of the air lock mechanism the o ring seals are designed so that there always exist at least two o rings in contact with the housing between each of the ports a and b and the port c as shown in fig4 a the product chamber a 39 is aligned with inlet port a while product chamber b is aligned with the outlet port c fig4 b illustrates a first incremental motion of the spool so that the two o rings to the right of chamber a 39 seal the pressure differential between ports a and c while the two o rings to the right of chamber b seal the pressure differential between ports b and c fig4 c and 4d illustrate further incremental movements of the spool toward the inlet port b and serve to illustrate that the product within the chamber a 39 is moved along the housing channel within the spool at the same pressure conditions existing at inlet port a fig4 e illustrates the first position at which chamber a 39 is totally sealed from the pressures at ports a and c in this position the innermost o rings on either side of chambers a 39 and b 39 seal the pressure differential between ports a and c and ports b and c fig4 f illustrates the first location in which the chamber a 39 is equalized with the pressure conditions existing at outlet port c in this position the two 0 rings to the left of chamber a 39 seal.