Biosystems® PhD Lite Confined Space Gas Detectors PhD Lite Gas Detectors are ideal for confined space. Powered by three AA batteries and housed in a completely sealed metal plated case, they monitor up to four atmospheric hazards simultaneously; oxygen, combustible gas, carbon monoxide and hydrogen sulfide. The innovative Duo-Tox dual channel sensor allows PhD Lite Detectors to measure both carbon monoxide and hydrogen sulfide simultaneously withou cross interference. All procedures necessary for day-to-day operation, including fully automatic calibration, are controlled through a single button.