The mean age and the mean transit time generally have different values : : it takes some time for particles to begin leaving the system. Examples include water in a lake with  inlet and outlet on opposite sides ; and a nuclear bomb test where radioactive material is introduced high in the stratosphere and filters down to the troposphere. : the frequency functions E and I are exponential , as in the CSTR model above. Such a distribution occurs whenever all particles have a fixed probability per unit time of leaving the system. Examples include radioactive decay and first order chemical reactions (where the reaction rate is proportional to the amount of reactant). : most of the particles  pass through quickly, but some are held up. This can happen when the main source and sink are very close together or the same. For example, most water vapor rising from the ocean surface soon returns to the ocean, but water vapor that gets sufficiently far away will probably return much later in the form of rain.
The mean age and the mean transit time generally have different values , even in stationary conditions: : examples include water in a lake with the inlet and outlet on opposite sides and radioactive material introduced high in the stratosphere by a nuclear bomb test and filtering down to the troposphere. : E and I are exponential distributions. Examples include radioactive decay and first order chemical reactions (where the reaction rate is proportional to the amount of reactant). : most of the particles entering the control volume pass through quickly, but most of the particles contained in the control volume pass through slowly. Examples include water in a lake with the inlet and outlet that are close together and water vapor rising from the ocean surface , which for the most part returns quickly to the ocean, while for the rest is retained in the atmosphere and returns much later in the form of rain.
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: : it takes some time for particles to begin leaving the system. Examples
, even in stationary conditions: : examples
A
the
R
; and a nuclear bomb test where radioactive material is
and radioactive material
R
and filters
by a nuclear bomb test and filtering
D
the frequency functions
R
, as in the CSTR model above. Such a distribution occurs whenever all particles have a fixed probability per unit time of leaving the system.
distributions.
A
entering the control volume
R
some are held up. This can happen when the main source and sink are very close together or the same. For example, most
most of the particles contained in the control volume pass through slowly. Examples include water in a lake with the inlet and outlet that are close together and
R
soon returns
, which for the most part returns quickly
R
but water vapor that gets sufficiently far away will probably return
while for the rest is retained in the atmosphere and returns