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I keep hearing the statement that breathing helium mixtures cause a diver
to lose more heat than breathing air. This is a major MYTH, and it is
totally wrong. Breathing helium mixtures at technical diving diving
depths does not cause significant increases in diver cooling as compared
to air (in fact, if we only look at the energy required to heat the gas,
helium requires less energy than does air).
One of the problems is that people look at values of specific heat that
are presented as heat capacity per gram of gas, and these numbers show
that the heat capcity (for a given mass of gas) is greater for helium
than it is for Air. However, when we breathe, we are moving essential
the same VOLUMES of gas in and out of our respiratory system, not the
same mass. It is this volume of gas that is important when considering
heat loss via the gas in the respiratory tract.
The values of specific heat [kJ/(kg)(C)] are as follows:
Air 1.01 kJ/(kg)(C)
Helium 5.19 kJ/(kg)(C)
Oxygen 0.92 kJ/(kg)(C)
Nitrogen 1.04 kJ/(kg)(C)
Thus, if we compare the heat(energy) required to change the temperature
of a kilogram of gas 1 degree C, it is obvious that helium requires
about 5 times as much energy as does air.
However, if we look at the Specific heat [cal/(mole)(C)], we see
a much different picture.
Some values for Specific Heat (cal/mole degree) are:
Hydrogen 3.39
Helium 4.968
Neon 4.968
Nitrogen 6.95
Oxygen 6.97
Air 6.96 (extrapolation from O2 and N2 values)
Argon 4.968
As the values are expressed in moles, at at a given temp and pressure
a mole of any gas will occupy a standard volume, then when comparing
comparable volumes we can use these specific heat values. For a
1 cu.ft. per minute ventilatory rate (surface consumption rate), we
are looking at approximately 70 calories/min for breathing air and
somewhere around 65 calories/min (if the breathing gas temp is 10 degrees
C colder than body temp) for breathing 80/20 heliox (again, these values
are strictly for heating the inspired gases, and ignore humidification
and conduction heat losses.
However, there is some fact in this myth, and it is when we start
talking about using Helium mixtures for drysuit inflation. In a
drysuit, it is not heat capacity, but heat conductance that is of
critical concern.
Thermal conductivity (in calories per degree per sq. cm per second)
is as follows:
-5
Hydrogen 39.7 x 10
-5
Helium 34.0 x 10
-5
Neon 11.0 x 10
-5
Nitrogen 5.66 x 10
-5
Oxygen 5.83 x 10
-5
Air 5.70 x 10 (extrapolated)
-5
Argon 3.92 x 10
-5
Heliox 80/20 28.37 x 10 (extrapolated)
As you can see, pure helium conducts heat approximately 6x faster than
air, and heliox (80/20) conducts heat approximately 5 times faster than
air.
Argon conducts heat 68.7% as fast as air, and only 13% as fast as heliox
80/20. Thus, argon is better than air, but air is definitley viable
alternative to argon in drysuit inflation except for the longest and
coldest hangs.
Hope this helps clear things up -
Take care and safe diving -
John
Submariner Research, Ltd.
(johncrea@de*.co*)
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