If this is not the case it is important to make some substantial adjustments in the Stella model. In the example each pipe has been split into five segments, each represented by a reservoir. Valves are located between these reservoirs in the direction of the fluid flow. More accurate models are created by using more segments, but more segments also makes for more congested Stella models. The valves connecting sections of the same pipe represent the movement of solute down the pipes as time progresses.
Valves also point from one column to the other.
These valves represent the diffusion of solute across the permeable membrane from one fluid to another. The formulas for these diffusion valves are identical to those found in the simpler model discussed earlier, namely a parameter k times the difference in concentrations, modified by rho . The flows between reservoirs of the same pipe are a little more complicated. It is important to remember that this Stella model, with its finitely chopped pipes and non-infinitesimal time steps is only an approximation to the real system, the system modeled with partial differential equations. Initial conditions must be specified for all reservoirs.