heh.
I have the answers I wanted.
The first shot shows the dry ice merrily bubbling away in the Iso ;
here's the last shot I took before it failed :
The temp sensors maxed at -48 and -40 as it shows here , and my exterior temp sensor maxd at -40 .
I estimate the liquid temp was about -52c when she bit the dust...
I'm going to let it slowly warm up , then try to find out whats died.