Objective of the study: Comparing the measurements carried out in the halogenated agent delivery device, with a mathematical model where the ideal values of halogenated fraction exhaled of agent for each event were found. Design of the study: Design of technology. Place of development: Rooms of surgery of a University Hospital, Bogota, Colombia. Interventions and measures: After designing and to join a semiopen anesthetic circuit type facial Mask with two one-way valves (an inspitratory and another exhalatory), proceeded to take the measures of inspired concentration of sevoflurane, and inspired fraction of oxygen, with exhalatory volumes from 70 to 170 mL with intervals of 10 mL, and with the dial of sevoflurane to 2, 2,2, 2,4, 2,6 vol%, the results consigned were compared with the values mathematically calculated for each event. Results: The measurements carried out in it Facial Mask are concordant with the calculations carried out in the mathematical model. The concentration exhaled of sevoflorane is slightly superior in each event with regard to It found in the mathematical model, It is not possible to assure the not reinhalación with the measurements carried out.