Where does effusion collide?

In physical chemistry, effusion is the escape of a gas through an opening so small that molecules pass one at a time without colliding in the hole, so a lighter gas effuses faster than a heavier one at the same temperature. Graham's 1846 On the motion of gases established the law that the rate of effusion varies inversely with the square root of a gas's density, later read as its molar mass. The referent is a molecular transport rate through a pinhole into vacuum, quantified in the laboratory, and it is unrelated to fluid pooling in a body cavity or lava pouring from a vent, sharing only the meaning of something flowing out.[1]

Unit

The rate at which gas passes through a small orifice, per square root of molar mass

Recovery means

Not applicable

Measured by

Measured as the time for a known volume of gas to escape through a pinhole, compared across gases by Graham's law

In clinical medicine, an effusion is an abnormal collection of fluid in a body cavity or space, such as the pleural cavity around the lungs, the pericardium around the heart, or a joint, produced when the balance of pressures and permeability that normally keeps such spaces nearly dry is disturbed. Light and colleagues' 1972 Pleural effusions: The diagnostic separation of transudates and exudates gave the still-used criteria that sort effusions into those driven by pressure imbalance and those driven by inflammation or leaky membranes. The referent is a pathological fluid accumulation inside a patient, sampled and analysed to find its cause, and it has nothing to do with gas leaving through a pinhole or magma reaching the surface.[2]

Unit

The volume and composition of fluid abnormally collected in a body space

Recovery means

Drainage and treatment of the underlying cause clear the fluid; the criteria classify rather than resolve it

Measured by

Assessed by imaging and by sampling the fluid, applying Light's criteria to its protein and enzyme content

In volcanology, effusion describes the relatively quiet outpouring of lava from a vent, as opposed to explosive fragmentation, and the effusion rate is the volume of lava erupted per unit time, a master control on how far and how fast a flow travels. Wadge's 1981 The variation of magma discharge during basaltic eruptions described the characteristic rise and decline of effusion rate through an eruption and linked it to the emptying of a pressurised magma reservoir. The referent is the surface discharge of molten rock during an effusive eruption, gauged from flow volumes and eruption duration, and it is an entirely different thing from gas effusing through a pinhole or fluid pooling in the pleura.[3]

Unit

The volume of lava erupted per unit time during an effusive eruption

Recovery means

Not applicable

Measured by

Estimated from erupted lava volume over time, and now from thermal satellite measurements of active flows