Monday, December 28, 2015

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.

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