Method and Apparatus for Ascertaining and/or Monitoring a Process Variable

A method for ascertaining and/or monitoring a process variable, comprising the steps of:

• feeding a time-limited exciting signal, which is described by at least one desired quantity, or by a desired signal form, to a first piezoelectric element or to a first polarized zone of the first piezoelectric element;
• registering a response signal, which is described by at least one actual quantity, or actual signal form, corresponding to the desired quantity, or desired signal form, by a second piezoelectric element or a second polarized zone of the second piezoelectric element;
• comparing the actual quantity, or the actual signal form, of the response signal and the desired quantity, or desired signal form, of the response signal with one another;
• modifying the exciting signal, in the case of a deviation of the actual quantity, or actual signal form, of the response signal from the desired quantity, or desired signal form, of the response signal that the actual quantity, or actual signal form, of an ultrasonic measuring signal transmitted from the first piezoelectric element, or from the first polarized zone of the first piezoelectric element, is at least approximately equal to the desired quantity, or the desired signal form, of the response signal;
• and
• ascertaining the process variable on the basis of the ultrasonic measuring signal, which is describable by the defined, desired quantity, or the defined, desired signal form, via a sound-entrainment method or an echo method.

An apparatus for ascertaining and/or monitoring a process variable, comprising:

• a first piezoelectric element having a first polarized zone;
• and
• a second piezoelectric element having a second polarized zone;
• said first piezoelectric element and said second piezoelectric element are embodied in a disk shape and each has two end surfaces lying opposite to one another, wherein:
• a time-limited exciting signal, which is described by at least one desired quantity, or by a desired signal form, to said first piezoelectric element or to said first polarized zone of said first piezoelectric element;
• a response signal, which is described by at least one actual quantity, or actual signal form, corresponding to the desired quantity, or desired signal form, is registered by said second piezoelectric element or said second polarized zone of said second piezoelectric element;
• the actual quantity, or the actual signal form, of the response signal and the desired quantity, or desired signal form, of the response signal are compared with one another;
• in the case of a deviation of the actual quantity, or actual signal form, of the response signal from the desired quantity, or desired signal form, of the response signal, the exciting signal is so modified, that the actual quantity, or actual signal form, of the ultrasonic measuring signal transmitted from said first piezoelectric element, or from said first zone of the piezoelectric element, is at least approximately equal to the desired quantity, or the desired signal form, of the response signal, and
• on the basis of the ultrasonic measuring signal, which is describable by the defined, desired quantity, or the defined, desired signal form, the process variable is ascertained via a sound-entrainment method or an echo method.