From: The examination and classification of aviators with special reference to the effects of high altitiude.  By JF Grant, California State Journal of Medicine, 1920, page 99.

“The Henderson Rebreathing Apparatus has been perfected for such tests [low O2 at altitude].  During the test the subject breathes the air in the tank.  He sits with a clip placed on the nose and comfortably adjusted mouthpiece in the mouth, which is suitably connected by means of inch rubber tubing with light automatic valves.  He inhales the air through the respiratory valve and exhales through the expiratory valve into a cartridge containing an absorbant for carbon dioxide , namely sodium hydroxide in cake form.  The exhaled air is thus freed from carbon dioxide as it is returned to the tank.  A spirometer compensates for changes in volume and writes a record of the respiration on the revolving drum of a kymograph.  By this arrangement the subject continues to rebreathe the air in the tank, from which he gradually absorbs oxygen.  As the percentage of oxygen decreases, the subject, in effect physiologically, is slowly ascending to higher altitudes.  The volume of air rebreathed is sufficient to require between 25 and 30 minutes to lower the amount of oxygen to 8 or 7 percent, which is equivalent to altitudes of 25,000 to 28,000 feet.”

From: The examination and classification of aviators with special reference to the effects of high altitiude.  By JF Grant, California State Journal of Medicine, 1920, page 99.

“The Henderson Rebreathing Apparatus has been perfected for such tests [low O2 at altitude].  During the test the subject breathes the air in the tank.  He sits with a clip placed on the nose and comfortably adjusted mouthpiece in the mouth, which is suitably connected by means of inch rubber tubing with light automatic valves.  He inhales the air through the respiratory valve and exhales through the expiratory valve into a cartridge containing an absorbant for carbon dioxide , namely sodium hydroxide in cake form.  The exhaled air is thus freed from carbon dioxide as it is returned to the tank.  A spirometer compensates for changes in volume and writes a record of the respiration on the revolving drum of a kymograph.  By this arrangement the subject continues to rebreathe the air in the tank, from which he gradually absorbs oxygen.  As the percentage of oxygen decreases, the subject, in effect physiologically, is slowly ascending to higher altitudes.  The volume of air rebreathed is sufficient to require between 25 and 30 minutes to lower the amount of oxygen to 8 or 7 percent, which is equivalent to altitudes of 25,000 to 28,000 feet.”