From “A Pointer Volume Spirometer system”, by L.I. Nemerovskii and A.A. Stakhov, Biomedical Engineering, Volume 20, Issue 2, page 54.

“The VS dry spirometer has three identical sections in parallel, each of capacity 2.5 liters, and correspondingly three ranges: 0-2.5, 0-5, and 0-7.5 liter, with three scales.

“The mobile covers for the sections are linked by a carriage running on guides.  The sections are switched manually with a valve mechanism connecting the hose with 1, 2 or 3 sections (then correspondingly 2 or 1 sections communicate with the atmosphere).

“The scales are switched at the same time as the valve mechanism, as they are attached to a drum kinematically coupled to the range switch.”