Why Argon

Why Argon Works, by Larry "Harris" Taylor

With thanks to E. Maiken's article Why Argon? in Aqua Corps.

(Thermodynamic people, engineers and p-chemist's ... before you flame in
the rec.scuba tradition, let me point out that discussions about "degrees
of freedom",  heat capacities (Cv and Cp) etc, are beyond the scope of this
forum and the discussion below is an attempt to remove the thermodynamics
from  the explanation ... besides, I am an organiker and hated p-chem! (g)

Basically, heat always flows from the warmest body to the coldest until the
temperatures of the two objects are the same (This is the second law of
thermodynamics) So, whenever you enter the water, you are trying to warm
the entire lake, river, quarry, ocean, etc to your body temperature. This
heat loss, in attempting to warm the entire ocean, is one reason why
repeated diving, even in tropical water, can lead to hypothermia.

You cannot, in this universe, stop the flow of heat from you to the water,
but you can slow it down enough to survive and enjoy immersion - even in
cold bodies of water. Insulation is any material that slows this heat loss
... insulation can be thought of as "resistance" to heat flow.

In water, most heat loss is conductive (direct molecule to molecule
transfer of heat from skin to water ... more on this later), so in water,
it is easiest to think only about conductive loss. This resistance can be
defined mathematically as:

R =   T / (K(gas) + K(uw))

where

R is the resistance (insulation value)
T is the thickness of insulation layer between diver and water
K(gas) is the thermal conductivity of the insulating gas
K(uw) is the thermal conductivity of the underwear you are wearing

The larger the value of R, the less heat the diver loses per unit of
surface area over a given time and temperature.

Looking at the formula ... for a given underwear material (keeping K(uw)
constant), resistance to heat loss can be increased by either increasing
thickness of the insulation (to the point of immobility due to bulk) or
decreasing the thermal conductivity (how much heat flows from a surface
area per unit time) of the insulating gas.

Thermal conductivity as a percent of air at 1 ata and 300 K
(300 K is 27 C .... 10 C cooler than body temp which is around 37 C)
The higher the thermal conductivity, the faster heat is lost and the more
rapidly the diver will chill.

Air        100
Nitrox     100
H2         704         (H2 is hydrogen)
He         586         (He is helium)
Ar          68         (Ar is Argon)
CO2         54         (CO2 is carbon dioxide)

So, hydrogen and helium take heat away from diver, faster compared to air
... i.e.. they will not make a good choice for an insulating gas.

That leaves Ar and CO2 as the easiest and cheapest to use.

CO2 is undesirable 'cause CO2 plus water (and you cannot stop perspiration)
forms a weak acid, carbonic acid ... this is mildly corrosive and can lead
to rashes (minor chemical "burns") ... depending on how much water is in
the suit, but the groin area and the underarms are most susceptible to rash
formation.

Argon is the conveniently available and relatively inexpensive ... and uses
commonly available gas connection devices ... it will provide about 50%
more insulation than air.

BTW, if you use this gas, its a good idea to alternatively inflate and vent
the suit (to remove air and make sure that the gas in the suit doing the
insulation is mostly argon) prior to diving.

Since someone mentioned "mono-atomic" I will add a bit, without going too
much into what's called the Kinetic Theory Of Gases, about why monatomic is
important.

Basically, gases are in constant motion ... heat (or with some
simplification, temperature) is a measure of this motion.
So, in conductive heat loss, the skin molecules warm up those molecules
nearest to the skin ... some of these molecules collide with other
molecules and transfer energy (heat) from the "warmer" to the "cooler" ...
and so on ... eventually, enough heat is lost by this conduction mechanism
so that skin temperature drop can be measured.


Argon is a monoatomic gas  (made up of only single atoms)  ... this means
it is spherical in shape and has a limited number of ways that it can carry
heat ... you can imagine that the heat energy they carry is uniformly
distributed about their spherical shape.

Molecules more complex than monatomic i.e.. have more than one atom, are
connected by chemical bonds (as if each atom were a ball hooked to another
atom by a spring  (This spring is called a chemical bond)) This spring can
vibrate. This vibration (which monoatomic gases cannot do 'cause they have
no bonds) is one way of carrying heat energy.

In addition, molecules of more than one atom "rotate" ... i.e. as they
tumble thru space, this rotation (like a lumberjack rotating a log in a
millpond) also takes energy ... called rotational energy.

Bottom line: gases more complex than mono-atomic have more ways of carrying
heat energy (by rotating and vibrating around their chemical bonds) ...
this increased capacity to carry heat makes them a poor choice for
insulation.