A sensor apparatus comprising a housing having an inner perimeter which defines an area through which gas may flow, the housing being provided with a first chamber which extends around the area through which gas may flow, an entrance being distributed around the first chamber, and a second chamber which extends around the area through which gas may flow, an exit being distributed around the second chamber, the first chamber being arranged to be upstream of the second chamber in use, wherein the sensor apparatus further comprises one or more sensors arranged to measure a pressure difference between pressure in the first chamber and pressure in the second chamber. Corresponding turbocharger and method of measuring a mass flow rate are also provided.