From a Fenyves & Gut sales brochure kindly provided by Emanuele Isnardi.
“One possible extension of the S basic unit is designed to determine the gas exchange or G or perform ergospirography, respectively, this representing a SG assembly. Unlike rival products this unit operates without valves, i.e. under optimum physiological conditions. For it should be noted that there are still systems on the market which require the subject to inhaled through a mask fitted with inspiration valves and exhale through a hose fitted to the mask. Of course such systems cannot work satisfactorily since they involve inspiration valves and because dead-space air is re-inhaled from the expiration hose. Our system has overcome these drawbacks.
Principle of measurement: the expired gases are continuously drawn off while changes in concentration levels occur. However, the average values necessary for further processing (e.g. O2 uptake) are drawn off in proportion to the expiratory flow rate by a special pump of our own design which can be modulated very rapidly. The expirate is then stored in a small collecting vessel. The resulting weighted samples are then analyzed in on-line analyzers for their O2 and CO2 concentrations (ΔFO2 and ΔFCO2 respectively).
Essentially the G apparatus consists of a slide-in unit for extraction proportionate to the flow of respiratory gases during expiratory phases. A device for averaging and converting the pulsating gas flow to a steady state required by the analyzers is included. The necessary equipment comprises O2 and CO2 and, optional, N2 analyzers (see under A), as well as a multi-channel compensated direct recorder (see under R). We recommend our slide-unit for determining the MV directly (unnecessarily if the assembly includes a computer) because it no only reduces working time, but also compensated for the time-lag occuring between the ΔFO2 and respectively, ΔFCO2 quantities on the one hand, and the minute volume on the other.”