Tire pressure monitoring systems are important safety features that the National Highway Traffic Safety Administration has mandated as standard on all cars manufactured after 2006. These systems can save lives and help prevent accidents on the road, so it helps to know exactly how they work.
These systems take advantage of the fact that vehicles nowadays have computer systems that monitor what's going on in the car. On cars without a tire pressure monitoring system, drivers may not be aware that they have a flat until its too late.
A direct tire pressure system will be able to monitor the actual air pressure. They are usually either installed on the valve stem or in the tire itself, so the sensor is actually testing the air inside the tire.
If the tire pressure gets too low, the sensor will catch this right away and send a signal to the dashboard light, which alerts the driver. Even if your car was made before 2006, you can still buy and install an aftermarket tire pressure monitoring system for your vehicle.
When it comes to aftermarket tire pressure monitoring systems, there are types sold other than the "direct" rotational sensors that are used by automakers. These "indirect' tire pressure monitoring systems do not techinically qualify with government regulations, which is why automakers do not use them. However, they are still widely considered to be effective and can make a smart purchase for drivers who do not have a pre-installed system.
Indirect tire pressure systems use rotation sensors on each wheel, which measure how fast each wheel is spinning. The sensors can detect any discrepancy in the rate at which the wheels spin. Typically, the reason for this discrepancy is that one of the tires is low on air and thus is moving more slowly than the others.
Another type of indirect system is one in which sensors detect the relative angle of the vehicle. This essentially works on the same theory - an underinflated tire will subtly adjust the angle of the car and trigger the sensor.
The one drawback to an indirect system is that it must be calibrated when the tire pressure is at the correct amount. If you attach a rotational sensor when the tires are already underinflated, for example, the sensor will think that the underinflated rotational speed is normal, which can be dangerous.