venturi: one construct, many systems

In fluid mechanics a venturi is a smoothly converging-then-diverging constriction in a pipe or duct: as the stream accelerates through the narrow throat its static pressure falls in step with Bernoulli's principle, and the pressure difference between the wide inlet and the throat becomes a direct measure of the flow rate. Herschel's 1888 The Venturi water meter turned the effect into a practical flow meter that is still used for water, gas, and ventilation air. The same low-pressure throat is also used to draw a second fluid into the stream, as in eductors and aspirators. The referent is the pressure drop produced by a flow constriction, quantified by the throat-to-inlet pressure difference.[1]

Unit

The static-pressure drop across a flow constriction and the flow rate inferred from it

Recovery means

Not applicable

Measured by

Measured from the differential pressure between the inlet and the throat, scaled by the venturi discharge coefficient

The Venturi mask applies the identical construct to oxygen therapy: oxygen flows through a narrow jet, the low pressure at the jet entrains a fixed proportion of room air, and the mask therefore delivers a precise, predictable oxygen concentration no matter how hard or softly the patient breathes. Campbell's 1960 A method of controlled oxygen administration which reduces the risk of carbon-dioxide retention introduced the device to give controlled low-dose oxygen to patients who retain carbon dioxide. It is the same throat-entrainment physics as a venturi flow meter, realised as a clinical air-entrainment device and specified by the entrainment ratio and the resulting fraction of inspired oxygen.[2]

Unit

The air-to-oxygen entrainment ratio and the resulting fraction of inspired oxygen

Recovery means

Not applicable

Measured by

Fixed by the jet and port geometry that set the entrainment ratio, checked against the delivered oxygen concentration