A system comprising: a substrate having a substrate top side and a substrate bottom side; a heating element on the substrate top side; an upstream temperature sensor on the substrate top side, wherein the upstream temperature sensor is positioned upstream of the heating element and is configured to sense a measure related to a first temperature of a fluid passing adjacent the substrate bottom side before the fluid reaches the heating element; a downstream temperature sensor on the substrate top side wherein the downstream temperature sensor is positioned downstream of the heating element and is configured to sense a measure related to a second temperature of the fluid after the fluid passes the heating element; a channel in the substrate forming at least part of the substrate bottom side under the upstream temperature sensor, the downstream temperature sensor, and the heater, such that a fluid can flow through the channel in a flow direction, a first end of the channel defining a channel inlet, wherein the fluid enters the channel at the inlet; the substrate having a membrane forming a top side of the channel and supporting the upstream temperature sensor, the downstream temperature sensor, and the heater above the channel; a thermal dam that thermally isolates at least part of the membrane from the remainder of the substrate; and an inlet temperature sensor on the substrate top side, upstream of the upstream temperature sensor and adjacent the channel inlet, characterised in that the thermally insulated part of the membrane supports the inlet, upstream and downstream temperature sensors and the heater and is insulated from the remainder of the substrate and that the system further comprises also a substrate temperature sensor on the substrate top and is positioned away from the channel. a substrate having a substrate top side and a substrate bottom side; a heating element on the substrate top side; an upstream temperature sensor on the substrate top side, wherein the upstream temperature sensor is positioned upstream of the heating element and is configured to sense a measure related to a first temperature of a fluid passing adjacent the substrate bottom side before the fluid reaches the heating element; a downstream temperature sensor on the substrate top side wherein the downstream temperature sensor is positioned downstream of the heating element and is configured to sense a measure related to a second temperature of the fluid after the fluid passes the heating element; a channel in the substrate forming at least part of the substrate bottom side under the upstream temperature sensor, the downstream temperature sensor, and the heater, such that a fluid can flow through the channel in a flow direction, a first end of the channel defining a channel inlet, wherein the fluid enters the channel at the inlet; the substrate having a membrane forming a top side of the channel and supporting the upstream temperature sensor, the downstream temperature sensor, and the heater above the channel; a thermal dam that thermally isolates at least part of the membrane from the remainder of the substrate; and an inlet temperature sensor on the substrate top side, upstream of the upstream temperature sensor and adjacent the channel inlet, characterised in that the thermally insulated part of the membrane supports the inlet, upstream and downstream temperature sensors and the heater and is insulated from the remainder of the substrate and that the system further comprises also a substrate temperature sensor on the substrate top and is positioned away from the channel.