SRX-CT33 % Oxygen Sensor is a drop-in replacement to Teledyne Class C-3, AII Model PSR-11-33 and AMI Model P4 % oxygen sensor. Though T90 of this sensor is 30 seconds to a step change but offers 24 months life in air. Recommended for applications where oxygen levels is above 2-3% and is often exposed to ambient air. Its proprietary electrolyte formulation ensures full utilization of Pb anode thus providing longer life without signal drift and minimizing periodic calibration requirement. Sensor is designed, developed and manufactured in the USA.