Patent ID: 7665357

Claim:
A field device for monitoring and/or determining a process variable of a medium, wherein the process variable is a fill level, viscosity or density of the medium, comprising: an oscillatable unit; a driving/receiving unit, which excites said oscillatable unit to oscillate, or which receives oscillations of said oscillatable unit; and a control/evaluation unit, which controls the oscillations of said oscillatable unit, or which evaluates the oscillations of said oscillatable unit wherein: said control/evaluation unit produces an accretion alarm, when the oscillation frequency of the oscillations of said oscillatable unit falls below an adjustable limit value; said adjustable limit value is determined and/or calculated from the smallest oscillation frequency as a function of the corresponding maximum (with reference to the field device) allowable process conditions and/or as a function of the maximum (with reference to the field device and with reference to the application) allowable process variable to be monitored and/or determined; said process conditions involve temperature and/or pressure and/or density and/or viscosity and/or fill level of the medium; the process variable is fill level; said adjustable limit value is determined and/or calculated as a function of the use of the field device, whether as a minimum switch or as a maximum switch; said control/evaluation unit produces a report, when the oscillation frequency of the oscillations of said oscillatable unit exceed an adjustable over-value; said adjustable over-value is determined and/or calculated from measured and/or calculated dependencies of the oscillation frequency on the process variable to be determined and/or to be monitored; and said adjustable over-value is determined and/or calculated from a greatest oscillation frequency as a function of corresponding maximum, in reference to the field device, allowable process conditions and as a function of said oscillatable unit oscillating uncovered.