A minimal jargon explanation, by Herr Direktor Funranium. Written while consuming a carbonated, no H2O Manhattan. Standards of insobriety, with corresponding misspellings and grammatical errors, are being upheld.
The stupid simple, “Go away, yer botherin’ me, boy” explanation of how a dewar works is that it’s like a really, really good scientific-grade Thermos(TM). The average person will nod and walk away, no better informed than they were a moment before. That kind of behavior encouraging ignorance is inexcusable and is dragging down the march of progress. We at Funranium Labs wish to enlighten…and drink. When combined, drunken enlightenment of others is also known as raconteurism. Telling a tale while wildly gesticulating, cocktail in hand, is the way we roll here. I seem to have gotten distracted. Where’s my drink?
Anyway, it would be more proper to describe a Thermos(TM) as a cheap commercial knockoff of a dewar. I’ll let the Thermos(TM) wikipedia article defame them properly for me. A beverage container that was a vacuum no better than a tenth of an atmosphere is hardly better than the old styrofoam beer can cozies. They rely primarily on the coefficient of thermal conductivity of select, proprietary insulating materials (usually a styrofoam-like substance) to minimize thermal transfer from the ambient atmosphere to your drink. Ah, I may have lost you there. Let us take a trip down High School Physics Memory Lane. Please make certain you are wearing pants before wandering the hallways and classrooms of yesteryear…*BEGIN WAVY LINES OF FLASHBACK*
There are three ways to transfer heat from one body/location to another. They are conduction, convection, and radiation. As we flip to the end of the chapter of definitions:
Conduction: transfer of heat directly from body to another by molecular motion. This is entirely dependent upon the physical/chemical properties of the materials in question on how well heat transfers from one body to the next.
Convection: transfer of heat by mass transport of warm fluids/gasses mixing with cooler ones. See Also: your oven (no, not your mom. I know you were expecting that joke though)
Radiation: transfer of heat by the emission of energy from a warm body as electromagnetic radiation (i.e. light).
Now, let’s review why dewars keep things at a constant temperature, not just cold. To the extent that you can reduce the three modes of heat transfer, you keep whatever at its starting temperature longer. The whole point is to keep temperatures as constant as possible; this is why another term for a dewar is “cryostat”. If it is hot coffee, it stays hot. If it is a frosty Manhattan, it stays refreshingly cold to the last drop. How does it reduce the transfer?
- It is made of two layers of material that are very good insulators (don’t conduct heat) with a seriously hard vacuum in the space between them.
- It comes with a lovely insulating foam lid.
- It is silvered on all possible surfaces containing the fluid.
Good scientific grade dewars are built of borosilicate glass which is fairly good about not conducting heat compared to metal and plastic (see also: most beer cans and cheap thermoses), reducing heat transfer by this route. Similarly, you need a vacuum to prevent both conduction and convection; if there is nothing there with which to conduct heat or convect, then you won’t have conduction and convection in those directions. Good quality scientific dewars will have a vacuum at least as low as .0000001 atmospheres of pressure, as opposed to the 1 to .1 atmospheres most Thermoses are (an atmosphere is defined as the outside air pressure at sea level at standard temperature, 68°F, and 20% relative humidity). The silvering is to help reflect all energy that your fluid could be absorbing from or emitting to the outside world, reducing radiation. Really, the only direction to thermal transfer via convection of warmed or cooled air, conduction to the air/fluid interface or radiation into open space is straight up from the top of your fluid. If you slap a decently insulated lid on top, well, you haven’t eliminated all of those but you’ve reduced them as much as you can such that you can still drink your beer.
How well does all this work, you might ask? At sea level, liquid nitrogen (AKA: LN2 for people in the biz) exists as a -196°C/-321°F fluid; it readily boils off in seconds when poured on the normal room temperature floor, which tends to crack the floor tiles and makes the maintenance staff grumpy. As a cryotech, I am very familiar with the situation where a bench top dewar by is sitting on the counter filled with LN2, lid on, and will still have LN2 in it three days later… much to my surprise when I bumped into it by accident.
THAT is why I use dewars to make these steins.