Cryogenics for English Majors
Table of Contents
Cold Enough for Ya?
The ice was here, the ice was there,
The ice was all around:
It cracked and growled, and roared and howled,
Like noises in a swound!
As we just learned, helium is the product of the sun’s nuclear fusion. But it’s a loooooong trip down the thermometer (way past Coleridge’s ice storm) before you get from the heat of the sun’s core – (millions of degrees, whether Fahrenheit or Celsius!) to the temperature at which helium will finally cease being a gas and turn into a liquid. That point is just a wee bit above absolute zero – the absolute coldest possible temperature. (Scientists, by the way, tend not to talk in terms of cold, but in terms of heat and the lack thereof; cold is simply the relative absence of energy in the form of heat. At absolute zero, there is absolutely no heat.)
PHYSICS FACTOID: Unlike helium, another cryogen, nitrogen, is abundant on Earth. In fact, it makes up about four-fifths of every breath you take!
Scientists, especially in cryogenics, talk about temperatures in terms of the Kelvin scale (plain old “K” for short), which starts at absolute zero (0 K). Helium boils at 4.2 K (-452.11 degrees Fahrenheit, or -268.95 degrees Celsius). That can be a little hard to picture, when everything else on the planet, at that temperature, has long since frozen solid. And unless you really subject it to high pressure – another variable that can affect phase changes – helium will never, ever even think about turning into a solid.
Not that you would find 4.2 K on Earth, except in a laboratory. In fact, those temperatures don’t even exist in our solar system. Far-flung Pluto, 38 K at its very coldest, is balmy in comparison. The table below gives an idea of the range of temperatures in the universe, depicted in the three common scales used to measure them.
Highs and Lows
|
|
Location
|
°C
|
°F
|
K
|
|
Sun's Core
|
15.5 million
|
27 million
|
15.5 million
|
|
Temperature required for thermonuclear fusion of hydrogen
|
2.8 million
|
5 million
|
2.8 million
|
|
Earth's Core
|
7,277
|
13,040
|
7,500
|
|
Surface of Venus (the hottest planet), at its hottest
|
467
|
872
|
740
|
|
Sun's Surface
|
5,538
|
10,000
|
5,811
|
|
Moon's surface, at its hottest
|
127
|
260
|
400
|
|
Boiling point of water
|
100
|
212
|
373
|
|
Highest temperature ever recorded on Earth (Libya, 9/31/1922)
|
58
|
136
|
331
|
|
Highest temperature ever recorded in US (Death Valley, 7/10/1913)
|
56.7
|
134
|
330
|
|
Average year-round temperature in Tallahassee, FL
|
20
|
68
|
293
|
|
Freezing point of water
|
0
|
32
|
273
|
|
Lowest temperature ever recorded in US (Alaska, 1/23/71)
|
-62
|
-80
|
211
|
|
Lowest temperature ever recorded on Earth (7/22/83, Antarctica)
|
-89.2
|
-128.6
|
184
|
|
Moon, at its coldest
|
-173
|
-280
|
100
|
|
Liquid nitrogen
|
-196
|
-321
|
77
|
|
Pluto, at its coldest
|
-235
|
-391
|
38
|
|
Liquid helium
|
-269
|
-452
|
4.2
|
|
Space between galaxies
|
-270
|
-454
|
3.15
|
|
Absolute Zero
|
-273
|
-460
|
0
|
|
Now that you appreciate the temperature extremes we’re talking about, you can begin to understand the effort it takes to make liquid helium.
First off, though abundant in our sun and other stars (in fact, after hydrogen, it is the second most abundant element in the universe), helium is relatively rare on Earth. Even after you do find the gas, you’ve got to turn it into liquid, into Coleridge’s precious drops, to get it cold enough for our magnets.
Next Page Good to the Last Drop: A Recipe for Liquid Helium
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