Mailing List Archive Mailing List Archive
"Michael J. Black" wrote:
<snip a whole pile of shit>
Here you go Black, this is a freebee lesson with citations for
you, so that you won't look like such a moron on your next 'helium'
post.
A french astronomer, Pierre-Jules-César Janssen, first obtained
evidence for the existence of helium during the solar eclipse of 1868 in
India when he detected new lines in the solar spectrum. No element at
that time was known to give these lines and so it was apparent that the
sun contained a new element. This initiated a search for the new
element on earth. In 1895, Sir William Ramsay discovered helium in
clevite, a uranium mineral. The identification of helium was left to
Sir William Crookes and Sir Norman Lockyer. It was discovered
independently in clevite by Cleve and Langley at about the same time.
Lockyer and Frankland suggested the name helium from the Greek word
"helios" meaning "sun" (1).
Helium is a very light, inert, colourless gas element, having
the lowest known boiling and melting points. Helium is hypothesized to
be produced in the earth's crust by the radioactive decay of uranium and
other elements. It is also hypothesized that helium was produced during
the fusion of hydrogen when the earth was first formed. Either way, it
then gradually works its way into the atmosphere. Helium would be more
plentiful in air if not for the fact that its atoms are so light that
they keep escaping from the earth's gravitational field and moving off
into space.
Concentration of helium in the atmosphere represents
approximately five parts per million. Due to low atmospheric
concentrations, commercial helium is obtained from natural gas deposits
which contain helium in concentrations above 0.03% by volume.
Helium's main purpose as a diving gas, is to reduce the fraction
of nitrogen in a breathing mixture to reduce narcosis (2). Helium is
blended into a diver's mix to give a desired equivalent narcotic depth
compared to air. By reducing narcosis, a diver can retain motor skills
and mental concentration, thus adding safety to the bottom phase of a
dive. Adding helium (instead of nitrogen), also helps to reduce the
fraction of O2 in a divers' mix to make it surface hypoxic. At depth,
the diver will have less oxygen exposure.
The best-known effect of breathing helium is its speech
distortion. The thinner gas passing across the vocal cords at
atmospheric pressure produce a comical high-pitched squeak reminiscent
of Donald Duck. The thinner gas affects the speed of sound as well as
flow turbulence (3). In fact, any change of air density can produce a
similar effect. Divers at a pressure of 40 m (132 ft) in a
recompression chamber will also produce distorted speech. Helium's
speech distortion is only relevant when through-water communications are
being used. Descramblers, called helium speech unscramblers/descramblers
(HSU or HSD) are commercially available to translate this distorted
speech (4-9).
There is an apparent chilling during breathing. Dilutant gases,
such as helium or hydrogen, have low density but high specific heat
compared to nitrogen. This is again due to the thinner molecular
density of the gas, which transmits heat faster by direct conduction,
compared to air. The gas entering the diver's lungs will not conduct
heat out of the body as readily as air, there being fewer molecules to
warm up. Air by comparison is denser and may feel warmer when inhaled
at any given depth, but will transmit more heat out from the lungs (and
by that contribute more significantly to core heat loss) than helium
mixtures (10-14). The greatest contributor to convective respiratory
heat loss (Cr) is the temperature of inhaled gas more so than gas
density (15-17). Lung resistance (RL) increases are also linearly
related to cold-induced changes (18,19).
Where helium-based mixtures can contribute significantly to heat
loss is when they are used as drysuit inflation gases, but overall the
use of trimix or heliox in drysuit inflation should be avoided at all
times.
Helium's low molecular density has other practical advantages.
The thinner molecular structure of helium-based mixtures produces a
better regulator performance at depth by direct comparison with air.
Helium is less dense at 300 feet (91 meters) than nitrogen is at sea
level. The reduced density also makes breathing easier, and may help to
flush CO2 out of the lungs. CO2 has been implicated in deep water
blackouts (the Bohr Effect), and DCS. An increased partial pressure of
CO2 (PCO2) is dangerous. High levels can be reached in the lungs with
increasing depth, by improper breathing, increased gas density affecting
regulator, and pulmonary performance. Trimix/heliox can help reduce,
but not eliminate the problem (20).
Some divers have a misconception that isobaric inert gas
counter-diffusion plays some adverse part when helium is used as a dry
suit inflation gas. Physical properties of skin as a diffusion barrier
for helium render such arguments void of merit. There are other more
pressing issues against helium's use in a drysuit.
High pressure nervous syndrome (HPNS) is possibly the most
significant limitation to the use of helium as a diving gas. We do
know a little about HPNS, though the physiological process that creates
this syndrome is still not entirely understood. Helium has the lowest
lipid solubility and the lowest narcotic potency at 4.26. Xenon, which
has the highest narcotic potency at 0.039 is actually an anaesthetic at
atmospheric pressure, while krypton causes dizziness.
Helium makes nerve cells "aggressive", which leads to HPNS,
nitrogen makes nerve cells "depressed" which leads to narcosis. Thus
trimix is created as a balanced or balancing gas.
Pure helium should never be breathed during the blending
process, or at any other time. Irreversible asphyxiation may occur
because of the rapid diffusion of the gas into the lung tissues,
essentially blocking the passage of oxygen once the helium source is
removed. Helium can be breathed for months without tissue damage (21).
Helium for breathing purposes must be HP (hyper pure) or medical grade.
I've invested 25 minutes in this, so I hope you appreciate it
Black. Before you disgrace the medical profession anymore with your
childish posts; try reading some of this research first. Man, I don't
know what medical school you went to, but I'm sure glad I got my
education somewhere else.
--
Randy F. Milak
Windsor, Ontario
(1) Lide DR, (ed.) in Chemical Rubber Company handbook of chemistry and
physics, CRC Press, Boca Raton, Florida, USA, 77th edition, 1996.
(2) Berghage TE, et al. Decompression advantages of trimix. Undersea
Biomed Res. 1978 Sep;5(3):233-42.
(3) Pasterkamp H, Sanchez I. Department of Pediatrics and Child Health,
University of Manitoba, Winnipeg, Effect of gas density on respiratory
sounds. Am J Respir Crit Care Med. 1996 Mar; 153(3):1087-92.
(4) Dejonckere P, et al. Mechanism of initiation of oscillatory motion
in human glottis. Arch Int Physiol Biochem. 1981 May;89(2):127-36.
(5) McGlone RE, et al. Vocal register "shift" identification in a
modified breathing atmosphere. J Acoust Soc Am. 1981 Feb;69(2):597-600.
(6) Rothman HB, et al. Speech intelligibility at high helium-oxygen
pressures. Undersea Biomed Res. 1980 Dec;7(4):265-75.
(7) Hollien H, et al. Voice fundamental frequency levels of divers in
helium-oxygen speaking environments. Undersea Biomed Res. 1977 Jun;
4(2):199-207.
(8) Barry SJ, et al. Comparison of speech materials recorded in room
air at ground level and in a helium-oxygen mixture at a simulated
altitude of 18,000 feet. Aerosp Med. 1969 Apr;40(4):368-71.
(9) MacLean DJ. Analysis of speech in a helium-oxygen mixture under
pressure. J Acoust Soc Am. 1966 Sep;40(3):625-7.
(10) Hart JL. Salicylate hypothermia in rats exposed to hyperbaric air
and helium. J Appl Physiol. 1975 Oct; 39(4):575-9.
(11) Piantadosi CA, et al. Thermal responses in humans exposed to cold
hyperbaric helium-oxygen. J Appl Physiol. 1980 Dec;49(6):1099-106.
(12) Jammes Y, et al. Bronchomotor response to cold air or helium-oxygen
at normal and high ambient pressures. Undersea Biomed Res. 1988
May;15(3):179-92.
(13) Naraki N, et al. Respiratory heat loss under hyperbaric
helium-oxygen environment (101 bar). Ann Physiol Anthropol. 1984
Jul;3(3):227-36.
(14) Clarkson DP, et al. Thermal neutral temperature of rats in
helium-oxygen, argon-oxygen, and air. Am J Physiol. 1972 Jun;
222(6):1494-8.
(15) Burnet H, et al. Relationship between inspired and expired gas
temperatures in a hyperbaric environment. Respir Physiol. 1992
Dec;90(3):377-86.
(16) Brubakk AO, et al. Heat loss and tolerance time during cold
exposure in heliox atmosphere at 16 ATA. Undersea Biomed Res. 1982
Jun;9(2):81-90. (17) Berezovskii VA, et al. Effect of helium on gas
exchange and tissue respiration. Fiziol Zh. 1982 May; 28(3):353-8.
Review.
(18) Jammes Y; Burnet H; Cosson P; Lucciano M. Bronchomotor response to
cold air or helium-oxygen at normal and high ambient pressures.
Laboratoire de Physiologie Hyperbare GS 15-CNRS, Facult´e de M´edecine,
Marseille, France. Undersea Biomed Res, 15(3):179-92 1988 May
(19) Burnet H, Lucciano M, Jammes Y. Respiratory effects of cold-gas
breathing in humans under hyperbaric environment. Respir Physiol. 1990
Sep;81(3):413-23.
(20) Lanphier EH. (June 1958). Nitrogen-Oxygen Mixture Physiology,
Phases 4 and 6. Research Report 7-58. Navy Experimental Diving Unit.
Panama City, Florida 32407.
(21) Dejonckere P, et al. Mechanism of initiation of oscillatory motion
in human glottis. Arch Int Physiol Biochem. 1981 May;89(2):127-36.
--
Randy F. Milak
Windsor, Ontario
~Friends help you move; Real friends help you move bodies!~
--
Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]