Puzzles/Logic puzzles/Scales and Weights/Solution 2

Weights are: 2, 6, 18, 54

Explanation

Put 2kg on one side, your item on the other. If item is less than 2kg, it is 1 kg. if it's more:

Put 6kg on one side, your item and 2kg on the other. If item and 2kg are less than 6kg, item is 3kg. Else:

Put 6kg on one side, your item on the other. If item is less than 6kg, it's 5kg.

...

This way you can weigh up to 81kg.