A sensor apparatus includes a heating element comprising an upstream side and a downstream side. A first heat sensing set is generally configured adjacent to the upstream side of the heating element and comprises a first sensing element and a second sensing element, the first and second sensing elements configured in a serpentine, interdigitated pattern. A second heat sensing set can be configured adjacent to the downstream side of the heating element and comprises a third sensing element and a fourth sensing element, the third and fourth sensing elements configured in a serpentine, interdigitated pattern.