Monday, August 28, 2017

Persistent LiveUSB Debian 9 Stretch

Live CD and USB operating systems are a great tool for working on computers with a slow or broken OS, they let you bring all your tools with you and don't need to make any changes to the host PC in order to operate. There are a number of Linux LiveCD distributions that are built for general use or specific tasks like file recover or drive partitioning but most of them lack "persistence." When you boot up a live CD/USB it is always the first boot, changes you made the last time you ran it do not carry over to the next.

Although there being a few live distributions that do have persistence, Debian, which I am most familiar with, is not one of them. After some moderate Googling I found a tutorial here: Creating a Debian Live Iso-Hybrid USB key with persistence which shows how to take the non-persistent Debian Live ISO and give it the ability to save any changes you make.

The process involves making 2 partitions on a USB stick, one to hold the operating system and one to hold any changes you make when you run the OS. The Debian Live ISO is extracted into the first partition and a few changes are made. Note in step 4 there is no longer a /syslinux/live.cfg and the word "persistence" was added to live boot option in /syslinux/menu.cfg instead. After booting the USB stick for the first time I also found it necessary to change the repository URLs in /etc/apt/sources.list to my local mirror in order to install new software.

With those two deviations from the original tutorial I was able to get a full system that can have software added and files stored and will run on nearly any PC.

Thursday, August 24, 2017

Fast growing Ice Crystals

Dry ice is a great material to have in a shop that can be a workable stand in for liquid nitrogen in most cases. When left unattended for a couple minutes water in the air will freeze on the dry ice's surface and form a furry coating. After the dry ice has completely sublimated the water ice begins to warm up to room temperature and melts a few seconds later. Normally the water ice crystals are so small that they appear to be a uniform fur but putting a small grain of dry ice under the microscope shows how complex the growing crystals really are.

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.