The needle valve is the most common means of regulating gas flow rate. As the valve is opened, the orifice around the needle becomes larger and flow increases.
Some valves incorporate a stop to prevent the valve being over-tightened.
Flowmeters Flowmeters on modern anesthetic machines consist of a tapered glass tube containing a bobbin or ball which floats on the stream of moving gas. As the gas flow rate increases, the float is carried further up the tube, so indicating the flow rate. Flowmeters are specifically constructed for each gas, since the flow rate depends on both the viscosity and density of the gas. Only the correct tube and bobbin or ball can be used to repair broken flowmeters. Since the bobbin floats in the gas stream, flowmeters will only function correctly if the tube is vertical.