A system is configured to generate a pressurized flow of gas comprised of a first gas having a partial pressure that varies in a predetermined manner. This may be used, for example, to simulate a previous and/or theoretical respiratory gas flow that was produced (or could have been produced) by a subject. The system is configured to deliver the pressurized flow of gas to a testing system configured to measure the partial pressure the first gas in flows of gas. This may provide an opportunity to determine the response of individual testing systems to various clinical circumstances.