In this simulation, the upper panels indicate respectively the concentrations of particles in the left and right compartments and the compartmental volumes that would result if water could diffuse freely down its activity gradient. The volumes don't change, however, because an equal and opposite pressure has been applied to the left compartment to prevent osmotic diffusion into or out of it: respectively, a positive or negative osmotic pressure.
The strip-chart recorder plots the osmotic pressure of the simulation (along the horizontal dimension) as a function of time (along the vertical dimension). Note that when the compartment contains equal solute concentrations, the pressure oscillates around the middle of the chart; this is the zero pressure position. When osmosis occurs into the right-hand compartment, a "negative" pressure (to the left of the zero line) is generated to prevent diffusion from occurring and the volume from changing. Alternatively, osmosis from right to left requires a "positive" pressure on the left-hand compartment to prevent it from expanding. Thus, all pressures are measured relative to the left-hand compartment in this simulation.
For several different simulations plot the pressure values as a function of the solute concentration in each chamber and of the difference in solute concentration, and describe the relationship. Which provides the best description? To explore this problem more precisely, proceed to the next simulation.