. . . . "To achieve the desired rigidity, the thickness of the wall of the test section 50 may be made to have a predetermined thickness, or the test section 50 may be made of a very rigid material, e.g., steel, titanium, Kevlar???, ceramic, or other material with a high modulus. [0058] It is within the scope of the present that the pressure sensor spacing may be known or arbitrary and that as few as two s" .