An elongated sensor (10, 110) having a proximal end and a distal end, and an inner portion therebetween, for detecting the presence of a fluid, the sensor comprising: a first conductor (20, 120) extending from the proximal end to the distal end of the sensor (10, 110); and, a second conductor (30, 140&150) extending from the proximal end to the distal end of the sensor (10, 110), isolated from the first conductor (20, 120) in the inner portion of the sensor (10, 110), comprising: a swellable conductor (40, 140), wherein at least a portion of the swellable conductor (40, 140) is swollen when the portion is in contact with the fluid, and wherein a conductance of the portion is at least 10 times less when the portion is in contact with the fluid than when absent contact with the fluid, and a subsidiary conductor (50, 150) in direct contact with the swellable conductor (40, 140) in the inner portion of the sensor (10, 110) and in electrical contact with the first conductor (20, 120) at the distal end of the sensor (10, 110), the swellable conductor (40, 140) is at least 50% more swellable than the subsidiary conductor (50, 150), and a ratio of the conductance of the subsidiary conductor (50, 150) to the conductance of the swellable conductor (40, 140) in absence of contact with the fluid is in the range of 0.0000001 to 1000; wherein the first conductor (20, 120) is at least partially covered with an isolative jacket for electrically isolating the first conductor (20, 120) from the second conductor (30, 140&150) in the inner portion of the sensor (10, 110). a first conductor (20, 120) extending from the proximal end to the distal end of the sensor (10, 110); and, a second conductor (30, 140&150) extending from the proximal end to the distal end of the sensor (10, 110), isolated from the first conductor (20, 120) in the inner portion of the sensor (10, 110), comprising: a swellable conductor (40, 140), wherein at least a portion of the swellable conductor (40, 140) is swollen when the portion is in contact with the fluid, and wherein a conductance of the portion is at least 10 times less when the portion is in contact with the fluid than when absent contact with the fluid, and a subsidiary conductor (50, 150) in direct contact with the swellable conductor (40, 140) in the inner portion of the sensor (10, 110) and in electrical contact with the first conductor (20, 120) at the distal end of the sensor (10, 110), the swellable conductor (40, 140) is at least 50% more swellable than the subsidiary conductor (50, 150), and a ratio of the conductance of the subsidiary conductor (50, 150) to the conductance of the swellable conductor (40, 140) in absence of contact with the fluid is in the range of 0.0000001 to 1000; wherein the first conductor (20, 120) is at least partially covered with an isolative jacket for electrically isolating the first conductor (20, 120) from the second conductor (30, 140&150) in the inner portion of the sensor (10, 110). a swellable conductor (40, 140), wherein at least a portion of the swellable conductor (40, 140) is swollen when the portion is in contact with the fluid, and wherein a conductance of the portion is at least 10 times less when the portion is in contact with the fluid than when absent contact with the fluid, and a subsidiary conductor (50, 150) in direct contact with the swellable conductor (40, 140) in the inner portion of the sensor (10, 110) and in electrical contact with the first conductor (20, 120) at the distal end of the sensor (10, 110), the swellable conductor (40, 140) is at least 50% more swellable than the subsidiary conductor (50, 150), and a ratio of the conductance of the subsidiary conductor (50, 150) to the conductance of the swellable conductor (40, 140) in absence of contact with the fluid is in the range of 0.0000001 to 1000; wherein the first conductor (20, 120) is at least partially covered with an isolative jacket for electrically isolating the first conductor (20, 120) from the second conductor (30, 140&150) in the inner portion of the sensor (10, 110).