Microprocessor-based Series 500's nondepleting electrochemical sensor does not decay over time and is factory calibrated to NIST traceable standards. Sentry Circuit checks instrument's calibration against system or condition changes, and triggers alert if changes become significant. Monitor measures 0-25% oxygen range with ±1% FS accuracy. Other features include adjustable oxygen alarm setpoints; audible and front-panel alarms; and open diffuser sensor that eliminates need for sample pump.