The Dewar's efficiency depends upon reducing the heat transfer between its outer and inner walls across an evacuated space.  Heat is transferred across this space by radiation and by conduction through the residual gas.

Dewars have been in use since the late 1800's and have served multiple purposes in many fields.  In the oil exploration field, Dewars are used differently than the original Dewar.  Although they serve the same purpose of keeping the external heat from reaching the internal contents, the Dewar used in the oil field is subjected to much higher temperatures than the Dewar used in cryogenics.  These higher temperatures produce "outgassing" of the surfaces of the materials contained in the vacuum space of the Dewar, thereby requiring different ways of keeping a high vacuum in this annular space. Most of N=K=W’s efforts in the research and development of flasks, has been spent in engineering flasks which are rugged, and conserve a high vacuum pressure.

The ends of the flasks are normally configured to interface with the customer's hardware. A basic simple flask (the same as the coffee thermos) has only one opening at one of it's ends, and require a cork (or 'stopper' or 'insulator') installed in this opening, so as to complete the thermal enclosure of it's contents.

Through-wire routing for the flask's electronic payloads require variations from the basic single ended design, and include:
(a) Wiring passages alongside the external tube (making the flask eccentric to the Housing's centerline).
(b) Wiring passages through an internal hole (enclosed by a bellows). This is the recommended way.

The relative cost of a simple flask is moderate, and like with everything else, the individual 'per flask' cost of a large batch, will be cheaper than ordering just one (and only) unit.