Net Safety Millennium II ST3 XChem Electrochemical Toxic Gas Sensors provide high specificity, fast response, and long life. These toxic gas sensors can detect hydrogen sulfide gas, carbon monoxide and oxygen levels. All Net Safety Millennium ST3 toxic gas sensors are compatible with the Net Safety Millennium II Basic (M2B) Universal Gas Transmitter, Net Safety Millennium II (M21) Single Channel Universal Gas Transmitter or Net Safety Millennium II (M22) Dual Channel Universal Gas Transmitter. The design employs a “SimpleClick” gas sensor element design and low power consumption.