44 for receiving the tapered mixing plate the lower chamber 33 is reduced to a de minimus volume the upper chamber 32 is at maximum size through the use of the valve 11 and the regulator 12 the upper chamber 32 is filled with a preliminary charge the pressure is limited by the strength of the structure and a typical maximum is in the range of 1800 psi pressure is charged to some specified level perhaps 800 psi this representative pressure level is sustained by operation of the regulator the valve 13 is opened so that the charging gas flows through the line 14 and the valve 15 this supplies the same charging pressure through the tee 16 no venting occurs because the valve 17 is closed this piston 47 is held up assume that sample is collected and forced into the chamber 33 it flows through the valve 18 it fills the lower chamber 33 it initially forces the interface piston up and the tapered mixing plug 35 is lifted also the sample collects above the sight gauge piston 47 which is in the up or raised position of fig3 if the initial charge placed in the topmost chamber is 800 psi then the sample that is introduced must be received at a pressure of 800 psi to force the piston upwardly in other words the sample pressure must exceed 800 psi in.