cholesterol statin drugs, can only occur at low temperatures
approaching -100°C. Special cryogenic chemical reactors are
used to remove reaction heat and provide low temperature
environments for these reactions. Fast freezing of foods and
biotechnology products like vaccines, require liquid nitrogen or
liquid helium for blast freezing in immersion freezing systems.
Certain soft or elastic non-metallic materials become hard
and brittle at very low temperatures for which a machining
process, cryogenic milling (or cryomilling) was developed as
an option for processing materials that cannot be milled at
higher temperatures. The five most commonly used gases with
sufficiently low boiling points for initiating and controlling a
wide range of cryogenic processes are:
• Helium, (mixtures of Helium-3 / Helium -4) boiling point
from -270°C to 269°C;
• Hydrogen, boiling point 252,88°C;
• Nitrogen, boiling point -196,06°C;
• Oxygen, boiling point -182,97°C (air, boiling point -194,25°C
as mixture of – not one of the five gases); and
• Methane, boiling point -161,45°C.
Various sections of cryogenics have become established under
headings such as Cryobiology – biology involving the study
of the effects of low temperatures on organisms. Much of
the work done has been to develop procedures for achieving
cryopreservation, and in particular, for establishing practical
methods for the conservation of animal genetic resources.
When the objective of cryopreservation is for conserving specific
breeds, it is referred to as ‘cryoconservation’.
CRYOSURGERY
Cryosurgery is now frequently used or included in standard
surgical procedures to remove undesirable tissue materials,
particularly the malignant tissues of cancer cells.
• Cryoelectronics
The word is self-descriptive involving studying electronic
phenomena at cryogenic temperatures, particularly those
involving superconductivity. Practical applications of
cryoelectronics are now known as ‘cryotronics’.
• Cryonics
A recently adopted cryo – word for long held beliefs
that cryopreserving humans and animals can be done
by methods and procedures which will enable future
revival. Probably due to the similarities of the words
‘cryogenics’ and ‘cryonics’, they are often erroneously used
interchangeably in popular culture and the media.
The list of ‘cryo’ headings continues to increase as further work
is done at low temperatures across the spectrum of physical
and chemical research and applied technologies. With regard
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Dry Ice blocks.
to people working and carrying out studies in any of the
various cryogenic fields the single title of ‘cryogenicist’ is most
commonly used.
“Stored gases such as liquid
nitrogen, are now used for
a large number of diverse
specialty chilling and freezing
applications.”
In order to simplify and standardise temperatures, many
scientists as well as other people working in the field of
cryogenics no longer use Celsius or Fahrenheit temperature
scales with arbitrary zero points but rather Kelvin or Rankin
scales both of which measure from absolute zero. Both scales,
therefore, begin at -273.15°C as 0°R and 0 K. Rankin uses
Fahrenheit degree intervals, so taking an example of a cryogenic
temperature at -196°C, the Rankin reading would be 140°R and
Kelvin would be 78 K. Note that degrees Rankin are denoted as
°R in the same way that Celsius is shown as °C, whereas there is
no degree symbol used for Kelvin; just the number of degrees
above absolute zero followed by K. In practice, however, it will
probably take several years or even decades before usage of
Celsius reduces to negligible levels.
Amongst the many notable low temperatures previously
not regarded as cryogenic prior to adoption of the upper
cryogenic limit at -50°C is the coldest temperature ever
recorded on earth; -89.2°C measured at Vostok, Antarctica on
July 21, 1983. Another substance found at a similar temperature
Continued on page 55
RACA Journal I January 2020
53