see a rubber seal and a small fixed pin. The rubber seal presses against the lip of the valve stem to prevent air from leaking during the measurement, and the pin depresses the valve pin in the valve stem to let air flow into the gauge. The air will flow around the pin, through the hollow passage inside the sphere and into the piston chamber.
When the pressure gauge is applied to the valve stem of a tire, the pressurized air from the tire rushes in and pushes the piston toward the right. The distance the piston travels is relative to the pressure in the tire. The pressurized air is pushing the piston to the right, and the spring is pushing back. The gauge is designed to have some maximum pressure, and for the sake of example let's say it is 60 psi. The spring has been calibrated so that 60-psi air will move the piston to the far-right of the tube, while 30 psi moves the piston half-way along the tube, and so on. When you release the gauge from the valve stem, the flow of pressurized air stops and the spring immediately pushes the piston back to the left.
To allow you to read the pressure, there is a calibrated rod inside the tube:
The spring is not shown in this figure, but the calibrated rod fits inside the spring. The calibrated rod rides on top of the piston, but the rod and the piston are not connected and there is a fairly tight fit between the rod and the stop. When the piston moves to the right, it pushes the calibrated rod. When the pressure is released, the piston moves back to the left but the rod stays in its maximum position to allow you to read the pressure.
For more information on tire pressure gauges and related topics, check out the links below.