Abstract: An anesthesia system wherein the respiratory system is fitted with interchangeable patient circuit modules that allows a generic fitting in the machine to have affixed, hereto, a module that has ports suitable for connection to various patient breathing circuits. As such, the anesthesia machine can have the patient circuit modules easily changed and thus be adaptable for use with patient breathing circuits such as the circle system, the open/nonrebreathing circuit and the Bain or Mapleson D circuits. A detector system is provided that identifies that particular patient module installed in the anesthesia machine to alert the main CPU and to thus provide the correct flow sensing schemes and flows to the patient breathing circuit then being used.