Abstract: Methods and embodiments are described for sampling and measuring oxygen concentrations of flowing samples drawn from the ullage of a chemical or fuel tank and from a source of inerting gas used for purging the tank. The sampled flows are drawn with a venturi pump or a pneumatically driven turbine pump and returned to the ullage of the tank. An optical absorption spectrophotometer is used having a diode laser sensor, and both sensor and the pump can be safely located near to the tank. Control systems and apparatus are disclosed for controlling the oxygen level within the ullage of the tank.