The central component of both sensors is the piezo-driven vibrating element. When this element is covered by bulk solids, the oscillation amplitude changes. The change is detected and processed by the oscillator and converted into a switching command. This measuring technique does not require adjustment with medium and offers comprehensive fault monitoring up to SIL 2 (according to IEC 61508). These sensors are also available for dust Ex areas of zone 20 (1D or 1/2D depending on the application).