An elongated sensor for detecting the presence of a particular fluid includes first and second conductors, both extending from the proximal end to the distal end of the sensor. The second conductor includes a swellable conductor, at least a portion of the swellable conductor is swollen when in contact with the fluid and its conductance at least 10 times less when the portion is in contact with the fluid than when absent contact with the fluid. The second conductor further includes a subsidiary conductor in direct contact with the swellable conductor. In one embodiment the first conductor is isolated from the second conductor in the inner part of the sensor, and the two conductors are electrically connected at the distal end of the sensor. In another embodiment, the first contactor is in direct contact with the subsidiary conductor within the inner part of the sensor.