Fig. 3. Control trace demonstrating successful identification of a thermal conversion factor for switching between N₂ and He using a single mass-flow controller. The stream flowing through the CO₂ analyzer was composed of 5 ml min^–1 of a span gas (505 p.p.m. CO₂ in N₂), 21 ml min^–1 O₂ and 74 ml min^–1 of either He or N₂ carrier. Using the identified conversion factor (1.37), we were able to switch between carrier gases without affecting the reading from the CO₂ analyzer (25 p.p.m. CO₂, which is 5% of 505 p.p.m.).