The nozzle flapper is a displacement transducer that translates displacements into a pressure change. A secondary pressure measuring device is required within the instrument. Fluid at a known supply pressure, Ps, flows through a fixed restriction and then through a variable restriction formed by the gap, x, between the end of the main vessel and a flapper plate. The body whose displacement is being measured is connected to the flapper plate. Motion of the flapper plate causes a change in the pressure in the measurement chamber, Po, which is almost proportional to x over a limited range of movement of the flapper.