# Mean AgeΒΆ

For open systems, users can automatically calculate the transient mean
age distribution using the *Calculate Mean Age* command. After selecting
this option from the Menu, users will be prompted to select an age
source from among the inlets, a calculation start time, and an age
diffusion coefficient. The age diffusion coefficient is related to the
molecular self-diffusion coefficient. The spatial variation in the mean
age is printed to volume and slice output files. The mean age at each
outlet is printed to output.

The transient mean age represents two physical values. First, it represents the average age of molecules at a given point inside the tank, relative to the time these molecules entered the system via the selected inlet. Second, the represents the mean residence time of molecules at a given point inside the tank. For systems with a single outlet, the mean age at the outlet is equal to the volume averaged mean-age. The volume averaged mean age is also equal to the mean residence time of the system.

- Start Time
Start Time, [seconds]

Time at which to begin the mean age calculation

- Diffusion Coefficient
Age diffusion coefficient, [m^2/s]

Diffusion coefficient of the age variable. Typically equal to the molecular diffusion coefficient.

- Selected Inlet
Selected inlet for mean age calculation