From “Methods in pulmonary physiology”, by Bertels H, Bucherl E, Hertz CW, Rodewald G, Schwab M. Translated by Workman JM. Hafner Publishing Co., 1963, page 24.
“In Dethloff’s apparatus, the difference in volume of expiratory gases before and after absorption of CO2 is used to measure CO2 output. To this end two gas meters are placed in the circuit, one proximal and one distal to the CO2 absorber. The difference in reading between the two gas meters provides a continuous measure of CO2 output.”