The anesthesia flowmeter is described as a constant pressure, variable orifice flow meter. Newer anesthesia machines may have electronic flowmeters; however, auxiliary O₂ flowmeters of the conventional type may be present.

The glass flowmeter or Thorpe tube is tapered, smaller at the bottom and wider at the top (Fig 80.1). Gas flows under the float raising it until the bobbin or ball stops as its weight is supported by the pressure difference above and below.

Flow tubes are specific to the physical characteristics of each gas.

Gas flow at low rates is laminar and viscosity of the gas is important. Laminar flow is predicted by the Hagen-Poiseuille formula:

where Q is flow, ∆P is the pressure gradient (unchanged), R is the radius (variable as tube widens), ῃ is the viscosity (characteristic of each individual gas), and l is the length of the tube.

At higher flow rates, flow becomes turbulent as the Reynolds’ number exceeds 2000. With turbulent flow, the density becomes more important than viscosity:

υ, fluid linear velocity; r, radius; ρ, density; and ῃ, viscosity [1]