Mass fluid flow sensor with low-loss bypass passage

A mass fluid flow sensor 10 includes an internal bypass passage 22 characterised by a first section 30 that converges along a first axis to define a nozzle, and a radially expanded cylindrical second section 38 coaxial with the first section defining a radial step. The passage further includes a semi-spherical third section 44 adjacent to the second section having the same diameter, and a converging fourth section whose axis is disposed at a right angle to the axis of the first and second sections. A fifth section 58 may be provided adjacent to the fourth section that includes a further ninety degree bend. The resulting U-shaped passage features reduced pressure losses and an improved velocity profile, whereby the performance of a sensing element 66, e.g. a hot-wire element, disposed proximate to the radial step is improved.