[0037] The refrigerant mixture should consist of R142b (22 oz.), R134a (20 oz.), R508b or R508a (18.2 to 19.7 oz.), R14 (16.7 to 17.5 oz.) and R740 (14.6 to 17.1 oz.) to achieve a freezer of -140.degree. C. to -154.degree. C.


Found this on a patent site and didn't pics yet of the physical setup - but I thought it would be interesting to share. They actually give the mix.

I didn't read everything so I'm not sure how stages it had to get these temps (2?) and I haven't found the diagrams yet - will look tonight if someone doesn't post them first.