A tire pressure gauge measures the air pressure inside a tire, which is important to maintain for optimal performance and safety. Pressure is typically measured in PSI, kPa, or bar. There are analog and digital gauges available, with the latter being more expensive but more accurate. Different types of analog gauges include the basic pen-sized device and the more advanced Bourdon gauge.
A tire pressure gauge is a small instrument that senses and displays the relative air pressure inside a tire. Tire pressure is important to monitor and maintain, as tires are designed and built to operate at a very specific level of pressure. This can lead to too much or too little pressure and negative effects, including irregular or accelerated wear and poor fuel consumption. Checking your tire pressure regularly with a tire pressure gauge can also help you catch slow leaks early on that need to be fixed.
Tire pressure is most commonly expressed in pounds per square inch (PSI). The metric equivalent of PSI is pascal (Pa) and kilopascal (kPa) units. Most air gauges read, in these two units, similar to the way speedometers usually include both miles per hour (MPH) and kilometers per hour (KPH).
Another unit of tire pressure is the bar, equal to 14.5 PSI (1,000 kPA). This unit is significant because one bar is the level of atmospheric pressure, or the normal pressure present at sea level. Tire pressure gauges are zeroed to one bar atmospheric pressure and actually measure relative pressure above it. This type of reading has a specific term, known as a gauge pressure.
In absolute terms, therefore, the zero reading from which an air meter starts is actually already 14.5 PSI (1,000 kPA). A comparison is how scales take gravity into account when they are calibrated, so zero technically isn’t really zero. Since atmospheric pressure drops about 0.5 PSI (3.4 kPa), for every 1,000 feet (305 m) gained in altitude, tire pressure gauges reset to one bar will increasingly overstate their readings as altitude rises.
While they all ultimately perform the same task, there are a variety of tire air gauge models available 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 the price. The biggest option is whether to get an analog or an electronic one.
Analog display air meters themselves come in different forms. The most common type, found in the glove compartments of cars around the world, is a pen-sized device with a bulbous end that is pushed onto the valve stem of a tire. An open end with a white plastic gauge usually sticks out to record the pressure being measured. This basic gauge has a simple piston inside that pushes the gauge out as it is pushed by the tire pressure.
A more advanced design is known as a Bourdon gauge. This type — named for French inventor Eugene Bourdon, who patented the design in 1849 — incorporates a small, flat tube that inflates when pressure is supplied. The tube is connected at one end to a hose and nozzle that exits and connects to the tire valve stem. At the other end is a needle that moves to display pressure on a calibrated analog display, similar to an analog dial or tachometer.
A digital pressure gauge couples a pressure sensor with a digital display to measure and communicate tire pressure. Instead of a mechanical part, such as a piston, a digital gauge houses a very small piece of silicon film or metal foil. This acts as a strain gauge, translating the strain due to pressure into electrical resistance. A small circuit can then translate that level of resistance into a usable digital output for the display. Digital models are typically more expensive than analog ones, but ultimately they’re an affordable tool that every driver should own.
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