The View Up Here

Random scribblings about kites, photography, machining, and anything else

Catching Up

Posted by Tom Benedict on 15/10/2010

I missed most of Worldwide KAP Week 2010.  A few weeks previous I’d come down sick with a cold, and just never quite shook off the cough.  It didn’t help that my job took me to the cold dry air at 14,000′ elevation over and over (and over!) during and after the cold.  In the end I came down with walking pneumonia.  This left me with no energy and a wracking cough through most of WWKW 2010.  I did get out, I did send pictures in to the book, but it wasn’t the happy time I’d planned.  C’est la vie.  Until next year.

In the end I was sick for well over two months.  I finally got my clean bill of health today, and should return to duty at the summit tomorrow morning.  Meanwhile I’ve been working on a cryostat at headquarters, and finally started flying kites again about a week ago.  Unfortunately all this came together in a not so comfy way today.  It all ended well, but getting to the end of the day was a trial.

At lunch I broke a spar on my Premier Kites Widow.  Plain and simple.  Dumb crash, dumber re-launch, and now I have to cut a new P-200 spar once I get home.  Thank goodness for spares!

The real fun has been this cryovessel.  I was testing the idea that you could boost the performance of a closed-cycle cooler by using thermoelectric coolers, or Peltier junctions, between the closed-cycle cold head and the cold surface.  It worked, after a fashion, and I think it bears re-examining with a properly sized cascaded Peltier junction.  But for our application I just couldn’t get enough cooling power out of the thing to get the delta-temperature I was after.

During this testing I wound up putting something like 30W of power through a two-stage cascaded cooler, but the cold head simply couldn’t remove the heat fast enough.  Within a few minutes I had a cold head at -150C, a cold surface at -100C, and a Peltier junction at a soaring 38C.  I killed power, brought everything back down, and let things hit steady-state before killing the power to the cooler.

What that told me, though, was that the biggest delta-temperature in the system was across the link between the cold head and the cold surface.  I also realized if I minimized the dT, I could switch the gas in my cooler and possibly hit my target temperature that way.  Time to make new parts!

So I replaced that link with a new one.  The old one consisted of ten strips of 6.5″ x 1.0″ x 0.010″ copper in a nice neat stack, or 6.5″ of 0.100″ sq in copper.  I replaced it with 2.8″ of 1.5″ diameter copper.  At 60W of load the first one got me about 52C dT.  With this new one I should be able to keep that under 2C of dT.  If the cold head reaches its ultimate temperature of -158C, this should give me a cold surface temp of -156C.  If that happens, I can change gases and potentially hit the 77K, or -196C temperature I’m after.  Time will tell.

For the record, I hate machining copper.  Oxygen free high conductivity copper is even worse.  It’s like machining stale bubblegum.  Tools dig instead of cutting, the copper oozes out of the way instead of making nice chips, and it takes all manner of tricks to pull off clean cuts.  I got the parts made, but the final steps of tapping the M3 holes in the thing gave me the willies.  After a thorough cleaning, I opened up the cryostat, installed the new parts, and closed it back up.  It’s on the pump now, and I should have a chance to start cooling it over the weekend so I can work with it on Monday.

But what I’m really looking forward to right now is a new spar for the Widow, a weekend of good weather, and a chance to get out and make up for what I missed during Worldwide KAP Week 2010.

– Tom

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