A tensile load is applied to a sensor element that will carry the tensile load in the absence of a substance and will fail to carry the tensile load in the presence of the substance. The sensor element is disposed in a flow of fluid, and a flow control member is actuated to shut off the flow of fluid when the sensor element fails to carry the tensile load. In a preferred implementation, a hydrocarbon sensing valve includes a polystyrene sensor element suspending the flow control member above a valve seat. The sensor element is disposed in a cavity in the top of the flow control member, and a channel member directs fluid from the inlet port to the cavity. The sensor element is pinned to the channel member, and the flow control member is pinned to the sensor element.