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.