An apparatus including a pressure sensing device in combination with a frequency converter, a displacement servo and a transducer in which the pressure sensing device incorporates a vibrating sensitive diaphragm in combination with an electrical drive circuit and a variable frequency oscillator for producing output voltages having frequencies which vary in a substantially linear manner with respect to pressure altitude. The frequency converter is coupled to the pressure sensing device and has a substantially linear transfer characteristic of variable frequency input with respect to amplitude of voltage output for transforming the variable frequency electrical output signal from the pressure sensing device into a variable amplitude output voltage that is applied to the displacement servo which provides an output shaft angle displacement. The transducer is coupled through appropriate gearing to the displacement servo which produces an output electrical signal in response to the output shaft angle displacement that is linearly proportional to the pressure altitude.