In industrial installations in which high temperature fluids circulate in ducts, it is very often necessary to measure the velocity or flow rate of those fluids in order to control or monitor the industrial installation.  For example, in the case of nuclear power stations incorporating a pressurized-water reactor, it is necessary to know the flow rate of the primary water to a very high degree of accuracy while the reactor is operating.  The flow rate can be measured by determining the velocity of propagation of the pressurized water in a primary duct, by measuring the the propagation time of ultrasonic waves in fluid circulating in the duct.  In order to do this, use is made of ultrasonic sensors, which can transmit waves into the circulating water and receive these waves after propagation and possible reflection on a wall of the ducting.  Prior art sensors actually have to be placed in an opening machined on the inside of the wall of the duct, to limit losses and amplify the amplitude of the received return echo. 
Such a sensor mounting, which is described as intrusive in the duct makes the use of these sensors very difficult in the primary circuit of a pressurizedwater nuclear reactor in which the water is at a pressure of the order of 155 10.sup.5 Pa and at a temperature in the range 280.degree.  C. to 320.degree.  C. For safety reasons it is in fact necessary to limit as far as possible the number of tappings in the primary ducts of the reactor. 
Also, sensors which incorporate a piezoelectric ceramic pellet and a wave guide coupled to that pellet use in their manufacture materials that are generally incapable of withstanding the very high temperatures of the fluid and of the primary ducting. 
In the case of a non-intrusive mounting of the sensor, the latter must, however, be in contact, via its wave guide, with the duct wall which is at a very high temperature.  Consequently, the wave guide must be designed so as to withstand the temperature without distorting and without excessive expansion, and in such a way that it has a homogeneous, resonant and easily machinable structure, so that its surface coming into contact with the duct wall has a very high surface finish, i.e., of very low roughness.  No wave guides having all these characteristics have been known to date.