Which device controls gas flow from the machine using a constant pressure with a variable orifice?

Which device controls gas flow from the machine using a constant pressure with a variable orifice?

• A. Flowmeters
• B. Ventilator
• C. Oxygen analyzer
• D. Capnograph

Answer: (A)

Gas flow control using a constant driving pressure and a variable orifice is the principle behind the flowmeter in an anesthesia machine. The flowmeter (Thorpe tube) sits in the gas path where the upstream pressure from the machine is kept essentially constant, and the needle valve creates a variable opening. As the orifice size increases, the resistance to flow decreases and more gas moves through, so the float in the tube rises and the corresponding flow is read on the scale. This setup makes the flow directly dependent on the size of the opening while the pressure pushing the gas stays about the same. Other devices do different jobs: a ventilator manages breaths and lung ventilation rather than regulating fresh gas flow via a variable orifice; an oxygen analyzer and a capnograph measure gas properties (oxygen concentration and CO2) rather than control flow. Hence, the device that controls gas flow with a constant pressure and a variable orifice is the flowmeter.

Gas flow control using a constant driving pressure and a variable orifice is the principle behind the flowmeter in an anesthesia machine. The flowmeter (Thorpe tube) sits in the gas path where the upstream pressure from the machine is kept essentially constant, and the needle valve creates a variable opening. As the orifice size increases, the resistance to flow decreases and more gas moves through, so the float in the tube rises and the corresponding flow is read on the scale. This setup makes the flow directly dependent on the size of the opening while the pressure pushing the gas stays about the same.

Other devices do different jobs: a ventilator manages breaths and lung ventilation rather than regulating fresh gas flow via a variable orifice; an oxygen analyzer and a capnograph measure gas properties (oxygen concentration and CO2) rather than control flow. Hence, the device that controls gas flow with a constant pressure and a variable orifice is the flowmeter.