A transit time ultrasonic flow meter estimates the flow velocity by transmitting acoustic pulses up- and downstream with respect to the direction of the flow. From the time of the pulses’ flight, the direction of the flow, its velocity and the velocity of sound in the fluid can be estimated . Fig. 1 shows a cross-section of such a meter. When the fluid is not moving, the up and down times of flight should be the same. If they are not, the meter detects it as a false flow. Hemp  shows that this error vanishes when applying the theorem of reciprocity. Willatzen  recently published a tutorial on transducer modeling with a general theory and applications to ultrasound reciprocal systems. He also relies on the fact that the transmitter and the receiver are identical for the system to be reciprocal. To literally quote Pierce , reciprocity refers to the situation for which a magnitude associated with an ‘‘effect’’ at a point is unchanged when the location of ‘‘cause’’ and ‘‘point of observation’’ are interchanged. The current work shows that reciprocity does hold when the two transducers are identical. It additionally shows that some measurement violations exist with small variations of the transducers. These variations are due to the manufacturing process of the transducers. In the next section, we present the simulation tool used to evaluate small changes of properties or dimensions in each of the layers of one of the two transducers as well as its wiring.