The Missing Grams

A mint has arranged coins into ten separate stacks. Nine stacks contain ordinary coins weighing exactly 10 grams each. Every coin in the remaining stack is counterfeit and weighs exactly 9 grams, but that stack looks identical to all the others. Each stack contains enough coins for any test you choose. You may place any selection of coins on a digital scale. The scale reports the exact total weight in grams, and all coins used in a test may be weighed together. What is the fewest number of weighings needed to determine with certainty which stack contains the counterfeit coins?