Auto questions: how tire pressure gauges work

Tire pressure gauges receive air from an inflated tire and the resulting pressure readings can easily be read by a driver.

What is the difference between a flat (but undamaged) tire and one that is properly inflated?

The obvious answer is how much air the tire contains. Air is a measurable gas with specific properties such as mass. When air is pumped into a tire, its individual atoms want to press against the outside of the container, especially when things get crowded. Most automobile tires can hold 30-35 'pounds' of air without becoming stressed. Any amount of air under 30 psi (pounds per square inch) can cause the tires to sag, which in turn can lead to excess pressure on the walls and less efficiency. Overinflating tires can reduce the amount of rubber actually contacting the road (called the contact patch) and lead to unexpected blow-outs. Some stunt drivers may deliberately overinflate tires for specific handling effects, but the average driver shouldn't attempt it.

Tire companies and car manufacturers emphasize the importance of keeping tires inflated to a proper psi. This information is almost always printed on the tire itself, as well as in an interior part of the doorwell and in the owner's manual. Some newer car models even have special sensors which can detect tire pressure problems. But most cars will not automatically alert drivers when tires are under or overinflated, so drivers need to routinely check their tires for damage and proper inflation. For this task, many drivers use a pen-shaped device called a tire pressure gauge.

A tire pressure gauge has a round knob on one end which fits tightly over the valve stem of a pneumatic tire. A driver twists off the valve stem protector and places the bottom of the knob squarely over the valve stem itself. By pressing down lightly, air should escape from the tire and into the chamber of the tire pressure gauge. Inside a smooth metal cylinder lies a soft rubber piston which glides along the internal walls. As air from the tire is released, this piston is forced to the right side of the chamber. The pressure from this piston pushes a spring-loaded rod containing a free-floating plastic or metal stem. When the rod moves as far to the right as the pressure will carry it, it springs back to its original position along with the piston--but the plastic or metal stem will remain in place until the driver pushes it back.

The sides of this stem have calibration lines which correspond to specific pounds per square inch. When the pressure gauge pushes this stem out, it stops at a measurable distance. The driver can look at the portion of the stem closest to the gauge's body and read the highest line visible. This information should be compared to the recommended inflation pressures listed in the owner's manual. If the manual recommends 35 psi and the gauge reads 25 psi, then the driver must add 10 additional 'pounds' of air. All four tires must be checked and inflated (or possibly deflated) until they all have 35 psi of air. Many service stations feature air pumps with tire pressure gauges built into the hose near the valve. These gauges work on the same principle as the pen-shaped model- tire pressure forces a calibrated stem out of its cylinder. Tire pressure can be checked as the driver continues to use the air pump.