The Delta F Series 500 breathing air monitor eliminates the problems of inaccurate oxygen readings due to drift or low false readings. These types of problems can cause personnel to ignore or turn off the monitor. With Delta F's non-depleting sensor technolgoy and unique Sentry Circuit, every Series 500 monitor provides years of consistent, reliable and accurate operation. The Series 500 offers: -Reliabiltiy: The non-depleting sensor does not decay over time. There are NO FALSE LOW READINGS. -Stabiltiy: The non-depleting sensor requires little or no recalibration over time. -Environmental Sensitivity: The Sentry Circuit technology automtically adjusts to minor changes in relative humidity and barometric pressure without affecting the true oxygen deficiency repsonse. -Falisafe Operation: The battery backup picks up when AC power stops so that you will always be safe!