An indirect tyre pressure monitoring system (TPMS) is based on the principle that an underinflated tyre has a smaller rolling circumference and thus rotates more quickly, as well as a different frequency and amplitude, to that of a correctly inflated (reference) tyre. The vehicle's onboard computer is setup and programmed to detect this difference via the wheel speed sensors, and alert the driver as necessary.

This system doesn't measure a tyre's inflation pressure, so it works with values relative to whatever the system has been set at. Thus, the system is dependent on the user to ensure the tyres are inflated at the correct pressure if the TPMS is to function as intended.

Unlike first generation systems, a second generation indirect TPMS is able to identify which individual tyre is punctured or underinflated.



Contrast this with a direct TPMS, which uses inflation pressure sensors attached to the air valve and installed inside each wheel on the rim flange (the spare wheel may or may not be included). These sensors communicate with the vehicle's onboard computer wirelessly and measures inflation pressure as well as temperature, so this system works with absolute values, eliminating the need for user calibration (read: miscalibration).

However, inflation pressure sensors (which aren't always inexpensive) need replacing every 8-10 years, because as a consequence of trying to protect the electronics and casing from excessive wheel vibration, the lithium batteries used to power them are not normally user replaceable.



For any TPMS according to EU regulations:
Upon adjusting or calibrating the inflation pressure, the system is to be given 20 minutes of learning time.
The puncture threshold is 20% below operating (warm) pressure, detected within 10 minutes.
The diffusion threshold is 20% below operating (warm) pressure, minus a further 7 kPa (1 psi), detected within 1 hour.
The system must be able to operate from a speed of 40 km/h.

For any TPMS according to US regulations:
The threshold is 25% below the recommended (cold) pressure, detected within 20 minutes.

To the best of my knowledge, there is nothing in the regulations for an upper threshold to be established.