Wednesday, January 30, 2013

CryoBUG Demo Unit Achieves -156°C

I ran a few different charges and kept hitting a ceiling around -148°C. I began to suspect that not only was the static heat load greater than the previous prototype (actually this was a no brainer), but that it was probably more attributable to radiant heat coming off of my vacuum vessel wall, and not conduction as I had originally suspected. This became more clear when I noticed that the bellows section of my vacuum vessel was also getting very cold (see image below).

Click image to Enlarge
In fact the entire bellows appeared to be evenly cold on both ends, something that seemed a bit odd. So I started thinking that perhaps I should give some super-insulation a try, in order to minimize the influence of radiation.
For those not familiar with super-insulation, it simply refers to a method of shielding or blocking the radiant energy emanating from an external heat source. In most cases this involves using multiple layers of a metallic foil like sheeting separated by an insulating material such as mylar or nylon.

Luckily a place that I subcontract to, had some old vacuum jacketed lines laying around that had been disassembled, which just happened to have an insulation material very similar to what is pictured to the left. Since my cold head is fairly small, I only needed a few feet to test out my theory. As it happened one of the lines had begun to unravel at one end, so I snipped off what I needed and wrapped this material around my CryoBUG's cold head, and then re-installed and evacuated the stainless steel bellows.

I then restarted the CryoBUG's compressor, and allowed it to cool down. Within 50-55 minutes I was seeing temperatures drop down below -150°C, and in just barely over an hour, it leveled off at -156°C. finally I was back in my target temperature range, and this was only with an 80-90 micron vacuum. Probably with a few more layers of super-insulation and a better vacuum, -160°C would be possible.

Mytek Controls Prototype Heat Load Controller
Knowing that I was now ready to run some heat load tests, resurrecting an old load controller project of mine seemed like a good idea.

This was a prototype of a 6000+ watt radiant heat load controller that I had stopped development work on about 2 years ago when I ran into problems with the linearity and sampling speed of the power transducer. I have since gotten a much better (as in also more expensive) watt transducer, but just hadn't gotten around to swapping it out for the bad unit, or finishing the embedded software code.

Now seemed like as good a time as ever, so I spent about two days making the hardware changes, and then embarked on setting up my laptop to communicate with the on-board processor chip. This of course took way longer than it should have, but eventually I got everybody talking to each other.

Next I started tweaking with the code, and was able to get a fairly stable output at the low wattage levels that I would be needing (10-100 watts). The only problem was that the new transducer required a divide-by-four circuit to adjust it's 0-10 Vdc output signal down to the A/D converter's 2.5 Vdc analog input limit. I used some 1% resistors to create a voltage divider, but with the transducer's 1.5 mv per watt output, the stability and drift became an issue. So I am presently waiting for some instrumentation op amps to arrive with which I can fashion a precise and stable voltage divider from.

Other things on my agenda, will be to come up with a vacuum electrical feed-through for connection to a load resistor attached to the cold head. Once this is done, I can run a heat load capacity curve on the CryoBUG.