The general aim is to develop a Venturi flow sensor for the inspiration line to be used in Mechanical Ventilation. This work is an advance for the development and construction of this sensor and to explain some of its characteristics in mechanical ventilation. The Mach number in this sensor grows with the pipe diameter, but it is less than 0.3 to diameters higher than 3mm, and according to the traditional bibliography [9] it can be used as incompressible fluid for the design. For this reason the simulations were done between 2:1 and 6:1 to simulation pressures from 15 to 16.5 Psi (Mechanical Ventilation range). The results of these simulations are: it needs to consider the gas compressibility levels for Mach numbers smaller than 03 because the error of flow measure can be between 5 and 15% for the pattern of ideal gas and enter 7.5 to 20% for the Van Der Waals model [6] above the incompressibility pattern, and these results were used for the construction of the small reduction the Venturi's pipe from 3 to 78 Lpm, taken from absolute pressure to complete the norm ISO9360 [5].