Sensor, EC, Hydrogen Sulfide (H2S) 0-200 ppm for GX-3R/GX-3R Pro/GasWatch 3 The replacement LEL sensor for the GX-3R and GX-3R Pro has dual active filiments to minimize chances of silicone poisoning. If one filiment is poisoned, the second filiment will continue to allow LEL detection of combustible gases.