Diffusion:

It is essential to have a higher pressure within the barrier tubes than outside in sequence to have a flow by the walls of the tubes.  This within pressure is known as the fore pressure or high side pressure (H.S.P.), and is measured at an arbitrary point within the tubes near the center of the second pass.  The pressure outside the tubes is the back pressure or low side pressure (L.S.P.), and is measured at an arbitrary point outside the tubes near the center of the second pass.

The fore pressure is regulated through a control valve in the "B" stream, or down flow, from the converter.  In fact, the control valve regulates the pressure instantly above it.  This is known as the control pressure and is the pressure that is denoted at the cell panel. It is slightly lower than the fore pressure because of the pressure drops within the converter and piping.

The rate of diffusion by the tubing walls for any given pressure drop across the barrier is determined through the permeability of the barrier. Mathematically, this is a dimensionless quantity that is the ratio of the rate of gas flow by the barrier to the rate of gas flow by the similar area that would take place if the barrier were not there.  The term, commonly known as permeability, is used to relate the actual permeability to the design permeability. For instance, if the flow by the barrier has decreased 10percent because of plugging of the barrier holes, the permeability would be 90 percent.