Figure 9-12 A schematic of the North American Dräger Oxygen Ratio Monitor Controller (ORMC) is shown. The ORMC is composed of an oxygen chamber, a nitrous oxide chamber, and a nitrous oxide slave control valve, all of which are interconnected by a mobile horizontal shaft. The pneumatic input into the device is from the oxygen and the nitrous oxide flow meters. These flow meters have resistors located downstream from the flow control valves that create backpressures directed to the oxygen and nitrous oxide chambers. The value of the oxygen flow tube's resistor is three to four times that of the nitrous oxide flow tube's resistor, and the relative value of these resistors determines the value of the controlled fresh gas concentration of oxygen. The backpressure in the oxygen and nitrous oxide chambers pushes against rubber diaphragms attached to the mobile horizontal shaft. Movement of the shaft regulates the nitrous oxide slave control valve, which feeds the nitrous oxide flow control valve. (Adapted from Schreiber P: Safety Guidelines for Anesthesia Systems. Telford, PA, North American Dräger, 1984.)