and 21 the average vane tip force represented by the line 571 in fig2 is about the same as the average vane tip force represented by the line 671 in fig2 thus for single chamber pumps like those shown at 500 600 in fig1 19 and like those shown at 300 400 in fig1 13 the modifications can be made without sacrificing vane life further the reader will also note that vane tip bouncing or skipping indicated at 173 in fig2 has been eliminated by the pump 600 as shown in fig2 fig2 and 24 illustrates a purely elliptical cavity 728 with a rotor 326 disposed therein the pump 700 illustrated in fig2 includes two pump chambers 758 759 disposed on either side of the rotor 326 the rotor 326 engages the inner wall 751 of the cavity 728 at two points 762 763 or where the minor axis 764 intersects the inner wall 751 not only are the pump chambers 758 759 of the same size dividing each pump chamber 758 759 into two using major axis 765 both u201c halves u201d of the pump chambers 758 759 are of equal volume or each quadrant of each pump chamber 758 759 is of equal volume in contrast turning to fig2.