SRX-CTR31 % Oxygen Sensor replaces Teledyne Class R-33S, R-33D, Analytical Industries PSR-11-37-D7, Analox 9212-0, NuvAir UBS O2 Stick and IT-G D-03. Proprietary electrolyte formulation ensures full utilization of Pb anode, thus providing longer life without signal drift and minimizing periodic calibration requirement. AST brand sensor are designed, developed and manufactured in the USA.