An anesthetic breathing apparatus and system, having a patient circle system for re-breathing exhaled gases by a patient a volume reflector, a fresh gas delivery line, and a gas sensor unit arranged to measure a gas stream upstream a fresh gas connection and downstream said reflector unit. The gas sensor unit provides a signal for detection of a reflector driving gas (RDG) crossing over the volume reflector during inspiration based on at least one property of the gas stream measured by the gas sensor unit. Appropriate action may be taken based on this measurement, for instance in disclosed methods.
