An apparatus for estimating a parameter of interest includes a conduit and a reactive media in the conduit. The reactive media interacts with a selected fluid component to control a flow parameter of the conduit. The apparatus also includes at least one sensor responsive to the flow parameter. The apparatus may be used for estimating a water content of a fluid flowing from a subterranean formation. The apparatus may include a flow path configured to convey fluid from the formation. The at least one sensor may be responsive to a pressure change in the flow path caused by interaction of the reactive media with water.