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

“The measurement unit in a water spirometer is divided by a baffle into two sections equal in volume, so it has two ranges (0-4 and 0-8 liter), and correspondingly two scales with different calibration.  When the 0-8 liter range is set, the patient respires into both sections through a collector and hose with mouthpiece.  On the 0-4 liter range, respires into one of the sections through the hose, while the other communicates with the atmosphere.”