A method and apparatus for monitoring liquid levels using pressure responsive instruments is described, in which an arrangement for compensating for the effects of errors including gas weights errors is provided. The required compensation factor for a particular installation is first calculated theoretically, and then used in order to raise the output signal of the instrument electronically by the calculated factor. In one arrangement, the output signal is amplified by the desired factor. Alternatively, the excitation voltage of the instrument itself, or of a potentiometer used to interface the instrument to electronic data acquisition devices, is varied in order to raise its output voltage by the desired compensation factor.