DIY X-ray backscatter imaging system (airport body scanner)

I built an X-ray backscatter imaging system from parts found on eBay. This system works by scanning a very thin beam of X-rays across the target, and measures the amount of backscatter for a given beam position. The beam is scanned mechanically by a rotating chopper (collimator) wheel, and by tilting the rotating wheel on an orthogonal axis. The output image is generated on an oscilloscope by matching the horizontal scan speed to the rotating wheel, and using a potentiometer to measure the vertical axis position. The scope's brightness (z axis) is controlled by the amount of backscatter signal received by a large-area detector. Thus, the image is constructed bit by bit. I used a long-exposure shutter on my camera to see the image formed by the moving oscilloscope trace.

http://www.google.com/patents/US5181234

Making Pop Rocks candy at home

Here I show how I made gassified candy (eg Pop Rocks) in my home shop. The basic procedure is to heat sugar water up to 280*F, carbonate it with 600 psi CO2 and vigorous mixing, then cool the mixture while it is still under pressure. After the mixture has solidified, rapid depressurization causes the candy to burst apart into small fragments. Each fragment has lots of tiny CO2 bubbles embedded in it. When placed on the tongue, the bubbles burst open as the candy dissolves and a popping sensation is felt.

The main difficulty is building a high-pressure chamber that allows precise temperature control as well as thorough mixing of its contents while under pressure. I've built something that meets these requirements, and I've finally made a batch of candy that would pass as Pop Rocks, but they are pretty weak. I still need to adjust the temperature and mixing times to get a better product.

Link to recipe: http://www.exploratorium.edu/cooking/candy/recipe-lollipops.html

How to design a transistor circuit that controls low-power devices (tutorial)

I describe how to design a simple transistor circuit that will allow microcontrollers or other small signal sources to control low-power actuators such as solenoid valves, motors, etc.

My Tektronix contest entry

Vote for me here: http://mytektronixscope.com/videos/

I am using my Tektronix 2246 analog oscilloscope to show the image generated by my DIY electron microscope. The 2246 is operating in X-Y mode, with the channels connected to a raster scan generator. The vertical scanning frequency is about 30Hz and the horizontal frequency is about 10KHz. The trace brightness (Z axis) is modulated with the secondary electron signal from the microscope.

DIY Scanning Electron Microscope - Image Quality Improvements 1

This weekend, I spent some time making incremental improvements to the image quality of my scanning electron microscope. I decided to abandon my own X/Y deflection amplifier in favor of the amplifier from a commercial analog oscilloscope. The oscilloscope's amplifier is extremely linear, easy to control with the front panel knobs, very fast, and has a "good enough" amount of deflection and offset, but it could be better. I will try to compensate for this by adjusting the deflection plate length and separation distance.


Here is a shot of the aluminum window screen that I have been using as a test target. The wires in the target are pretty straight, indicating that the system is relatively linear, and can produce an undistorted image.


This photo shows an integrated circuit -- this one is a MEMs gyroscope. The wild black/white pattern on the hard-edged object at lower right is the actual silicon die. I imagine the strange pattern is caused by the varying conductivity of the die's surface. The die's bond wires can be seen clearly as well as the metal pads that connect the bond wires to the chip's external leads.


I spent a good part of the day redesigning my filament power supply. It is now DC and fully regulated. The original design was AC, which caused huge focus problems (the beam would fluctuate at 60Hz, so I rectified it to unregulated DC. Later, I found that the ripple in the output voltage was also causing image problems. Since my image scan rate is on the order of 30Hz (in vertical), I would see the problems caused by 60Hz noise as rolling bars in the output image. The ripple in the DC filament voltage caused strange black bands to roll down the video image. I suspect this happened because of slight changes in the electron gun's bias. I am really surprised that the 2V filament voltage would have any effect on the bias voltage of hundreds of volts, but it apparently does.

The next problem I found was apparent at higher magnifications. This photo shows a close-up of one of the chip's bond wire pads. The pad in reality is very straight, but it is quite curvy in this image. At first, I thought this problem was caused by a ground loop between the X/Y amplifier and the rest of the microscope's power supplies. No, it turned out to be caused by the oscillating magnetic field created by the diffusion pump's cooling fan and the pump's heater.



I switched off the pump and fan, and the problem went away. I thought about adding magnetic shielding to the chamber, but I think I will just accept with the wavy image while focusing/panning, then turn off the pump to record an image.

My SEM design is ideal for teaching and exploring electron beam control, but has several major design flaws, which I will summarize for those interested in creating homebrew SEMs with actual utility value:

1. The photomultiplier tube is extremely sensitive to ambient light. In my SEM, I use a black plastic light-tight box to cover the glass bell jar and keep it protected from room lighting. However, the SEM's own filament produces enough light that escapes through the electron gun's vent holes to cause big problems. I can only run the photomultiplier tube at about 700V, before exceeding the average maximum rated anode current (.1mA). The SEM imaging system is AC coupled so the offset is not a deal-breaker, but I believe the DC offset also creates a lot of AC noise in the output image, and would also damage the photomultiplier if I should run it for a long time or greatly exceed the max average anode current. The solution is to build the microscope column from an opaque material (metal), and arrange the electron gun such that there is no optical path from its vent holes to the specimen chamber of the SEM (like all commercial designs).

2. Remove magnetic disturbances or build the SEM column from a magnetically-shielding material.

3. Use very clean, tightly regulated DC for all supplies involved with the project. This is a sensitive analog system, and it picks up every kind of noise.

4. The main accelerating voltage need not be very high (eg 3KV works fine). Lower-energy electrons are easier to deflect and focus, lowering the requirements for those respective power supplies. So far, I haven't seen any benefit to using higher accelerating voltages.

5. Minimize the use of all insulators inside the chamber. Try to build structures that stand on their own without insulator support, or shadow insulators from the electron beam with conductive surfaces. I will talk about this more in a future video, but it caused me many problems in the early weeks of the electron column design.



My next actions will target the overall softness of the image. A youtube commenter pointed out that my aperture disc is much too thick (.010" thick, with a 100um hole). In the pinhole photography world, this would be considered a silly setup and would lead to soft images. I am inclined to believe a similar thing is happening with the electron beam, and will try the same aperture size with a thinner sheet.

I will also try to reduce the amount of light coming out of the electron gun and/or reduce the amount of light entering the photomultiplier. This should reduce the grainy high-frequency noise in the image.

PS, I also think that all of my images are inverted with respect to the normal secondary-electron images from SEMs. All of the images that I have posted so far have dark regions where secondary emission was high. I will probably correct this by adding another inverting opamp the signal chain.

DIY Scanning Electron Microscope - Sources, Costs and References

Metal aluminum window screen

Just a few linearity problems ;)

I used an oscilloscope's X and Y amplifiers for these images. It has much better linearity than my own, but not enough differential voltage or offset range.


The sum total of the big-ticket items shown in the video is $1485. This does not include hoses, wiring, raw metal, teflon, screws, a cabinet, etc. It also does not include an oscilloscope, which can be a very simple model (under $100 on eBay) as long as it has a z axis (brightness) input. Your diffusion pump or diffusion pump baffle may also require a water chiller.

Here are a list of information sources that helped me with this project:

Teralab - Homebuilt electron gun and other great projects
http://www.teralab.co.uk/Experiments/Electron_Optics/Electron_Optics_Page1.htm


Popular Mechanics video on commercial desktop SEM
http://www.popularmechanics.com/technology/gadgets/4218957


Hamamatsu - Supplier of PMTs
http://sales.hamamatsu.com/assets/applications/ETD/pmt_handbook_complete.pdf


TV to oscilloscope circuit
http://www.electronixandmore.com/project/14.html


CRT oscilloscope clock circuit
http://web.jfet.org/vclk/


Charged particle optics simulation program
http://www.electronoptics.com/

"A Simple Scanning Electron Microscope" P.J. Spreadbury -- Advances in Imaging and Electron Physics Vol 133 Chapter 2.5 (no link).


ISI SEM refurb at home
http://members.tm.net/lapointe2/Scanning_Electron_Microscope.html


Great technical info on cathodes and wehnelt cup spacing. Most of the article concerns LaB6 cathodes, but there is a short paragraph on tungsten cathodes.
https://www.kimballphysics.com/cathode/support_PDF/Cathode_ES423_LaB6_info.pdf

Numerous websites that gave background and operational information about SEMs.
Lots of web searches



Nearly all raw materials for this project were purchased from McMaster-Carr. All power supplies were purchased on eBay, or I already had them, in which case they came from a surplus store or flea market. Nearly all of the electronic components came from Jameco.

DIY 10-finger flex sensor gloves for possible VR or video game control

I built a pair of flex sensor gloves for capturing the motion of all ten fingers. This system uses individual flex sensors made by Spectra Symbol and a National Instruments analog capture device to record the flex sensors' values.

DIY searchlight housing for 1000W xenon arc lamp

Original test run of 1000W arc lamp:
http://benkrasnow.blogspot.com/2010/04/first-test-run-of-1000w-osram-xenon.html

I finally finished the 1000W xenon searchlight project that I started earlier this year. The power supply is a slightly modified arc welder coupled with an automotive ignition coil for the starting pulse.


This is the searchlight's beam shooting skyward behind a large pine tree in my back yard. The beam is very difficult to capture on video.

Improved level sensor for the DIY aquarium top-off project

In my original post regarding an automatic water top-off system for aquariums, I designed a sensor head that consisted of a plastic rod with a set of pocket holes drilled at the tip. The holes were diametrically opposed and angled so that they intersected at a point about 1cm in front of the plastic rod face. I inserted a plastic fiberoptic into each of the holes, and the system would allow sensing a liquid level surface by measuring the amount of light reflected off the surface. If the liquid covered the two probes, all of the light would be scattered off into the liquid and the signal would be almost zero. When the level fell below the fiber ends, the reflected light would trigger the top-off pump.

This system worked very well until nearly flooding my house a couple weeks ago. By extreme luck, I happened to be sitting near the aquarium when an air bubble got trapped between the two fiberoptic ends. This caused the top-off pump to run even after the water level had risen higher than the sensor head. I heard the tank dripping water and quickly shut off the pump. Following this event, I decided to improve the reliability of the sensor.

I did some searching for commercially-built liquid level sensors and found that many of them operate by submerging a prism and measuring the amount of total internal reflection. When the prism is submerged in liquid, the light will pass out of the prism and into the liquid. When dry, the prism will reflect most of the light internally. By positioning the fiberoptics symmetrically, the light signal will be drastically changed by the liquid surrounding the prism.



I machined and polished a piece of acrylic into a point. Then I drilled two holes that would snugly hold the fiberoptics. I mounted the whole thing on a Delrin rod.

This sensor should be much less sensitive to air bubbles, snails, dirt, etc than the previous model. For the next week, I'll be monitoring the top-off system and manually controlling the pump. If it looks good, I will connect the pump and let everyone know how it works.

DIY stainless steel conical beer fermenter Pt.1

Please search my blog for "fermenter" to find all of the posts regarding this project.

I am building a stainless steel tank that will eventually become a very unique beer-brewing vessel. My idea is to make a tank such that the entire process can take place without ever having to transfer the beer from one tank to another. This vessel will boil the wort, chill the wort, provide a temperature-controlled fermentation period, allow the trub to be removed, and provide a secondary fermentation. This tank was designed with my experience in brewing about 30 5-gallon batches of beer using the extract process. I don't have much inspiration to do all-grain brewing yet.

Having said all of that, I am also learning to TIG weld, and this project will provide many different welding setups -- all in stainless steel.


I bought a stainless steel conical hopper, model TMS14514 from
http://www.toledometalspinning.com/products/hoppers/priceList.asp

Toledo Metal Spinning sent the item very quickly, and I am impressed with the quality. The edges are extremely flat, and the overall finish and dimensional tolerances are great.

It holds 6.4 gallons total, so a 5 gallon batch of beer should fit pretty well. The hopper is a continuous piece with no hole in the bottom. I will be mounting a butterfly valve at the apex, so I need to cut the tip off to match the diameter of the valve housing. I knew before I ordered the hopper that I would only need to slice off about 1/8" off the end.

I used a slitting saw in my milling machine to do the job. This left me with a super flat clean edge. 70 RPM, 0.5 inches per minute, however the feed rate is measured at center of the saw, and I programmed a G2 circular path. This means the feed at the cutting point is probably lower. I had problems with chatter, thus necessitating this low feed rate.

This is one half of the butterfly valve housing after I welded it to the cone. The blue hose is silicone, and is carrying argon to the backside of the weld. In addition to the foil on top, I have made a dam with aluminum foil and tape inside the neck of the cone to trap the argon in the space around the weld.

Since the first weld went so well, I decided to weld on the inside of the fitting as well. Ultimately, this was not a great idea, but the weld itself went well. I used a copper tube with a line of tiny holes drilled in it to disperse backing argon to the outside of the cone. I had a fair bit of room inside the valve fitting for the TIG torch and filler rod.

I used a die grinder to smooth out the interior weld. After putting it together, it leaked! I had used the die grinder too much, and made the metal thin enough where a tiny pinhole in the weld made it all the way through the metal. I re-welded the outside bead, and then realized that I should have just made a couple of passes on the exterior to build up material. Then I could die-grind away the inside until I ground into the weld bead. No need to weld the interior. This would provide a nice smooth surface inside the tank and ensure there was enough material to keep it structurally sound.


It looks good now.


I originally started to cut this hole with a high-quality hole-saw in a corded drill. After a few seconds, I realized it was probably not going to work. Stainless is just such a tough metal, cutting tools just bounce off it. I used a free-hand plasma cutter to make the hole.

I made another aluminum foil/tape dam around the wall on the interior.


The weld went pretty well.



This time, I learned how to do it. Instead of welding on the inside, I just built up a nice bead, then used the die-grinder on the interior until I ground into the bead. It's nice and smooth on the inside.

DIY aquarium chiller is a success

Today was the first warm day of the season here in Sunnyvale, CA. In my living room, the air temperature was close to 90*F


The aquarium temperature was a steady 80*F throughout the day.
The temperature controller claimed to be using only 10-15% of the chiller's capacity, but I think this might be misleading because of the way the system is setup. The chiller (water cooler) has its own thermostat, and attempts to keep its water temperature around 45*F. The fishtank's temperature controller turns a pump on and off that pushes the chilled water through a heat exchanger with the aquarium water. The amount of time that the pump is running is the "percentage of capacity" that I have been listing here and in other posts. I am sure the system has a non-linear response such that the amount of cooling delivered at %100 would not be ten times the cooling delivered at %10. This is because the chiller's water temperature would be rising (making it less effective at cooling the tank) as the pump runs more often. Nonetheless, I think the chiller system has plenty of headroom, and it only rarely gets hotter than 90*F in my house, so I am feeling pretty good about the project.

Even more detail for aquarium DIY top-off system

In this post, I'll describe the machines and methods that I used to make the optical sensor head for the aquarium top-off system.

Step 1: Cut off 2" of 1/2" diameter black Delrin rod. Delrin is a brand name for a specific type of acetal copolymer plastic. You can get it at www.mcmaster.com


Step 2: Square-off the end of the rod on the lathe.


Step 3: Drill two holes at 30* to the rod's major axis so that they intersect a few mm in front of the rod's face. You may need to use a punch or dremel to prevent the drill bit from wandering on the sloped face of rod. I like to use the vise-within-a-vise method for drilling angled holes on the drill press.

Step 4: Taper the end of the rod with a coarse file.

Step 5: Use a smooth file to tidy everything up.

I found that the optic fibers should be cut square, and then recessed into the sensor head so that nothing sticks out. This allows air bubbles to form harmlessly in each of the spaces created by recessed fiber ends. If the fibers stick out too much, an air bubble might become trapped between them, and the system will have a "false positive".

DIY aquarium chiller

See update here:
http://benkrasnow.blogspot.com/2010/04/titanium-heat-exchanger-for-diy.html

My nano reef aquarium is usually 2-3*F hotter than the ambient room temperature (after the heater setpoint has been reached). This is a problem, since the temperature in my living room is often higher than 82*F in the summer. This puts the tank water at an uncomfortably high temperature (84+), and I think that the corals suffer from the temperature swings as well as the overall high values.

So, how to lower the tank temperature? For a 5 gallon tank like mine, a peltier heat pump like the Coolworks Ice Probe would seemingly be a good choice. I tried building just such a device a few years ago, and it was a big failure. I learned that peltier heat pumps cannot be controlled by raw pulse width modulation (PWM) signals, and they don't do well in thermostatic (on/off) systems either. One reason is that the semiconductors inside the Peltier device do not like the thermal shock of the constant on/off switching. Also, Peltier heat pumps are already horribly inefficient, and using PWM or on/off control makes things even worse. During the "off" cycle of either the PWM pulse or the on/off cycle, the heat will flow backward though the device -- the same heat that the device just pumped during the "on" part of the cycle. Think of bailing out a sinking boat with a bucket that has a huge hole in the bottom. The best way to control the peltier modules is to generate high-frequency PWM, then smooth it out with an inductor/capacitor filter. There is still the problem of the peltier junction's inefficiency, and the hot-side heatsink must be massive with a massive fan to make the system viable. Anyway, I haven't heard anything great about the Ice Probe, nor any other Peltier cooling systems designed for any application that requires a good amount of cooling. I have a thermoelectric refrigerator that is just marginally good enough for its purpose.

So, today's design for a new aquarium chiller will NOT use Peltier junctions, as much as I love the idea. I bought a $99 water cooler that uses a conventional compressor and r-134a refrigerant.
I filled the cooler with tap water, and mounted a Rio pump with an outgoing hose and return line.


The two hoses connect to a stainless steel coil. I've had this thing laying around my shop for a long time. It came out of junked, expensive lab equipment. It is non-magnetic, which indicates 3-series stainless steel. I'm guessing it's 316, which is highly corrosion resistant. Of course, the aquarium purists would insist on titanium, but I don't have any, nor do I think it's really necessary. I'd love to hear from anyone who saw a stainless steel chiller coil corrode, or definitively caused tank poisoning.

I melted a couple slots in my hang-on cheapo protein skimmer (it's not a Skilter, but very similar). I would have used a dremel, but I didn't feel like taking the filter off the tank, and I also wanted to avoid getting plastic shavings in the water. The stainless coil sits down into the slots and is just held by gravity.

The cooler is plugged in all the time. It keeps its insulated water chamber around 47*F. The Rio pump in the cooler is turned on and off by the PID temperature controller that I mentioned in a previous blog post. The controller can be configured to use a longer cycle time (eg 30 seconds) since it is controlling a pump, and it would not make sense to turn a pump on and off once per second as it would be for a heater.

The cooler is rated 86 watts. If this is what the compressor draws while normally running (I didn't check it). I would estimate the cooler can pump about 170 watts of heat (about 580 btu/hr). The coefficient of performance is around 2 for small compressor systems. For comparison, a large peltier device can move around 70W under ideal conditions, at a very specific current/voltage. The coefficient of performance for Peltier devices usually tops out around 1, and is often about 0.5 for realistic situations. So, a peltier pump drawing 86 watts, would only pump about 43 to 86 watts of heat.

I just installed this chiller today, so I'll monitor it on hot days and make another post about its performance.

DIY wave-maker plans

I have provided a basic schematic and parts list for the aquarium wave maker circuit. Here is what you will need:

*Blue plastic "double" electrical box (for household wiring) $1
*Standard electrical outlet and outlet/switch faceplate $2
*12 volt transformer and full-wave bridge (cut up an old one that you are not using. $6 for Jameco #100095)
*Power cord (cut up an old one)
*558 timer chip (Jameco #27457) $1.20
*Solid state relay Kyotto KB20C02A (Jameco #175214) $6.55
*7808 or 7809 voltage regulator (Jameco #876352) $0.56
*two 100K pots (Jameco #29103) $2.18
*two 3,300 uF capacitors (Jameco #93666) $1.22
*PN2222 transistor (Jameco #178511) $0.12
*perf board, wire, maybe a 16-pin DIP socket, misc caps and resistors $2.00

Total parts cost is about $23. Here's a link to Jameco

Notes: My circuit provided times of about 1 to 6 minutes for the on and off periods. You may want to put a 10K resistor in-line with the 100K pots. If either pot is turned too far down, the circuit will stop oscillating, so the additional fixed resistance will prevent this from happening.

This circuit does not provide the "soft start" that many commercial wavemakers tout. These motors likely use shaded poles for starting, and a soft-start controller would have to control the frequency of the AC power, not just the voltage. I find it pretty unlikely that this is what any wavemaker does, and for the price they charge, it's probably cheaper to buy a new powerhead every year! I haven't used my wavemaker long enough to know if it will kill the powerhead. I'm using an Aquaclear powerhead, and it clicks loudly upon startup, but there is no chatter. I may make another post about modifying the impeller to cope with the repeated startups.

I've experimented with using dimmers, current-limiters, etc, to control the speed of powerheads. The speed can only be reduced %10 or %20 before the motor stalls. It's not really practical without control over the frequency of the AC waveform.

The Solid-state relay in this circuit has a current limit of 2A. This mean it can control up to about 200W of powerhead.

DIY liquid nitrogen generator

Maker Faire 2010
http://www.youtube.com/watch?v=14B8LynojI4

I have always been intrigued by science demonstrations using liquid nitrogen, and often made trips to a local welding supply store with my stainless steel vacuum flask to purchase liquid nitrogen and satisfy my cryogenic craving at home. After a few fill-ups, I wondered about the possibility of making liquid nitrogen on demand. Some companies have already produced self-contained liquid nitrogen generators that are designed for small laboratories (http://www.elan2.com/). The Elan2 would be ideal for home experimenters, but the cost is over $10,000, so I decided to build a similar device with less total output, lower purity, and at much lower cost. The device that I built cost less than $500 and produces 1 liter of liquid nitrogen per day.

Nearly all large-scale liquid nitrogen is made by compressing, cooling, and expanding air. This process removes heat from the air and can be repeated until the air liquefies. The condensing gasses are then separated using fractional distillation. This process cannot be easily scaled down because it relies on maintaining a complex, large distillation column to separate nitrogen from the other gasses in air. To avoid using a distillation column, one could use a nitrogen separation device to strip out the nitrogen from air at room temperature. Then, the room temperature nitrogen can be liquefied via the standard compression and expansion method. This is likely the process used in the Elan2 generator. However, it still requires the use of a very high pressure compressor and heat exchanger, extensive insulation and many other custom parts.

Another approach to producing small-scale liquid nitrogen is to use a self-contained cryocooler, which is a specialized refrigeration device that is designed to pump heat across a high temperature differential. In many cases, the devices are specifically designed for small-scale use and designed for spot-cooling in electronics. The benefit of using a cryocooler is that the device requires almost no maintenance and can liquefy gasses at atmospheric pressure. A compressor would not even be necessary in a gas liquefier using a cryocooler, but is helpful for removing water from the air and isolating nitrogen from air’s other component gasses. There are a few different basic types of cryocooler, but this article will highlight free-piston Stirling cycle cryocoolers. These devices are built with an internal piston that is driven by an electrical coil – a linear motor. The piston expands a working fluid (usually helium) in the device while a separate displacing piston moves the fluid to the tip. The piston then reverses direction, compressing the fluid as the displacing piston forces the fluid toward a heat-rejection area of the device. This process is repeated so that the working fluid is constantly being expanded at the tip, and compressed at the heat-rejection area. This causes heat to be pumped from the tip to the rejection area. The rejection area is cooled with atmospheric air, or other fluids that exchange heat with the environment.

Stirling cryocoolers are not relatively common devices, but they are used for RF filters that contain superconducting components. Such RF filters with their integrated cryocoolers can be found on eBay for under $300. One particular unit is the Superfilter built by Superconductor Technologies Inc. It contains a cryocooler that is rated at 140 watts of input power, and is extensively documented here (http://books.google.com/books?id=POLgG5mma6IC&pg=PA75).

I purchased the Superfilter on eBay and extracted the cryocooler. In order to test the device, I attached a small heatsink to the cooler’s cold tip, placed the tip into a household vacuum flask, and powered up the unit. After 30 minutes, I took the cryocooler out of the flask, and noticed a small amount of liquid air had collected at the bottom. Inspired by this success, I continued construction of a more complete liquid nitrogen generator. I already owned a 30-liter dewar (large vacuum flask) and fabricated an acrylic plate that would seal the top of the dewar while the cryocooler was also mounted to the plate with its heatsink hanging down into the neck of the flask. I also removed the cryocooler’s finned heatsink on its heat rejection area and replaced it with a liquid-cooling manifold. Liquid cooling lowered the heat rejection area temperature more effectively than forced air cooling, and this ultimately lead to higher system efficiency.

The liquid nitrogen generator has two basic sections, the dewar with cryocooler, and the air processing equipment that creates dry nitrogen from atmospheric air. The dry nitrogen is fed into the dewar at just above atmospheric pressure where the cryocooler chills the nitrogen until it liquefies and drips off the heatsink. Surprisingly, most of project’s time budget was spent designing and building the equipment to produce dry nitrogen from air. There are some companies who make dry nitrogen supply devices, but even small units are meant for much higher throughput than what is needed by this liquid nitrogen generator. Each liter of liquid nitrogen requires about 700 liters of room temperature nitrogen gas. 700 liters per day is only 0.5 l/min, a very modest flow rate. One popular, but unnecessary use for relatively low-purity nitrogen is filling car tires. I tried to purchase such a machine, but the cost and flow rate were much higher than anticipated. Instead, I found a very small nitrogen separation membrane on eBay. It’s original use was unknown. The separation membrane is the actual component inside commercial nitrogen generators that perform gas separation. The membrane is formed into a large bundle of hundreds of 2mm dia tubes. Air is fed under high pressure into one end of the bundle. The tube walls are semi-permeable and allow oxygen, water vapor, and other “fast” gasses to permeate relatively quickly. Nitrogen and slower (larger molecules) gasses do not permeate as quickly, so the concentration of nitrogen is much higher at the exit end of the tubes than it is at the input end. Higher purities of nitrogen can be achieved by restricting the flow rate through the tubes, thus allowing plenty of time for the unwanted gasses to permeate the tube walls and leave the system. The resulting nitrogen will contain trace amounts of argon and even smaller amounts of other noble gasses.

I also built a dessicator from aluminum cylinders filled with silica gel and plumbed this into to the system before the air reaches the separation membrane. These units are available commercially, and the one that I built is not particularly specialized. Separation membranes also exist for removing water, and this would be an improvement over silica gel dessicators, which require the gel to be dehydrated in an oven after it becomes saturated with water.

The liquid nitrogen generator has proved to be a reliable, but fairly slow method to produce small quantities of liquid nitrogen at home. The initial cool-down of the dewar takes about 12-18 hours, after which liquid nitrogen is produced at a net rate of 1 liter per day. The generator uses about 300 to 400 watts of electricity (includes the water chiller, which cycles on and off), so the energy cost for producing one liter of liquid nitrogen is about 8.5 KWh, or $1.10. This is substantially less expensive than having a thermos filled at a local welding supply store.

Here is a system overview.

This photo shows liquid nitrogen dewar (on the right) with the cryocooler mounted on top. The device on the left is a window air conditioner that was converted into a water chiller with liquid cooling lines running to the cryocooler. The small compressor on top of the air conditioner pulls air in from the atmosphere and sends it through the nitrogen separation equipment.

The cryocooler with custom heatsink. When the unit is running, the heatsink gets cold enough to condense nitrogen, and the newly formed liquid will drip off the heatsink.

This is the power supply for the cryocooler. The original control board from the STI Superfilter requires 27VDC, so I found a switching power supply from eBay and use that to power the control board.

This is a silica gel desiccator that I built from aluminum. It also contains coalescing filters and carbon filters to remove oil droplets and vapor from the compressed air stream.

This is the nitrogen storage tank with pressure sensor, valve and gauges.

Liquid nitrogen experiments:

Make Ice Cream

Mix a standard ice cream recipe in a large bowl.

4 cups half-and-half

½ cup heavy cream

¾ cup white sugar

2 teaspoons vanilla extract

pinch of salt

Add liquid nitrogen slowly while stirring the mixture. As the nitrogen boils, it will help froth the ice cream as it freezes the mixture very quickly. The rapid freezing produces small crystals and a fine texture in the ice cream.

Freeze a balloon

Inflate a standard latex balloon with air, then submerge in liquid nitrogen. The balloon will deflate dramatically as the internal gasses contract and even condense. After removing it from the nitrogen, it will reinflate as it warms. This process can be repeated many times.

Perform magnetic levitation on a superconductor

Certain high-temperature superconductors can be used at the boiling point of liquid nitrogen – 77 K. Once the material is cooled, it will exhibit “magnetic mirroring”, so that a permanent magnet can be levitated above the superconductor as its magnetic field is reflected. The best type of magnets for this are small (5mm dia or less, by 2mm long) neodymium-iron-boron magnets.

Make liquid oxygen

A variety of common gasses such as oxygen can be liquefied by passing them through a copper tube submerged in liquid nitrogen. Liquid oxygen can accelerate the combustion of common objects by creating a localized pure-oxygen environment.

Ping-pong ball spinner

Use a needle to puncture a ping-pong ball, then bend the needle to make the hole somewhat tangential to the ball. Repeat this on the other side of the ball with the hole “facing” the opposite direction as the first like a rotary garden sprinkler. Submerge the ball in liquid nitrogen for about 30 seconds, then remove it and place on a large flat surface. The ball will begin spinning as the captive nitrogen boils and streams out through the holes.

Effect on semiconductors

Connect various LEDs to a 9V battery with an appropriate current limiting resistor, eg 1Kohm. Submerge the LED in liquid nitrogen and note its color and brightness. As the semi-conductor cools, the band gap changes, causing a color shift. Some have also suggested the color shift comes from the spacing of the crystal lattice changing due to the very cold temperatures. Different LEDs will show varying degrees of color shift, so try a few from different manufacturers.

Old blog post:


You can generate liquid nitrogen (LN2) in the comfort of your own home with some parts found on eBay. I have proven that this is possible by purchasing surplus equipment and assembling it as described in this post. I spent over a year searching eBay, so these parts are not really easy to find, but the total bill for the whole system was under $500. The device consumes about 300 to 400 watts of electricity and needs no consumables (just atmospheric air). The LN2 is produced at a net rate of about 1 liter per day. This comes out to 9.6 kWh/liter or $1.15/liter, which is substantially cheaper than having the local welding store fill up a thermos (granted the thermos must be cooled as it is filled, thus requiring more than its capacity of LN2).

The most important part of this system is the cryocooler. This is a device that employs a thermodynamic gas cycle to pump heat through a very high temperature gradient. Many of these devices are self-contained and require only an electrical input to start pumping heat. The crycooler that I used was removed from a surplus RF filter which used the cryogenic temperatures to maintain a superconducting RF filter. http://www.suptech.com/home.htm
The crycooler itself has been fairly well documented:
http://books.google.com/books?id=POLgG5mma6IC&pg=PA75&lpg=PA75&dq=sti+cryocooler&source=web&ots=ZTMqWVv8Pu&sig=HbbSzGgnD3fIFxyKJjxFLuNEa9E&hl=en&sa=X&oi=book_result&resnum=1&ct=result

I converted the cryocooler to be water-cooled on the hot end and attached a heatsink to its cold end. In operation, the cold end with the heatsink is inserted into the top of a large dewar. Eventually, the interior of the dewar gets so cold that the air will condense into a liquid and drip down to the bottom.

The second key part of this system is the nitrogen separation membrane. The is a device that accepts normal air, and produces relatively pure nitrogen. The waste products (mostly H2O, O2 and CO2) are vented into the air. Information regarding these membrane units is easy to find on the internet, but good luck buying one! They are nearly all produced for huge industrial installations, and those manufacturers will not even return phone calls from interested hobbyists. Asses! I spent a LONG time searching eBay, and eventually found a very compact unit, which was perfectly suited for this project. The nitrogen purity is dependent on the mass flow rate through the device. This means the flow must be carefully monitored and controlled. I will make another post that describes some fun stuff to do with LN2.