I think you can do it in 5 weightings, because a 0 eliminates both balls. Worst case:
scale(1,2) = 0
scale(3,4) = 0
scale(5,6) = 0
scale(7,8) = -1 # If it was zero, the counterfeit would be ball #9.
scale(6,7) = 0 # Must compare to a known-good ball. Here #8 is counterfeit.
