The anesthesia machine has a couple of safety systems that alert the user and stop the flow of other gases when the oxygen supply runs out (for example, when an oxygen tank becomes depleted). An auditory alarm sounds and a visual message appears to alert the user when the oxygen supply pressure falls below a predetermined threshold pressure of _ 30 psig (207 kPa).
A failsafe valve in the gas line supplying each flow controller-meter assembly, except oxygen, stops the flow of other gases. The failsafe valve is either an on–off valve or a pressure-reducing valve that is controlled by the pressure within the oxygen line. When the oxygen supply pressure falls below the threshold level, the failsafe valves close to stop the flow, or proportionally reduce the supply pressure, of all the other gases.
This prevents administration of hypoxic gases (e.g., nitrous oxide, helium, nitrogen, carbon dioxide) without oxygen, which could rapidly cause injury to the patient, but it also prevents administration of air without oxygen. The failsafe mechanisms do not prevent delivery of hypoxic gas mixtures in the presence of adequate oxygen supply pressure; the gas proportioning system, described below, prevents this.