Tire-pressure gauge
A tire-pressure gauge is a pressure gauge used to measure the pressure of tires on a vehicle.
Environmental conditions can introduce a 13% to 15% variability in pressure due to temperature (0 °C to 40 °C), and additional changes can result due to altitude. Most car owner manuals do not state rated pressure as a function of temperature or altitude and leave it to the user to make appropriate measurements.
Since tires are rated for specific loads at certain pressure, it is important to keep the pressure of the tire at the optimal amount. Tires are rated for their optimal pressure when cold, meaning before the tire has been driven on for the day and allowed to heat up, which ultimately changes the internal pressure of the tire due to the expansion of gases. The accuracy of a typical mechanical gauge as shown is ±3 psi. Higher accuracy gauges with ±1 psi accuracy can also be obtained.
[edit] Built-in tire pressure sensors
Many modern cars now come with built in tire pressure sensors that allow all four tire pressures to be read simultaneously from inside the car. Tire-pressure monitoring systems are also fitted as an option to the latest generation of BMW motorcycles.
In 2005, most on-board TPMS used indirect pressure monitoring. The anti-lock brake sensors detect one tire rotating faster than the rest and indicate a low tire pressure to the driver. The problem with this method was that if tires all lost the same pressure then none would show up against the others to indicate a problem.
[edit] Regulations on tire pressure
Since September 2007 all new automobiles below 10,000 pounds in weight sold in the United States are required to incorporate a Tire Pressure Monitoring System (TPMS) which is capable of monitoring all four tires and simultaneously reporting under-inflation of 25% of cold plackard pressures in any combination of all four tires. TPMS known as Direct TPMS are capable of TREAD Act legislation requiring simultaneous pressure measurement for each tire pressure.[1]
Early TPMS sensors required batteries but the latest TPMS technology eliminates all sensor batteries.[2]
