HOW TIRE PRESSURE GAUGES WORK?

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 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.