I came up with a scenario in which I think I am violating Heisenbergs uncertainty principle. Say I build a velocity selector, like the ones found in mass spectrometers, and I fire electrons from an electron gun through it. By measuring the Electric and magnetic fields, I can measure the velocities of the exiting electrons very accurately. The selector is in vacuum, so there is no friction slowdown of the electrons. I now let the electrons pass through a bubble chamber, leaving a trail. Assume the bubble chamber does not slow the electrons down. Now I know that at a certain time, an electron had a certain position (from the trail in the bubble chamber) and I know it had a certain velocity (from the velocity selector), so I know its position and its momentum. Am I not violating Heisenbergs uncertainty principle now? If not, or if there are errors in my scenario, please explain.