A method and apparatus of determining gas transport time from an input port to first and second ultrasonic detectors in a flow conduit for an aspirated smoke detector are provided. The method includes first measuring a transit time of ambient air between the detectors. Then, a different gas is injected into the conduit, and the time of injection is stored. The transit time is measured at least intermittently between the two ultrasonic detectors until a change therein is detected. A time of detected change is subtracted from the time of injection to establish a transport time for the aspirated smoke detector.