含羞草

There is a myriad of shapes, sizes, and costs to consider when shopping for a gauge, however, they all boil down to 3 basic types. The dial, digital, and stick. The gauges may look different, and display results differently, but all use the same principle. When the tire gauge is pressed onto the stem of a wheel, the air pressure in the tire pushes against some type of plunger which in turn actuates the readout section of the gauge assembly.

There are three simple steps involved in measuring a tire's pressure with a pressure gauge:
1 Get in a steady position to apply the pressure gauge to the valve stem.
2 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.
3 Read the pressure from the gauge.

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.

There is a calibrated rod inside the tube,to allow you to read the pressure.The calibrated rod fits inside the spring and it 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.