Abstract: These substitution kits are provided for field-installed volumetric differential pressure flow sensors (101, 102, 103, 104, 105, 106) comprising bores (111, 121) for sensing the fluid pressure and an orifice plate (13) of standard thickness which is fixed fluid-tight between two pipes (1, 2) conducting a fluid to be measured. A first variant of the kit comprises an annular disk (23) which replaces the plate (13) and whose thickness is equal to that of the plate as well as whose opening has a diameter equal to the inside diameter of the pipes. A single bluff body (2311, 2312, 2313, 2314, 2317, 2318, 2319) is arranged in the lumen of the annular disk. The bore (111) of the pipe (1) is fluid-tightly closed by a device (18) after removal of the pressure sensor of the bore (111). A vortex sensing element (19) is inserted fluid-tightly in the bore (121) of the pipe (2) after removal of the sensor of this bore. Acc.