Tuesday, April 4, 2017

Caching a "Bullet"

In my quest to get a ping pong ball up to speeds normally only achieved by military aircraft and light arms I ran into a metering problem. With my 1200 frame per second camera the ball only appeared in frame for 1 to 3 frames depending on how lucky I was. By having a length reference and knowing how long the shutter was open it was sometimes possible to calculate how fast the ball was moving by measuring how long its "streak" appeared to be in an image. This was only somewhat accurate and relied on the chance that the streak both started and ended in frame during one exposure so I came up with a much more accurate (and cheap) measurement apparatus.

I put two strips of aluminum foil 30cm apart and held tightly in a frame, using some alligator clips and 3.5mm stereo jacks I fed the output of my audio card into one end of each strip. On the other end I used more gator clips to feed the signal (from the audio card, and through the foil) back into the input of the audio card. When the ball is fired through the two strips they break a couple fractions of a second apart, by playing a tone through the strips and recording the result it is possible to count the number of audio samples (at 48,000samples per second) between the breaks. Now I have both an accurate measurement of the distance traveled and an accurate measurement of the time it took, letting me measure the speed of my ping pong balls with much more precision than my 1200fps camera.

Sunday, January 8, 2017

Paul Particle Trap

Normally a Paul Trap is used to catch individual ions, single nuclei or charged molecules, and then sort them according to their m/z ratio (mass to charge). However it is possible to scale the whole idea up to a table top size in order to catch finely ground powders like flour or corn starch.

The trap works by alternately pulling and pushing the particles in 2 or 3 axis. For one half of the cycle they are pulled up and down while being pushed inward from the sides, when the polarity of the trap reverses the forces reverse as well. The particles move according to the forces they feel but never get far before the reversal, trapping them in a small volume of space. When the voltage on the trap is increased the particles with the most charge relative to their mass will pick up more speed and eventually be flung from the trap, when the voltage is decreased the particles with the most mass relative to their charge will be overcome by gravity and will fall from the trap, in this way ions are sorted for used in mass spectrometers.