From: The respiratory exchange of animals and man by August Krogh, 1916, page 40.

“Krogh’s apparatus (fig. 15) [1913] which is a modification of an instrument constructed by Haldane and Douglas [1912], is furnished with valves and the air is circulated by the respiratory movements of the subject.  Carbon dioxide is absorbed in a vessel containing a charge of soda lime sufficient to absorb 1000 liters of carbon dioxide, a quantity produced by a man at rest in about 70 hours.  The recording spirometer gives a quantitative record of the respiratory movements and governs the admission of oxygen by closing an electric circuit at (5).  The oxygen from the cylinder is measured by the meter which records electrically by closing a circuit each time a certain quantity has been admitted.  Whenever an experiment has to be extended over a long period, or if the absorption of oxygen is very rapid as during heavy muscular work, the oxygen admitted must be nearly pure to prevent the oxygen percentage in the small apparatus from falling.

“The apparatus in its present form does not allow the direct determination of carbon dioxide.  When such determinations are desired samples of expired and inspired air are drawn from the vessels (2) and (10).  The respiratory quotient is determined by analysing these samples for carbon dioxide and oxygen.  The total respiratory exchange can also be measured over short periods by by multiplying the analytical results by the ventilation as measured from the graphic record.”