Flow Meters and Gas Flow Regulation
Flow Meters and Gas Flow Regulation
Flowmeters
Flowmeters on the anesthesia machine precisely control and measure gas flow to the common gas inlet (Figure 2.5). Flowmeters on modern anesthetic machines consist of a tapered glass tube containing a bobbin or ball which floats on the stream of moving gas. 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. The valve cartridge itself is usually removable so it can be replaced if it is damaged.
Gases mix in the manifold and flow to an outlet port on the anesthesia machine, where they are directed into either a vaporizer or an anesthetic breathing system.
Gas flow into the flow meter tube raises a bobbin or ball shaped float
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.
- Flowmeters will not function correctly if the tube is cracked.
The Oxygen Flash Valve
The oxygen flush valve allows direct communication between the oxygen high-pressure circuit and the low-pressure circuit. The spring-loaded oxygen flush valve stays closed until the operator opens it by depressing the oxygen flush button. For emergency purposes, provision is made for delivery of a large volume of oxygen (35 to 75 L/ min) to the outlet port through an oxygen flush valve that bypasses the flowmeters and manifold. The transmission of high airway pressure to the patient's lungs has the possibility of barotraumas; therefore avoid pushing the oxygen flush button while the breathing system is attached to a patient.
The Common Gas Outlet
Is where the oxygen/medical gas/anesthetic agent mixture exits the anesthesia machine to the breathing system, which can be either a rebreathing or non rebreathing system. These systems will be explained in session III.