From “Untersuchungen uber den Stoffverbrauch des normalen menschen” by Max von Pettenkofer and Carl Voit, published 1866, labeled page 246.

The illustration shows a mouthpiece attached to a pair of Muller valves. Exhaled air was directed through a chamber containing caustic soda (potassium hydroxide) which absorbed the exhaled carbon dioxide and from there to a precision gas meter.  The CO2 absorbant was weighed after each experiment to determine how much carbon dioxide was absorbed.  The gas meter allowed the CO2 output per minute to be determined.