The flow nozzles, more costly than other orifice due to their structure, are suited for determining the flow rates of fluids flowing at high termperature. Under the same measuring conditions, a flow nozzle has a hgither mechanical stength, can permit the flow of more than 60 percent greater volume of a fluid, and can measure the flow rates of fluids containing solid particles less disturbed, than an orifice having the same bore. Thus, they usited, in addition, for high-speed flowing fluids.