on the background pressure line indicates the occurrence of a pressure disturbance in the air flow this pressure disturbance is due to a vortex adjacent to opening 26 of pitot tube 20 the pressure variation is transmitted along pitot tube 20 to pressure transducer 24 which produces electrical signals in response to both the background pressure and the superimposed frequency pulsations indicative of passing vortices the electrical signal is coupled from transducer 24 to microprocessor 28 wherein the signal is processed to provide the separate indication of the background pressure used in density calculations and the frequency of vortex shedding that is a velocity signal that is it is possible to compute just the velocity of the air flow or to compute the mass of the air flow when background pressure and fluid temperature are taken into account the separate indication of the background pressure used in density calculations and the frequency of vortex shedding can also be obtained by placing a 34 tee 34 in the tubing carrying the signal from the pitot tube and connecting a slow response pressure sensor to one side of the 34 tee 34 to get the background pressure and a microphone to the other side of the 34 tee 34 to get the vortex frequency which is proportional to the velocity both of which provide the two separate electrical signals to the microprocessor 28 referring to fig1 a slow response pressure sensor.