A fluid sensor (1) consists of a hollow cylindrical casing (2) containing a large number of solid spheres (3) of identical diameter, packed tightly together. Fluid inflow and outflow blocks (4, 5) are mounted to each end of the casing (2), forming a fluid-tight seal. The inflow and outflow blocks (4, 5) each enclose a generally conical fluid chamber (6, 8) tapering from where it meets an end of an interior of the casing (2) to a respective inlet or outlet passage (7, 9). Circular grilles (10) divide the casing (2) from each fluid chamber (6, 8) and retain the spheres (3) in place. When a fluid, such as an anaesthetic gas mixture, is passed through the sensor (1) from the inlet to the outlet passage (7, 9), it must follow an indirect path between the spheres (3) packed into the casing (2). There is hence a resistance to the flow, which is manifested as a pressure differential across the casing (2). The pressure differential is measured via side passages (13) extending laterally from each fluid chamber (6, 8). For a given fluid and a given casing and sphere diameter, this pressure differential can be converted to a fluid flow rate.
