Although our testers don't NEED testing needles, you can use them if you want.
The method is as follows:
Remove any surface plating by rubbing the test-item vigorously on a touchstone then wash the residue of the stone and dry the stone (alternatively, file the item firmly with a steel needle file). Now draw the test-item across the stone heavily so that it leaves a streak behind. Next, draw lines (to make streaks) with each of the test needles alongside the unknown streak. Then place the acid across all the streaks at once. See which known streak dissolves (fades away) at the same rate as the unknown streak - that's the carat.
If you have any problems with the above, call us and we'll tell you that our testers don't need a touchtone and to follow the instructions that came with the testing set instead.