There is theoretically shown that pressure is equal to half of flow depth at state of speed flow, i.e .It is equal to half of critical depth. Knowing it, authors offer a device that is designed to finding critical section and critical depth in open flows which are defined by consecutive measurements of dynamic pressure and depth of flow by means of the device in various cross sections and section findings where the dynamic pressure is equal to a half of flow depth, and depth is critical depth, i.e. equality is observed. As a result of the use of such a device increases information density due to the direct determination of the critical section and the critical depth of flow and increase ease of operation.