2 1 . The fluid flow sensor of claim 20, wherein the flow oscillator further comprises: a flow diverting portion configured to oscillate the flow of the fluid between the 

fluid inlet and the fluid outlet to generate an oscillating pressure differential; and 

an oscillating portion downstream of the flow diverting portion and configured to oscillate in response to the oscillating pressure differential. 

22. The fluid flow sensor of claim 20 or claim 21, wherein the flow oscillator is configured to oscillate the flow of the fluid above an oscillation threshold in a first flow direction and not oscillate the flow of the fluid above the oscillation threshold in a second flow direction. 

