A heat exchanger system comprises a heat exchanger 100; and one or more sensor(s) 90 for measuring characteristics of a fluid flow field across a cross-section of a flow path in the heat exchanger 100. Each of the one or more sensor(s) 90 comprises multiple conductivity sensing elements 110 distributed across multiple locations in an array extending over the cross-section of the flow path for obtaining measurements of the fluid flow field at the multiple locations.
