Liquid Nitrogen Packet
Temperature: A measure of how “hot” or “cold” something is in degrees.
Also a measure of the average Kinetic energy of a substance.
Temperature does not seem to have an upper limit. (Interior of sun = 27 million ˚F)
Ice is hot to objects that are colder than the ice. (below 0˚C)
Temperature Scales:
There are four temperature scales used in the world.
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Celsius scale: based on the melting and boiling point of water.
Kelvin scale: similar to the Celsius scale adjusted so the 0 measurement is equal to absolute zero.
Fahrenheit scale: original temperature scale that seems to work well for human functions like cooking or
weather.
Rankine scale: similar to the Fahrenheit scale adjusted so the 0 measurement equals absolute zero.
Absolute Zero: The theoretical temperature at which the atoms or molecules of a substance have the least
possible energy. (Lowest possible temperature)
Equal to - 273.15 ˚C or - 459.67 ˚F
Liquid Air:
A product made by greatly reducing the temperature of air until it turns to a fluid.
Nitrogen (78%) , Oxygen (21%) and Argon (1%) are the principle components of air.
Liquid air cannot be measured with an ordinary mercury or alcohol thermometer.
A platinum resistance thermometer is used which measures the effect on the electrical resistance
of platinum.
Platinum’s conductivity changes with temperature changes.
Making Liquid Air:
The process of making liquid air is based on the fact that compressed air becomes cooler when it
expands.
Procedure:
1. Compress air with a compressor. (This creates heat)
2. Remove heat produced by compressing air with a heat exchange system.
3. Send compressed air to an expansion engine (a piston that rapidly increases the volume of
the gas) or a larger open area.
4. Some of the air liquefies and the remaining air is recirculated through the process until
enough liquid air is collected.
5. Separate the Nitrogen from the Oxygen using their differences in boiling point.
6. You now have liquid nitrogen.
Cryogenics:
The study of matter at temperatures much colder than those that occur naturally on earth
Cryogenic temperatures are those below the boiling point of methane (-285.3 ˚F or -176.1 ˚C)
Some cryogenic fluids are:
Oxygen (-183 ˚C), Nitrogen (-196˚C), Hydrogen (-253 ˚C), and Helium (-269˚C).
Cryogenic Articles
Temperature is how hot or cold something is as measured on a particular scale. The concept of temperature
is closely related to the flow of heat between two connected objects of different temperatures. Heat always
flows from the hotter object to the cooler one.
Instruments that measure temperature are called thermometers. A scale marked on the thermometer indicates
each level of “hotness.” The two most common temperature scales used on thermometers are Fahrenheit and
Celsius. Temperatures on all scales are based on the International Practical Temperature Scale of 1968.
Scientists often speak of thermodynamic temperature, a fundamental physical quality completely
independent of the properties of a substance. The unit of thermodynamic temperature is the Kelvin, indicated
by K. This unit was agreed upon by scientists from many nations. All temperatures are based on their position
above or below the freezing point of water (273.15 K or 0C). The triple point of water happens when water is at a
temperature of 0.01 ˚C and a pressure of 0.61 kPa. At this temperature and pressure, water, ice, and water
vapor all exist together in equilibrium. In common practice, a temperature is actually expressed in terms of its
difference from the melting point of ice at standard pressure conditions of one atmosphere of pressure (760 mm
Hg or 101.3 kPa).
Temperature does not seem to have an upper limit. Scientists believe the temperature at the center of the sun
is about 15 million degrees Celsius (27 million degrees Fahrenheit). However, the interior of any star larger
than the sun is probably much hotter. On the other hand, there does seem to be a lower limit to temperature.
This theoretical limit, called absolute zero, has a value of – 273.15 C or – 459.67 F. At this temperature, the
molecules and atoms of a substance have the least possible energy.
Absolute zero is the theoretical temperature at which the atoms and molecules of a substance have the least
possible energy. This temperature, which scientists believe is the lowest attainable, equals–273.15C or
-459.67F. The use of these values for absolute zero is based partly on observations of the relationship between
the temperature and pressure of a gas. When the temperature of a gas confined in a fixed volume is lowered, its
pressure decreases in direct proportion (as though it would become zero at –273.15C).
A temperature scale that has absolute zero for its zero point is called an absolute temperature scale. One such
scale is the Kelvin scale, the international standard for scientific temperature measurement. On the Kelvin scale,
absolute zero equals zero Kelvin (0 K). The word degree and the degree symbol () are generally not used with
Kelvin temperature readings. For example, 20C equals 293.15 K. An absolute temperature scale related to the
Fahrenheit scale is the Rankine scale. This scale is used in the United States for certain kinds of engineering work
(like the aerospace industry). A Rankine temperature is obtained by adding 459.67 to a corresponding
Fahrenheit temperature. For example, 68F equals 527.67R.
Physicists theorize that it is impossible to attain a temperature of precisely absolute zero. The lowest
temperature recorded so far, about 0.00001 K, was reached by cooling certain metal alloys. The nuclei of these
alloys were magnetized at extremely low temperatures in a magnetic field. When the magnetic field was
removed, the nuclei became demagnetized and their temperature dropped to near absolute zero.
Liquid Air is a product made by greatly reducing the temperature of air until it turns into a fluid. Air
becomes liquid at about -190C. Liquid air is so cold that it boils when poured over ice, which is almost 200C
hotter. The liquid is bluish and looks like water. Liquid air, like the air we breathe, consists of about 78%
nitrogen, 21% oxygen, and 1% argon.
Scientists use liquid air in cryogenics, the study of temperatures of -100C and lower (see Cryogenics). Liquid
air is considered a cryogenic fluid because of its extremely low temperature. It is a primary source of liquid
forms of nitrogen, oxygen, and other gases. Scientists use liquid nitrogen in biology, chemistry, and physics
research. Liquid oxygen and hydrogen are primarily used in compact, high-energy fuels for rocket engines that
power spacecraft.
Properties of Liquid Air: Liquid air affects different substances in a variety of ways. For example, liquid
mercury becomes as hard as steel when liquid air is poured over it. A racket ball that has been dipped in liquid
air will shatter when bounced. A lead bell, which normally makes a dull sound, will temporarily produce a
clear tone after being exposed to liquid air.
Scientists use liquid air to study the effects of extremely low temperatures on the strength of certain
substances. Such materials as iron and plastics temporarily become brittle after being dipped into liquid air.
However, copper and brass become tougher upon immersion in the fluid. Exposure to liquid air also makes
metals better conductors of electricity and increases the strength of certain types of magnets.
Scientists measure the temperature of liquid air with special thermometers because mercury and alcohol
thermometers cannot be used. The contents of such thermometers freeze at temperatures much higher than that
of liquid air. The most accurate and widely used thermometer that measures the temperature of liquid air is the
platinum resistance thermometer. This type of thermometer measures temperature by determining its effect on the
electrical resistance of platinum. Platinum becomes a better or poorer conductor of electricity as its temperature
changes. Another kind of special thermometer, the constant-volume gas thermometer, measures the effect of
temperature on the pressure of a gas kept at a certain volume. Such gases as helium or neon are used to
measure the temperature of liquid air because they turn into liquid at lower temperatures than air does.
Nitrogen and oxygen, the two major parts of air, can be separated and used in their liquid form by distilling
liquid air. When liquid air is heated, the nitrogen turns into a gas before the oxygen does because the boiling
point of nitrogen is lower. After the nitrogen has been removed, the remaining substance consists mostly of
liquid oxygen. The high oxygen content of undistilled liquid could cause an explosion if a flammable material
came into contact with it.
Making Liquid Air: The process of making liquid air is based on the fact that compressed air becomes cooler
when it expands. This cooling effect was described in detail in 1853 by two British physicists, James Prescott
Joule and William Thomson, and it later became known as the Joule-Thomson effect. In 1877, Louis-Paul Cailletet,
a French physicist, liquefied air for the first time.
In 1895, the German chemist Carl von Linde invented a commercial process for liquid air production based
on the Joule-Thomson effect. Linde’s method is still used today but with many improvements. Compressors
raise the air pressure in a chamber to about 3,000 pounds per square inch (210 kilograms per square centimeter).
Compression heats the air, and so water jackets on the compressor, plus a device called a heat exchanger, are
used to lower the compressed air’s temperature before it is liquefied.
Air can be liquefied in one of two ways. In one method, called Joule-Thomson expansion, the compressed air
flows through a series of throttling valves into increasingly larger chambers. The pressure and temperature of
the air decreases in each chamber as the air expands. In the final chamber, some of the air has become cold
enough to condense into a liquid. The cold vapor from this chamber is circulated around the other chambers to
help cool the air that is still going through the liquefying process.
In 1902, Georges Claude, a French engineer, developed the second method of liquefying air. This method
resembles the Joule-Thomson expansion but is more efficient because it makes use of work done by expanding
air. In the Claude method, air enters a chamber and pushes a piston as it expands. As the piston moves, the
volume of the chamber increases, which causes the air pressure and the temperature in the chamber to decrease.
The air is sent through a series of these piston-equipped chambers, called expansion engines, until it becomes a
liquid.
Special containers called Dewar flasks protect liquid air from heat and evaporation. A Dewar flask is a bottle
made of two layers of glass. There is space between the layers of glass to insulate the contents. The flask may
be coated with silver to reflect heat. Large quantities of liquid air for industrial use are stored in huge insulated
tanks. Most liquid air is manufactured by the Claude process. (See Diagram)
CRYOGENICS: The study of matter at
temperatures much colder than those that occur
naturally on earth is called cryogenics. In the
cryogenic temperature range, air becomes a liquid
or even a solid and living tissue freezes instantly.
At the very lowest temperatures of the cryogenic
temperature range, matter behaves strangelyliquids run uphill and electric current never stops
flowing.
Since steel becomes brittle at even Arctic
temperatures, scientists make their lowtemperature equipment out of such metals as
copper or aluminum. Also, silicone rubbers replace
natural rubber, which is as brittle as glass at liquid
nitrogen temperatures.
One of the most widely applied cryogenic
changes is the liquefaction of gases. Liquid air has
been distilled to produce pure oxygen, nitrogen,
neon, and other gases. These are much easier to
transport and handle as liquids, since as gases they
must be kept under great pressure. Rockets would
be far heavier if their hydrogen-oxygen fuel
supplies were gases rather than liquids.
Nonreactive liquid nitrogen is one of the safest
cooling agents available. It is used to control the
temperature of scientific experiments and to
preserve biological samples. In cryosurgery, liquid
nitrogen is used to kill unhealthy tissue by freezing.
Temperature is a measure of how energetically
atoms are moving—traveling, rotating, vibrating.
As a substance gains heat, its atoms move more
quickly. As it loses heat, they slow down. If the
substance were cooled far enough, its atoms would
cease to move (except for a theoretical “zero-point
motion” that is a permanent property of matter).
At that point the substance would have no motion
or energy left to give up.
The lowest possible temperature is the same for
all substances. Called absolute zero, it occurs at 273.16˚C, or -459.69˚F. The Kelvin and Rankine
temperature scales use it as their zero point.
Degrees on the Kelvin scale, commonly used in
cryogenics, have the same range as degrees of the
Celsius or centigrade scale. Degrees on the Rankine
scale, used in the aerospace industry, have the same
range as degrees of the Fahrenheit scale.
On an extremely cold Alaskan winter night,
temperatures might drop to – 70˚F (around 252 K).
The cryogenic temperature range begins at an
indefinite point much lower than this. The boiling
point of oxygen (90˚K) is in the high end of the
range. Since even liquid hydrogen and helium are
now produced commercially, scientists can begin
cryogenic experiments with liquid helium, which
has a boiling point of 4.2 ˚K. Temperatures below
0.001 ˚K have already been achieved. The
theoretical limit for one method is 0.0000005 ˚K.
Although such temperatures appear to be very
close to absolute zero, that temperature can never
be reached. The third law of thermodynamics
states this to be impossible. Just as there is no
largest number—since no matter how large a
number is written, another digit can be added to
make it even larger—so too there is no smallest
number. No matter how low a temperature is
reached as 0.001 ˚K shrinks to 0.0000001 ˚K and
below, temperatures even lower will always exist.
Reaching Supercold Temperatures:
Under natural conditions a substance is cooled
by placing it next to something colder. However, if
the substance is to be made colder than any source
available in nature, work must be done to remove
heat, and insulation must be provided to keep the
substance cold.
Cryogenic cooling techniques use similar energy
transfer principles. Energy must first be introduced
into a system (by compressing a gas, for example).
This raises the temperature of the system. The
excess heat energy flows out to the surrounding
area, which cools the system to its original
temperature. Then the system is insulated and
made to do work (by allowing the gas to expand,
for example). This step uses up some of the
remaining heat energy and cools the system below
the temperature of its surrounding.
Special techniques are used to reach
temperatures around the millidegree (0.001 ˚K)
level. These techniques make use of the
relationship, described by the second law of
thermodynamics, between the amount of energy
and the amount of disorder, or entropy, in a system.
According to the second law, disorderly atoms
within a system may be made more orderly (the
entropy may be decreased) only if energy from
outside the system is applied to the system.
Cooling by demagnetization depends on this
relationship. A strong magnetic field provides the
outside energy to line up the molecules of a
paramagnetic crystal. This also raises the
temperature. When the crystal cools back to the
starting temperature, the field is removed and the
molecules swing back to a disorderly arrangement.
The energy they use to move comes from the heat
energy of the crystal, so the temperature drops.
Unusual properties of helium at low
temperature make other cooling methods possible.
For example, at certain temperatures liquid helium
is more ordered (has less entropy) than solid
helium, which can be formed only under pressure.
If liquid helium is compressed at one of those
temperatures, it solidifies. But, unlike normal
substances, helium at these special temperatures
must absorb heat (increase its entropy) causing the
temperature to drop when it solidifies. This
technique does not require a magnetic field, which
could interfere with some kinds of experiments.
Superfluidity and Superconductivity:
Above 2.2 ˚K liquid helium acts like a normal
cryogenic fluid, but below that temperature,
helium-4, the most abundant isotope, acquires
superfluidity. Superfluid helium runs quickly
through tiny holes that would slow normal liquids;
defies the laws of gravity by creeping up the sides
of a container; and conducts heat a thousand times
more efficiently than copper.
At varying temperatures below about 20 ˚K,
some metals and metal alloys become
superconductors. When the transition temperature
for a given metal is reached, its electrical resistance
completely disappears. Presumably an electric
current could run forever through an aluminum
ring cooled to a temperature of about 1 ˚K or
through a lead ring cooled to about 7 ˚K. The
transition temperatures of some alloys are even
higher—an alloy of niobium, aluminum, and
germanium is superconducting at 20.7 ˚K.
One industrial application of cryogenics is the
superconducting electromagnet. Most strong
electromagnets require far too much electricity—
enough to supply an entire town—to be practical.
Once started, however, a superconducting
electromagnet needs no further electrical input.
Practical problems include maintaining low enough
temperatures throughout a large installation and
keeping the magnetic field from destroying the
superconductivity.
Superconducting computer elements are another
industrial application of cryogenics. Cryotrons, the
original cryogenic computer elements, consisted of
a straight core wire that had another wire coiled
around it. Both wires were superconducting; a
strong current in the coiled wire set up a magnetic
field that stopped current flow in the core wire.
More recently developed cryogenic computer
elements switch from one superconducting state to
another, a faster process.