A method of measuring a volume in a fluid flow micromechanical device includes: a) providing and positioning an optical apparatus for measuring the profile of a surface; b) providing a device for acquiring and processing the images coming from the optical apparatus for measuring a profile; c) placing the moving member in a first position and then in a second position, and activating the optical apparatus for measuring the profile of the surface to direct a light beam on the reference face, and activating the image acquisition and processor device to obtain a first image in the first position of the moving member and a second image in the second position; and d) comparing the second image with the first image of the reference face to determine the variation in the volume of the cavity generated by the deformation of the moving member.