Abstract: A low-oxygen alarm regulator assembly including a pair of diaphragms (40, 44) to which oxygen and nitrous oxide pressures are applied, and a displaceable intermediate member (46), the diaphragms having areas inversely proportional to desired flow rate percentages, to maintain the intermediate member in a neutral position if the desired flow ratio is maintained. The intermediate member actuates an alarm if the desired percentage flow of oxygen is not attained. In regulator operation, the assembly employs a valve (10) which is actuated with the alarm to apply an independent flow of oxygen to the oxygen diaphragm and thereby bypass an oxygen flow control valve and maintain a guaranteed oxygen flow ratio. The regulator action can be overriden by operation of a pneumatic switch (98a), which includes various safeguards to preclude inadvertent operation. A variable nitrous oxide flow restrictor 20 compensates for non-linearity in the pressure-flow characteristic for nitrous oxide at very low flows.