unit in an implementation the arbitrary count unit may be the servo motor index of the servo motor employed to control the up down movement of plunger 131 the horizontal axis shows the chamber gap in millimeter mm plot line 210 shows a linear relationship between confinement ring position and chamber gap for a given pressure fig3 shows in accordance with an embodiment of the present invention a plurality of empirically derived conductance curves which illustrates conductance versus confinement ring position for different chamber gaps as shown in fig3 the vertical axis is shown as conductance in liter per second l s the horizontal axis is shown as the confinement ring position crp again in arbitrary count unit plot line 310 is the conductance curve for a chamber gap value of 1 88 centimeter cm plot line 320 is the conductance curve for a chamber gap value of 2 34 cm plot line 330 is the conductance curve for a chamber gap value of 2 8 cm plot line 340 is the conductance curve for a chamber gap value of 3 1 cm a few observations may be made from fig3 first the curves are substantially linear in the region of chamber operation i e above 4 liters sec second these curves are substantially parallel illustrating that the linear relationship between the conductance and the confinement ring position is substantially preserved when the gap is changed third for any given desired.