In biological fluid analyzers of the type described in U.S. Pat. Nos. 4,287,155, issued Sept. 1, 1981, and 4,340,390, issued July 20, 1982, perfusion has been an occasional but persistent problem. "Perfusion" refers to the movement of dispensed liquid up the exterior surface of the dispensing container, rather than down onto a test element designed to receive the liquid. Perfusion is a problem because liquid contacting such exterior surface alters the volume of liquid that is subsequently dispensed. Perfusion often results in no fluid being dispensed to the slide surface. Because of this problem, studies have been made to locate the cause and to provide solutions. Unfortunately, it appears that there may be a number of possible causes, so that a solution to one of the causes is not necessarily effective in preventing perfusion caused by a different cause. For example, one problem involved process steps that caused a scraping of the exterior surface of the dispensing tip that tended to encourage perfusion. This was a problem even when using tips as described in U.S. Pat. No. 4,347,875, issued on Sept. 7, 1982, that were designed to reduce perfusion by pulling the exterior sheath of liquid away from the tip. Alteration of the analyzer to prevent such scraping prevented perfusion arising from this particular cause, but not necessarily from other causes. It is speculated, however, that all of the causes of perfusion relate to maintaining a continuous film of liquid over a large portion of said exterior surface adjacent the tip aperture.
Thus, prior to this invention there has been a need for a solution that would have more widespread application to the problem of perfusion, whatever its cause.