From: The respiratory exchange of animals and man, by August Krogh, 1916, page 42.

“The apparatus of Benedict (fig. 16) [1909, 1912] is arranged to measure both carbon dioxide and oxygen, and the recording spirometer has an attachment (a “work adder”) which automatically adds the excursions together and so records the rate of ventilation. The instrument has no valves, but a rapid circulation of air is maintained by the blower. This is necessitated by the great resistance of the water-vapour absorbers.  If this resistance were avoided the apparatus could be simplified considerably.”