Joseph Herbert on fri 6 jan 06
“ I doubt that anyone can categorically state that either of the given
examples is inherently more dangerous than the other. What is curiously
common in them is the use of 0 C as an endpoint.”
I am a little hesitant to wade into this pond, things can get pretty
philosophical pretty quickly.
Temperature scales, in and of themselves, are harmless things; arbitrary
divisions of the range of molecular velocities that we perceive as heat or
temperature. Fahrenheit chose “blood heat” for 100 degrees and some other
common “standard” and ended up with a degree size that had water freezing at
32, water boiling at 212 and cone 10 at 2380. The more “rational” Celsius
scale chooses water freezing for 0 water boiling for 100 and the two scales
meet at –40. All measuring the various degrees of molecular velocity. Once
the idea of no molecular movement sprang forth, no movement must mean no
temperature or absolute zero and we have the Kelvin (Celsius sized degrees)
and Rankin (Fahrenheit sized degrees) scales with the absence of temperature
as their starting points.
None of this matters until materials of construction and people become
involved. We have expectations of how things will behave based on our life
experiences and most of those take place in a pretty narrow temperature
band. Potters often deal with higher temperature, as do glass makers and
metal workers. Even so, our expectations are built on the things we touch
and move and eat.
An example of the problem with our perception and how we loose our way when
conditions are outside our experience is the famous “Null ductility
transition temperature.” People had been building things of steel for many
years when WWII came along and got them to building more things faster.
Also some of those things were sent places that there had not been so much
of a need to go before. It happened that a ship, built of some particular
composition of steel, was to join a convoy of ships going from Halifax, Nova
Scotia to England one winter. The ship, which had had some life in warmer
climes already, came to the harbor, sat in the 30 (or –2) degree water and
broke in half just sitting there. The investigation showed that the square
port that someone had made didn’t help because it concentrated the stresses,
but the real reason the ship cracked apart and sank at the pier was the
temperature of the water. The particular steel the ship was constructed of
had a peculiar property that caused it to loose ductility, the ability to
deform without breaking, at a relatively high temperature. It turns out
that many steels have this characteristic but at lower temperatures, not
normally found in the world as we experience it. There was a feeling among
sailors at the time that some ships that were lost in the North Atlantic
failed in this way rather than being lost to the weather or military action.
Certain stainless steels are mixed phases, two different structures in the
one piece of metal. The proportion of these phases changes with
temperature. Surprisingly, to us inhabitants of the narrow molecular
velocity band, the change in proportions continues to change even as the
metal is cooled to liquid nitrogen temperatures. These odd changes of metal
characteristics with temperature change and rate of temperature change has
produced a myriad of names for heat treatment processes and their products.
In the case of glass, and ceramic glazes are mostly glass, the material is
already a super-cooled liquid at “normal” temperatures. The molecular
structure is disordered as in a liquid, and the properties of the material
vary more or less continuously as it cools, there are no phase changes as
glass cools. In that vein, there are no particular effects as the glass
cools further. It is brittle at room temperature and is brittle at lower
temperatures. One might find that the flexibility of glass, when cooled to
cryogenic temperatures, is less than that at room temperatures. After all,
at 1700 degrees, glass is pretty flexible.
Anyway, my point here is that the concern is not with the changes in
temperature, per se; it is with the effects on the particular materials that
are there when the change takes place. For some materials, it will matter
not a bit; for others, it is catastrophe.
The concept of temperature and the way we think about it is interesting. It
really is a measure of the velocity of molecular motion. For this reason,
the temperature in outer space is often quoted as being rather high, like
50,000 degrees. Well, the occasional molecule that goes by is traveling at
a velocity that does translate to that temperature. The temperature of that
molecule is indeed that big number. We more often experience molecular
motion in bulk, gasses that have enough molecules close together that we can
feel them on our face. Just in case you were thinking that the wind blowing
made the temperature hotter, molecules having “normal” temperatures are
traveling at thousands of miles per hour. 3600 comes to mind.
Our molecular velocity here is depressed slightly tonight. It will be about
20 degrees.
Joseph Herbert
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