Taking requests, I will talk about tire pressure sensors. I will talk how they work and systems used in vehicles. Monitor the tire pressure increase the fuel efficiency and safety.
How it works?
The most used air pressure sensor are the capacitives, because they are the most reliable. One of the capacitor plates are linked to a thin diaphragm. The pressure in the diaphragm causes contraction in the diaphragm and consequently the capacitance variation between the plates and the central diaphragm.
The sensor installed in a car wheel.
This sensor has a transmitter antenna (the metallic part in the middle) to transmit sensor`s data to a central module. Also has a lithium battery which has an average life of 10 years.
The tire pressure monitoring system (TPMS) serve to remind the driver the time to calibrate the tires. There are two types: the indirect and the direct. The direct TPMS has a pressure sensor in each tire and transmit data to the module by low frequency radio waves.
The indirect TPMS replace air pressure tire sensors by a speed sensor in each wheel and transmit the signal to control module from ABS system.
The direct TPMS is more accurate than the indirect because it measure directly the air pressure, but it is more expensive. The indirect TPMS can be inaccurate if the tire has a different size ou has been worn. The indirect TPMS must be reset after calibration or tire rotation. By other side, the resynchronization and maintenance by passing time is more expensive with the direct TPMS.
Temperature and altitude influence the pressure on tires. The increase of temperature expand the air in the tires, therefore increase the pressure and the decrease of temperature lower the pressure. Higher altitude means less atmospheric pressure and more pressure come from inside the tire. The indirect TPMS can give wrong measures in low aderence terrain.