There are many instances where it is necessary or desirable to determine the average velocity of a cross section of a fluid, particularly as flowing through some type of containment vessel.  Average velocity is needed in such an instance in order to determine volume of flow, as by simply multiplying average velocity by the cross section of fluid.  However, as is well known, the velocity of flow often varies within a discrete volume, particularly as between levels of a given cross section of it, making it difficult to determine the average velocity. 
One prior system, which is described in U.S. Pat.  No.  4,083,246, employs an electromagnetic sensor, this being placed in the flow stream typically near the bottom of a channel of flow.  Its output varies as a function of a sensed electromagnetic field, sensing being limited to the region closely adjacent the sensor, and thus it senses the velocity at its particular level in the flow.  In accordance with this patent, liquid level, height of liquid, must also be sensed and the output of the electromagnetic unit modified as a function of liquid height to obtain a velocity signal which it is asserted representative of average flow of the fluid in which the sensor is immersed. 
A second type of flow velocity measurement device is of the Doppler shift type wherein an acoustic signal of a known frequency is emitted into a moving fluid, and reflected signals, particularly from objects in it, are sensed and their frequency compared with the emitted signal.  With this method, a difference frequency or frequencies, Doppler frequencies, are thereby obtained, each representative of a discrete velocity.  A modification of this approach employs several sensors, and their outputs are averaged. 
One significant problem with electromagnetic type sensors is that accuracy often tends to fall off fairly rapidly with time of usage, particularly where there is any foreign matter which can adhere to the sensor.  This is a significant problem in the case of usage in sewage lines. 
A distinct problem with acoustic-type devices is that they respond to multiple acoustic reflections, each from a distinct floating object, particle, or other moving interface with the fluid, and as these reflectors may be moving at different velocities depending upon such variables as their height and/or other distance from a containment wall, a variety of velocity indicating signals are received.  In the past, different approaches for selecting particular reflected Doppler signals as indicating velocity have been attempted.  Perhaps the most prominent one has been that of choosing the signal having the greatest amplitude or the average frequency of several discrete highest amplitude signals.  The problem with this approach is that there is no assurance that it is derived from a reflecting source or sources which yield a true average velocity.  For example, the echo or echoes may be from a large object which is substantially submerged and is moving at a much less than average velocity.  On the other hand, it, or they, may be from an object moving on the surface and thus at a higher than average velocity.  Thus, this approach is inherently susceptible of significant inaccuracies. 
It is, accordingly, the object of this invention to overcome the aforesaid and other problems relating to prior art devices and to provide an improved velocity measurement system which is significantly more accurate and reliable.