Lighting a BBQ with liquid oxygen

http://www.youtube.com/watch?v=1bjvj5FjUPE
After watching the famous liquid oxygen BBQ lighting video on the internet, I knew it was time for me to try it myself. In this video, I am pouring 1.5 liters of liquid oxygen on a bunch of smoldering charcoal briquettes. The oxygen rapidly turns to gas and accelerates the rate of combustion at the surface of the briquettes. Some of the charcoal fractures and shoots off tiny pieces that burn with a bright light.

I made the liquid oxygen by passing gaseous oxygen through a copper tube that was submerged in liquid nitrogen. The liquid nitrogen is colder than the boiling point of oxygen, so it causes the oxygen to condense and be ejected as a liquid from the other end of the tube. Liquid nitrogen can be bought at welding supply stores, however I made a batch myself with a cryocooler.
http://benkrasnow.blogspot.com/2008/08/diy-liquid-nitrogen-generator.html

Improved automatic cat feeder

I recently set up my automatic cat feeder (http://benkrasnow.blogspot.com/2009/02/automatic-cat-feeder.html) for a long-weekend vacation. This was probably the sixth time that I have used the feeder with great results each time. I also setup a motion-activated webcam that posted frame captures to a web server so that I could see the feeder (and cat) while I was away.

One day during the trip, I checked the web server and noticed the feeder had dispensed a huge amount of food. I've never seen my cat eat his fill and walk away from a dish with food in it, but he did this time so there much have been a truly massive amount. When I returned home, I saw what had happened: The cat had knocked over the darkroom enlarger timer, and by strange chance, the timer knob was turned from 3 seconds (an interval that dispenses a single-serving of food) to 20 seconds! The feeder emptied the entire storage chamber of food into the serving dish. It's not possible the cat knew what he was doing by messing with the darkroom timer, but it's also very strange that this happened by mere chance.

I never liked the timer approach to the cat feeder because it assumes the motor turns at a constant speed, and the darkroom timer is not easy to adjust accurately in the range of 2-4 seconds. I thought about adding a shaft encoder to make the system closed-loop. I also thought about using a mechanical means to control the shaft (ie a Geneva Wheel). Finally, I decided to use a strong stepper motor in open-loop control. I also wanted to keep the same gear reduction box that was originally coupled to the food dispenser. It's torque amplification is helpful, and it has also become a structural part of the feeder, so not easy to change.


I removed the old DC motor and pulled the armature off the shaft. I cut the shaft, then built a press-fit delrin coupler to mate the stepper motor shaft to the remaining piece of original shaft (with an integral helical gear on the end). I then made some spacers out of delrin and mounted the motor to the gearbox.


I used an Atmel AT-Mega8 AVR to turn the stepper motor through four shaft rotations when power is applied. I used PN2222 transistors to switch the unipolar motor windings. The transistors are driven through resistors by half the D port in the AVR. The gear reducer's output will spin about 1/6 of a turn, which will advance the "butterfly valve" in the feeder enough to dispense a single serving. I coded the Atmel chip using Atmel's IDE in C with WinAVR. I used PonyProg to download the code into the chip via a simple parllel port interface. I would normally use an Arduino or similar microcontroller for projects like this, but this one is so simple, a full-fledge microcontroller would be overkill and too expensive. The AT-Mega8 is about $6 and needs a $1 crystal. The Arduino is around $30.




My cat stuck his head into a pile of plastic shavings the garage and managed to get a couple scraps stuck in his whiskers.

Fixing a misfire during acceleration in a 1992 Honda Civic

I took some packages to the post office today, and was surprised to find my 1992 Honda Civic was having a very bad day. It was misfiring during acceleration at low-mid RPM. Idle was smooth and cruising was fine, but any normal amount of acceleration from a stoplight or stop sign produced an extremely loping, uneven amount of power. The car was drivable, but just barely.

I took out a couple spark plugs, and was surprised to find the gap had become huge -- around .075"! The plugs appeared to be worn out. I figured this had to be problem. I got some new NGK plugs, and unfortunately the problem persisted. I checked the compression in each cylinder, and they were all 150 psi +/- 5 psi. After a short consultation with my dad, I decided to replace all of the common ignition parts with real Honda replacements: distributor cap, rotor, and plug wires. This did the trick! The car was back to normal, and running strong. The original cap and rotor were Honda parts, but the plug wires appeared to be aftermarket (Prestolite). Dad and I agreed that aftermarket plug wires should be avoided altogether. Just bite the bullet, and buy OEM wires. They'll last much longer.

Even though it is difficult to see in this photo, the old plug (right) has a gap almost twice that of the new plug (left). This too-large gap in all four plugs might have contributed to the destruction of the plug wires.

Finishing the refrigerator conversion project

In my last post, I had just finished charging the "new" refrigerator with R134a. I was surprised that it worked at all, and it was still working the next day, so something must be correct. I used some expanding foam to fill up the large square hole left by the thermoelectric module in the bottom of the fridge. The clear vinyl tube is a hose that I connected to a drain on the inside of the fridge. If (when) the evaporator defrosts, I am sure there will be a fair bit of condensate dripped out. I put a small stainless tray on top of the compressor to catch the water.

I also used the fridge's existing thermostat (the white tube on the right). The manufacturer setup the thermostat so that it would regulate the temperature on the evaporator itself. The thermostat has a capillary tube that is clipped onto the evaporator and a large knob with numbers 1 to 7 (like any refrigerator). I also filled up the hole through which the refrigerant lines and thermostat enter the fridge with expanding foam.



While the existing thermostat works well, the table has a nice cutout for a two-digit 7-segment display and four buttons. The thermoelectric fridge came with a digital thermostat, and I made the cutout in the tabletop specifically for it. I would like to keep the digital thermostat feature, so I am building one with an Arduino microprocessor.

Here is the rear of the completed table with white thermostat control knob at lower left. I've already written some Arduino code for the digital thermostat, and will probably be implementing that soon. In the meantime, my table refrigerator is now frosty cold, and using less power than the thermoelectric model.

Retrofitting a thermoelectric refrigerator with a conventional compressor system

A couple years ago, I built a nice wooden table that housed a small refrigerator. The idea is that it keeps drinks handy in the living room for parties and guests. It's also cool, and fits with my idea of what "functional furniture" should be.

I chose a refrigerator meant for storing wine because it had a nice glass door, was pretty small, was very quiet, and could operate facing upwards because the thermoelectric device doesn't care about its orientation. I knew that thermoelectric refrigerators generally suck at actually refrigerating, and this one was not exception.
On really hot days, the fridge would get up to 5o-60*F, which is cool, but not nearly as nice as drinking 40*F beverages. The other problem is that it draws 70W, essentially constantly. This comes out to 613 KWh/year. Most small conventional refrigerators use around 300 KWh/year. Also, I like to tinker and wanted to mess around with a refrigerant system.



I bought a small 1.8 cu ft Haier fridge off craigslist for $15. Geez, can't beat that! My first task was to pull out all of the system components from the insulated box. This required draining the system of refrigerant through a small hole that I drilled in the compressor fill tube. Before doing this, I checked the label, and it indicated the system used 1.6oz of R134a. I can buy R134a at the auto parts store, so I will be able to refill the system.


I de-soldered the tiny capillary line from the accumulator / dryer and de-soldered the suction line on the compressor. I used an oxy-acetylene torch to heat the joint, then I just pulled the tubes apart when the solder became molten.

Here is the evaporator liberated from the fridge.



Luckily, with one flap unfolded, the evaporator fit perfectly in the thermoelectric cooler (TEC) fridge.


I mounted the compressor underneath the table (also getting very lucky that there was enough clearance). I cleaned the copper tubes carefully, then re-soldered them with silver solder and some paste flux. I had to extend the suction tube by a few inches, and so I just got some tube that fit around the existing line and soldered it to the outside.


I soldered the capillary tube back into the dryer, and put a valve on the compressor fill tube. The system was sealed at this point. Now, I connected my vacuum pump (Welch 1400) with a micron vacuum gauge and a tee that connected to a can of R134a. I positioned the can on a sensitive scale so that I could meter out 1.6 oz of refrigerant.

I pulled a 375 micron vacuum in about 15 minutes or so. I even ran the compressor while the system was under vacuum. It raised the pressure just a couple hundred microns, then it settled back down quickly, so I felt the system was dry and sealed.


After I metered out 1.6oz, which only took a few seconds with the valve just cracked open, I tightly closed the valve that I added to the fill tube, and was very pleased to see feel the evaporator getting very cold.

Tomorrow, I will re-insulate the TEC fridge and hopefully give it a final test.