An acid gas removal plant includes an absorber that provides a rich solvent to two regenerators that independently generate a lean and a semi-lean solvent, wherein the semi-lean solvent is produced in one of the regenerators using heat andor steam derived from the other regenerator. A method for calibrating according to the present teachings includes generating a transmit signal that includes a series of frequency ramps and determining a timing of gain peaks in a receiver in response to the frequency ramps in the transmit signal. When the electrode duty of a one-terminal pair surface acoustic resonator is defined by the following equation :.