Chris Nicholas wrote in message ...
All the installations I have seen use a standard vacuum flask with no
additional insulation, but they were all buried either beneath/behind
the instrument panel, or inside the fuselage - anyway in shadow.

A key element I have read about, and all mine have been fitted with, is
copper pan scourers loosely filling the interior of the flask, so that
temperature variation within the capacity is elinimated.

It's interesting to note that these two measures have very different
effects. The insulation ensures adiabatic expansion, which means that
the temperature of the air drops as the pressure falls. The copper is
there to make sure that the temperature stays the same.

Isn't thermodynamics fun?

Ian