Liquid air!

Here is part 2 of the Cryocooler Saga! You can see Part 1 and Part 3 also. 

It works! Here's my first liquefied air! I'd removed the cryocooler from the Superlink and mounted it in a wooden frame to hold above a thermos to collect the air in. Here's how I put it together.

After removing the cryocooler from the Superlink, I built a wood frame to allow the stock fan on the Superlink to blow air through the cooling fins on the heat rejection side. He's a scrappy fellow.

This thing's walls are probably more sound than some of them in my house. Not that that's saying much.

The hole in the base is (approximately) the size of the cooling fins, forcing air to flow through them.

I made a trap door on the bottom to divert air blowing out the bottom away from the cold finger. It's just a few pieces of aluminum sheet screwed to the wood, with 1/8" luan for the trap door. The luan sheets can be separated to insert the cryocooler flange, then closed around it to divert most of the air away. I didn't make any special effort to make it a tight seal. We aren't trying to freeze all of Gotham City here, absolute efficiency isn't really one of my goals at the moment.

The cooler's control program requires that the cold side temperature be measured to control the power input to the cooler. I figured out that the only pins on the wiring harness that goes to the Dewar that tell the control board what the temperature is are the pair on the fourth from the bottom. The control board appears to have a constant-current circuit that attempts to put 0.1 mA through the pins and measures the voltage. At ambient temperature, the resistance is roughly 5.6 kilohms, so my best guess is that the temperature sensing element in the dewar is a thermistor that is specifically designed for accuracy at about 80K. Putting a 10K potentiometer between the pins allows you to gradually lower the "measured" temperature of the cold side for the control board to allow it to cool down. I plugged a couple stackable headers into the connector to use as sacrificial pins so I don't damage the original circuit board's pins.

I don't want to damage the cooler by trying to ramp it down too aggressively though, so I tried a few ways to measure the temperature of the cold head. A kitchen thermometer gave up at about -40 degrees. I had a Type K thermocouple with a MAX6675 amplifier, but that chip can't read below zero C. I ended up buying a MAX31855K from Sparkfun, which allegedly can go to -200 C, right about where we want it. More on that later. I put a cheapo coffee travel mug under it and fired'er up! 

Aaaaand, not much happened, other than the mug sweating like a cynophobe at the Westminster Kennel Club. Turns out that insulation is a feature that some mugs lack, albeit they compensate with sweet wood paneling. I tried supplemental insulation (a layer of bubble wrap, four layers of floor underlayment insulation, and a towel for good measure), and that allowed the cold finger to get down to 133 K as measured by the newly improved thermocouple, but nothing was in the mug upon removing it from the cooler.

I broke down and bought the mug that claimed the longest time to keep its contents cold/hot that was available at my local Wally World, which ended up being a Thermos brand. I put it in and cranked it down. It hit a brick wall about 133 K also, which I though was strange, because the temperature had been dropping fast when it hit that temperature. I let it go about half an hour before guessing that perhaps the thermocouple calibration wasn't quite on and perhaps it was indeed in the 80-90K range and was stuck because it was condensing air.

Sure enough, when I took it off, I had a good bit of liquid in the bottom! Cold diggity dogg!

A few improvements in the works - I plan to calibrate the thermocouple by placing it in the original Dewar and then cooling it down with liquid air and monitoring the "real" measured temperature as reported by the cooler board when connected to the dewar to make a calibration curve for this thermocouple. Ultimately, I'd like to send a proper signal back to the control board so it can control the cooldown itself, by either finding the proper thermistor to connect or spoofing the signal to the control board. I also plan to fabricate some sort of assembly to decently isolate the cold head and thermos from the atmosphere, so I can introduce gases I want to liquefy into the thermos without air contamination.

Cryocooler

HO-LY-FA-ZO-LI. I'm on the brink of accomplishing one of my long-time desires - having a device capable of creating liquid nitrogen OUT OF THIN AIR (or 253-foot-above-sea-level air at least). Why? Why not, of course! How? Let's go there.

I'm now the proud owner of a Stirling cycle refrigerator (or cryocooler) capable of cooling something down to the neighborhood of 77 Kelvin, or 320 degrees Fahrenheit below zero. Assuming you're reading this on Earth, your atmosphere consists of something like 78% nitrogen, 21% oxygen, and 1% argon and other gases. By cooling down air to somewhere between 77 K (nitrogen's boiling point) and 90 K (oxygen's boiling point), air will begin to condense into a liquid mixture of nitrogen and oxygen. Trouble is, the coldest day in the chilling depths of South Texas winter is about 280 K, or about 200 degrees K or C too hot. Luckily, at the intersection of cell phones and superconductors, there was reason for a company called Superconductor Technologies to manufacture a fine piece of thermodynamics known as a Stirling Cycle Cryocooler.

Now that everybody has fourteen cell phones and smart watches apiece blasting out radio waves constantly, cell towers must be able to filter out all of the noise in order to provide service for each device. It turns out, for reasons I won't even pretend to give a hilariously naïve explanation of, that superconductors can help do a fantastic job of filtering only the target radio signal and amplifying it. Back in the 80's, some sharp grad students discovered that certain materials lose all resistance at temperatures as high (hah) as liquid nitrogen's boiling point. This finally culminated in the purpose for the folks at Superconductor Technologies to attempt to create a compact, reliable, low-ish cost refrigeration device capable of getting down to those temperatures, in order to cool superconducting filters. They did it so well that today you can buy ten year old devices on Ebay for $600 or so that are still perfectly functional. The ghost of Carl von Linde is probably beaming with joy. That these pieces of surplus telecom equipment had within them these cryocoolers was discovered by tinkerer extraordinaire Ben Krasnow and documented over at his blog, which is where I got my inspiration.

I bought a Superlink 850 superconducting filter off of ebay, which includes a Stirling cryocooler rated up to 150W input power. Like any refrigerator, it has a hot end and a cold end. The working fluid is high pressure helium. The piston and displacer are shuttled back and forth by a solenoid linear motor. Here's a paper that describes the workings.

The Superlink - some disassembly required

When I first received the cooler, I powered it up with a variable power supply (it needs about 27VDC to run) but the unit didn't cool. The seller was able to put me in touch with another cryocooler enthusiast who was able to help teletroubleshoot the problem down to a blown power transistor on the control board - shout out to Andrew for graciously donating his Friday night helping out a total stranger.

The toasted transistor kindly left a chalk outline around its dead body to identify himself.

After the seller shipped a new board, the cooler worked just as intended. Besides the troubleshooting, Andrew also offered some valuable tips about the cooler's operation, including a suggestion to use a 36V power supply adjusted down to the acceptable range of the cooler. I bought one off ebay capable of 11 amps output and able to adjust down as low as 28V out of the box, which is in the acceptable input voltage range.

I have some work to do to get it into a functioning air condensing machine, but it'll be fun along the way!

(Edit: Here's Part 2 and Part 3 of the Cryocooler Saga. Part 2 is pretty darn cool!)