Direct measurement systems are considerably more precise, but also involve more outlay, and are therefore more expensive. Here, a battery-powered sensor is located in each wheel. This measures the temperature and the pressure of the tyre, and transfers the measured values wirelessly to the tyre pressure monitoring system control unit and/or the display unit. One or more antennas transmit the radio signal.
Direct systems compare the tire pressure with a reference value stored in the tyre pressure monitoring system control unit. This has the advantage that pressure losses of several tyres can be detected at the same time. This may mean that, following a tyre change, a re-adjustment (calibration) or re-coding of the sensors is required.
A further disadvantage of the direct measuring systems is that the batteries have to be replaced after approx. 5 – 10 years. As, depending on the manufacturer, these form a unit with the sensors, this often means complete replacement of the sensor unit.
If the batteries have to be replaced, this is shown in a timely manner by the display unit, meaning that a sudden system failure is avoided. When changing from summer to winter tyres, it should be ensured that additional wheel sensors are attached, or that existing sensors are modified. Certain important points should be noted so that no damage or functional problems occur during tire fitting.