Devices that operate on the principle of measuring the volume and flow of gases through the displacement of a liquid volume (Drum type meters) or air. (Bellows type meter). Various models are available suitable for different flow ranges and quantities of passing gases, as well as with different resistance to increased pressures depending on the material of construction.

The applications mainly refer to biogas measurements, in organic chemistry laboratories, and in the production-industrial sector.

Ritter, from Germany, holds a global patent for this specific equipment.