Patent ID: 7981193

Claim:
A method of controlling an oxygen concentration of a product gas produced by an apparatus for separating product gas and non product gas from an air supply and for delivering the product gas to a user for breathing, the oxygen concentration of the product gas produced by the apparatus being variable depending a) on a control signal and b) on the efficiency of the apparatus in producing product gas which depends upon an environmental parameter which changes and which effects the efficiency of the apparatus, the method comprising the steps of: sensing the oxygen concentration in the product gas produced by the apparatus, determining an instantaneous demand for oxygen concentration in the product gas for breathing by the user, providing a process variable signal indicative of the sensed concentration of the oxygen in the product gas, and providing a demand input signal indicative of the instantaneous demand, sensing the environmental parameter and providing an environmental parameter input signal, comparing the process variable signal indicative of the sensed concentration of the oxygen in the product gas and the demand input signal indicative of the instantaneous demand to generate an error signal indicative of a change in oxygen concentration in the product gas required to match the instantaneous demand, and conditioning the error signal to derive the control signal and using the derived control signal to control the apparatus to satisfy the determined instantaneous demand, wherein said conditioning step includes compensating the error signal which is used to derive the control signal depending upon changes in the environmental parameter input signal which changes are correlated to the efficiency of the apparatus.