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A tire air gauge is a small tool that senses and displays the relative air pressure inside a tire. Tire pressure is important to track and maintain, as tires are designed and built to operate at a very specific pressure level. Too much or too little pressure and negative effects can result, including uneven or accelerated wear, and bad fuel mileage. Checking tire pressure regularly with an air gauge can also help in early detection of slow leaks that need to be repaired.
Tire pressure is expressed most commonly in pounds per square inch (PSI). The metric equivalent of PSI is units pascal (Pa) and kilopascal (kPa). Most air gauges read, in these two units, similar to the way that speedometers usually include both miles per hour (MPH) and kilometers per hour (KPH).
Another unit of measurement for tire pressure is the bar, equal to 14.5 PSI (1,000 kPA). This unit is significant in that one bar is the level of atmospheric pressure, or the normal pressure present at sea level. Tire gauges are zeroed to the atmospheric pressure of one bar, and actually measure the relative pressure above it. This type of reading has a specific term, known as gauge pressure.
In absolute terms, therefore, the zero reading that an air gauge starts at is actually already 14.5 PSI (1,000 kPA). A comparison is the way scales take into account gravity when tared, so zero is technically not really zero. Since atmospheric pressure drops by roughly 0.5 PSI (3.4 kPa), for every 1,000 feet (305 m) gained in altitude, tire pressure gauges zeroed to one bar will increasingly overstate their readings as altitude goes up.
Though they all ultimately perform the same task, there are a variety of tire air gauge designs on the market. Some may have extra bells and whistles, but, for the most part, the differences come down to the type of display and price. The biggest option is whether to get an analog or electronic one.
Analog display air gauges themselves come in several different forms. The most common type, found in automobile glove compartments around the world, is a pen-sized device with a bulbous end that is pushed over a tire's valve stem. An open end with a white plastic meter usually pops out to register the measured pressure. This basic gauge has a simple piston inside that pushes out the meter as it is propelled by pressure from the tire.
A more advanced design is known as the Bourdon pressure gauge. This type — named for French inventor Eugene Bourdon, who patented the design in 1849 — incorporates a small, flat tube that inflates as pressure is fed into it. The tube is connected on one end to a hose and nozzle that feeds out and connects to the tire valve stem. On the other end is a needle that moves to display the pressure against a calibrated analog display, similar to an analog watch face or speedometer.
A digital air gauge couples a pressure sensor with a digital display to measure and communicate tire pressure. Instead of a mechanical part, like a piston, a digital gauge houses a very small piece of silicon or metal foil film. This acts as a strain gauge, which translates the strain due to pressure into electrical resistance. A small circuit can then translate that level of resistance into usable digital output for the display. Digital models are typically more expensive than analog ones, but they are ultimately an affordable tool that should be owned by every driver.