required for each cycle of the second or higher pressure pumping unit 46 as stated hereinabove the heating of a chamber may be provided by other devices known to those skilled in the art fig3 illustrates another embodiment of the gas transfer mechanism 18 including a micromechanical pump 88 having an alternative implementation of valves whereas the valves 62 74 and 78 described with respect to fig2 are active requiring input from the control unit 16 and power from the power source 12 the valves of fig3 are passive requiring neither power or command inputs each valve 90 92 94 includes a sealing surface 96 biased against a valve seat 98 to block the aperture 100 defined by the valve seat 98 in the exemplary embodiment of fig3 each of the valves is shown in its biased or closed state each valve is piston shaped having a helical spring 102 disposed around a shaft 104 the spring 102 being compressible between a rear portion of the sealing surface 98 and a spring support such as another valve seat other biasing means known in the art are also suitable an air filter 106 is provided between the opening 56 in the plenum wall 54 and the aperture 100 leading to a first pumping unit 107 provided with a resistive element 108 adapted for electrical connection with a power source 12 via wires 110 a second pumping unit 112 is provided as required and.