Lance’s Nitrox Notes
Some readers have asked for my Nitrox notes. So here they are:
What is Nitrox and how is it different from air?
Air: 21% O2 79%N. Therefore air is Nitrox (NO2).
But for our purposes diving Nitrox has between 22% and 40% O2.
O2 is actively metabolized by the body while N is
inert and always being absorbed by the tissues of the body (blood is a tissue).
More N gets absorbed during diving and this is directly related to the risk of
DCS and is a critical factor in determine dive depths and times.
Advantages of
Nitrox
Due to the depleted N:
·
Nitrox may allow for extended bottoms times
compared to air at the same depth
·
Nitrox may provide decreased surface interval
times allowing for more dives to be completed in the same time period compared
to air.
·
Nitrox may decrease the risk of DCS due to the
decrease N in the breathing gas.
·
Nitrox may decrease the no fly time incurred by
diving air.
Other Names
·
NO2
·
EAN 32 or EAN 36 (The two most commonly used
blends.)
·
NOAA I and NOAA II
·
De-Nitogenated
Air
·
Safe Air
·
Oxygen Enriched Air or Enriched Air
(Experiments with Nitrox started in the 1800s but it was
NOAA and the US Navy which made is a useful dive tool. It was not until 1995
that is Nitrox was widely accepted by recreational training organizations.)
Principals of
Pressure
1.
Ambient Pressure is the force pressing on an
object, diver or gas. At the surface of the ocean we have the weight of the
atmosphere pressing downward on us. It is measured in the metric system as 1
bar equivalent to 1 Kilogram per sq cm. The Imperial System abbreviates it as 1
atm equivalent to 14.7 psi. For practical purposes these are considered equal.
While at depth the diver must take into
account both the weight of the atmosphere and the water. Because water is much
denser than air even very minor adjustments in depth will make significant
changes in pressure. The total weight of the atmosphere and water are known as absolute
pressure or ambient pressure.
Rather than thinking in depth it is
advantageous to think in pressure. (All depths represent depth in seawater.)
|
Depth-Pressure
Relationship
(Increments based on Pressure)
|
||
|
Depth
|
Pressure
|
|
|
Metric
|
Imperial
|
bar/atm
|
|
0 m
|
O
ft
|
1/1
|
|
10 m
|
33
ft
|
2/2
|
|
20 m
|
66
ft
|
3/3
|
|
30 m
|
99
ft
|
4/4
|
|
40 m
|
132
ft
|
5/5
|
The above table is useful for increments
equal to one atm. But we do not dive at these preset depths.
Depth
to Pressure Calculations
As we are used to Imperial Calculations I
will present in that format.
Remember 33 feet of sea water = to 1 atm
PLUS the atmosphere itself. So simply divide the depth by 33 and add one for
the atmosphere.
P = (D/33)
+ 1
P =
(99/33) +1 = 4 atm
Pressure
to Depth Calculations
Pressure to Depth Calculations are just a
straight forward; just take a way 1 from your pressure and multiply by 33 to
find your depth.
D =
(P-1) x 33
D =
(4-1) x 33 = 99 feet
Boyle’s
Law
This is the first law that a Nitrox diver
should understand. Boyle’s Law describes the affect of ambient pressure on a
diver’s breathing gas. Simply stated; the volume of a gas in a flexible
container is inversely proportional to the pressure being exerted on the
container. It is important to note here the amount of gas molecules remains
constant in the container.
A simple example of Boyle’s Law may be
demonstrated like this: If the pressure on a flexible container is doubled the
volume will be halved. Conversely, if the pressure is halved, the volume will
double. Divers usually think of this in terms of over expansion injuries. Nitrox
divers need to think about Boyle’s Law in terms of on and off gassing (which is pressure dependent).
Boyle’s Law can be mathematically expressed
as:
P1V1=P2V2
Where the subscript is used to designate
the beginning and ending values of Pressure and Volume.
|
Pressure-Volume-Density
Relationship
|
||||
|
Depth
|
Pressure
|
Volume
|
Density
|
|
|
Metric
|
Imperial
|
Bar/atm
|
|
|
|
0 m
|
0
ft
|
1/1
|
1
|
x1
|
|
10 m
|
33
ft
|
2/2
|
1/2
|
x2
|
|
20 m
|
66
ft
|
3/3
|
1/3
|
x3
|
|
30 m
|
99
ft
|
4/4
|
1/4
|
x4
|
|
40 m
|
132
ft
|
5/5
|
1/5
|
x5
|
An important fact that is being
demonstrated is; in a gas filled flexible container, the pressure of the gas
inside the container is equal to the ambient pressure outside the container. It
is this homeostatic nature that allows gas to expand and contract. And of
course, Bole’s Law has no effect in a ridged container, such as a SCUBA
cylinder.
The tissues of the body are largely a
non-compressible liquid, and are not directly impacted by Boyle’s Law. The
lungs and connected spaces are a flexible container. The instant the gas leaves
the regulator it becomes subject to Boyle’s Law. For Nitrox divers, the most
important issue is the gas inside the diver’s lung will be equal to the ambient
pressure surrounding the diver.
Dalton’s
Law
For the Nitrox diver, Dalton’s law tells us
each individual gas in a mixture has its own specific pressure. This is called
the Partial Pressure and for oxygen is abbreviated PO2 and for
nitrogen PN2. Pressure is
expressed in terms of atmospheric pressure as bar or atm.
Dalton demonstrated the total pressure
exerted by a mixture of gases is equal to the sum of the partial pressures
exerted by the sum of the total gases in the mixture. And each component gas
accounts for its fraction of the total pressure, in direct proportion to the
fraction of that gas present in the total mixture. In other words; in a Nitrox mixture of 32%
oxygen and 68% nitrogen the oxygen exerts 32% of the pressure and nitrogen
exerts 68%of the pressure. In a SCUBA cylinder of EAN 32 at 3,000 psi oxygen
exerts 320 psi and nitrogen 680 psi. This is expressed as FO2 and PN2.
The other important part of Dalton’s Law
for divers is; gases will move to an even pressure throughout a space. That is
they move from an area of higher partial pressure to equalize with an area of
lower partial pressure until the gases are equally co-mingled.
Understanding these two aspects is
important for the discussion on Henry’s Law and membrane pressure gradients.
|
Partial
Pressures of EAN 32
|
||||
|
Depth
|
Pressure
|
PO2
|
PN2
|
|
|
Metric
|
Imperial
|
Bar/atm
|
Bar/atm
|
Far/atm
|
|
0 m
|
0
ft
|
1
|
.32
|
.68
|
|
10 m
|
33
ft
|
2
|
.64
|
1.36
|
|
20 m
|
66
ft
|
3
|
.96
|
2.04
|
|
30 m
|
99
ft
|
4
|
1.28
|
2.72
|
|
40 m
|
132
ft
|
5
|
1.60
|
3.40
|
While the fraction of a gas in a mixture
remains constant in a mixture, its partial pressure varies dramatically.
·
As you can see in the table above partial
pressure of oxygen, in a flexible container, at 3 atm is roughly equivalent to
breathing 100%oxygen at the surface when using EAN 32.
·
Breathing EAN 32 at 3 atm is approximately
doubling the amount of nitrogen compared to the surface.
If we were to look at air on the same table we would see
that the oxygen exposure reaches an approximate equivalent of 100% at
approximately 5 atm. Nitrogen exposure from air at 3 atm is roughly equivalent
to the nitrogen exposure of EAN 32 at 4 atm.
This is the key to the advantages of using Nitrox. There is
simply less nitrogen exposure at depth with Nitrox than with air. Therefore the
physiological effect of nitrogen at depth is diminished.
Determining
the Partial Pressure of a Gas
Simply
multiply the total partial by the fraction of the gas. (G represents the gas
e.g. oxygen or nitrogen.)
PO2 = P x FO2 ~or~ PN2 = P x PN2
Henry’s Law
Henry’s
Law illustrates the mechanisms by which gas moves in and out of tissues of a
diver’s body. Henry discovered the specific quantity of gas that will dissolves
into a liquid is dependent on two factors:
1.
The partial Pressure of the gas
2.
The coefficient of that gas in a particular
liquid (This coefficient is a mathematical variable that demonstrates different
liquids absorb the same gas, into solution at different rates and in different
quantities.)
According to Henry’s Law, when a partial pressure of a gas
is increased, additional gas will be dissolved into the liquid. This happens
where the gas comes into contact with the liquid, normally with the surface of
the liquid or through a membrane such as an alveolar wall. This takes time. Where
there is a chemical interaction the time can be significantly shorter. The
actual rate will vary depending on the pressure gradient. (This is the
difference in partial pressure in the gaseous gas and the dissolved gas.)
As long as the total partial pressure remains constant, any
sudden change in the partial pressure of an individual gas will merely
accelerate the on-gassing or off-gassing. However, if there is a sudden
decrease in the total pressure exerted upon a liquid, it may cause some of the
dissolved inert gas to come out of solution while still within the liquid and
from bubbles.
The solubility of any gas in liquid is directly proportional
to the pressure exerted on it. When the pressure is doubled, the amount of gas
that can be dissolved is also doubled.
|
Gas Solubility in a Liquid
|
|
|
Partial Pressure of Gas
|
Maximum Quantity of Gas
|
|
bar/atm
|
In solution
|
|
1
|
x1
|
|
2
|
x2
|
|
3
|
x3
|
|
4
|
x4
|
|
5
|
x5
|
Gas Dynamics
Gases continually travel between our lungs and tissues,
transported by the blood, moving from areas of higher partial pressure to lower
partial pressure. They are effectually following the laws of physics seeking to
equalize their own partial pressure.
The gases enter the blood stream via the lung’s alveoli, and
are transported to tissues where the lower partial pressure of the gas “draws”
the newly arriving gas in. When the partial pressure of the gas in the blood is
less than that of the tissues the gas is drawn into the blood and exits the
body via the same alveolar pathway. This off gassing will begin to occur at
some point during the diver’s accent to the surface and will continue until
there is no longer a pressure gradient between the diver’s tissues and the
ambient partial pressure of the gas.
Nitrogen Dynamics
Nitrogen is not used by the body in any fashion. It is
merely absorbed by tissues according to the laws of physics. When a diver
spends sufficient time at sea level (1atm) the partial pressure of nitrogen in
the his tissues will equalize to 1 atm. This is referred to as “saturated”.
That is, the tissues have absorbed all the nitrogen possible under these
circumstances.
When the diver descends, the partial pressure of nitrogen
will increase with depth. It will be greater in the lung space than the blood
creating a pressure gradient. Now the “on gassing” process will begin. The
amount of on gassing will depend on the depth and time of the dive. If the
diver was to stay a given depth sufficiently long, the partial pressure of
nitrogen in his tissues will equalize with the ambient partial pressure of
nitrogen, and again, he will be “saturated”. In the case of tissue saturation,
off gassing will immediately begin when the ambient partial pressure of
nitrogen is less than the partial pressure in the diver’s tissues. This is why
there is a “no fly time” after diving.
Most aircraft are pressurized to equal an 8,000 foot elevation. That is
approximately equal to ¼ atm or 8.25 feet of sea water. (It is a difficult to
be exact because 3/4of the earth’s atmospheric mass is within 36,000 feet while
the upper boundaries of the atmosphere are approximately 400 miles with no
definite boundary with space!)
Decompression
Sickness
At the end of every dive the diver is “super-saturated”.
That is, he is carrying more nitrogen in his tissues than the ambient partial
pressure. … He is not done off gassing. There is a limit to the body’s ability
to carry this excess nitrogen in tissues. This is where the dive tables and
computers come in. Generally, staying
within these parameters will prevent DCS.
DCS occurs when the partial pressure of nitrogen absorbed
nitrogen is suddenly greater than the ambient partial pressure of nitrogen. The
time for nitrogen to cross membranes and exit the body is insufficient for the
partial pressures to equalize. Now the laws of physics cause the nitrogen to
come out of solution into its gaseous state. At first, the smallest amounts of
gaseous nitrogen begin to form micronuclei. As the micronuclei bump into each
other they eventually form bubbles. These bubbles cannot escape the tissues and
become lodge therein. These bubbles may interfere with bodily functions causing
symptoms. This is Decompression
Sickness.
There are predisposing factors to DCS which make predicting
it very difficult. They include: body fat (not just obesity), dehydration (your
body needs fluids to carry the gases), elevated Carbon dioxide levels (from
working hard on a dive or poor lung function from smoking or other lung
disease), old age and diminished fitness.
Oxygen Dynamics
The movement of oxygen from your lungs to your tissues is
completely dependent on pressure gradients created by partial pressures. Oxygen
is readily metabolized by your body and is completely vital. It is accordingly
very difficult to saturate your tissues with oxygen. Because the byproducts of
these metabolic process are CO2 and H2O, pressure gradient for oxygen is towards
the tissues. Due to the lack of CO2 in our breathing gas, its
pressure gradient carries it out of the body.
Haldane’s Theory
The British scientist was approached by the Royal Navy to
come up with depth and time limits for the Royal “Hard Hat” divers. Haldane
theorized nitrogen loading by creating multiple “compartments” that load and
unload (on gas and off gas) in half times. A half time here is the time, in
minuets, it takes for a compartment to go for the starting saturation half way
to a new saturation based on the new depth. This is an exponential progressing.
Haldane used algorithms mimic the tissues absorption. As it turns out he was
pretty darn accurate. His work is the bases today for all decompression theory,
dive tables and computers. I have included the tables showing his original half
life theory.
|
60 Minuet Compartment
On Gassing
|
|
|
Elapsed Time
|
On Gassing Completed
|
|
Start
|
0%
|
|
1 Hr
|
50%
|
|
2 Hrs
|
75%
|
|
3 Hrs
|
87.5%
|
|
4 Hrs
|
93.8%
|
|
5Hrs
|
96.9%
|
|
6 Hrs
|
98.5%
|
|
5 Minuet Compartment On Gassing
|
|
|
Elapsed Time
|
On Gassing Completed
|
|
Start
|
0%
|
|
5 Minuets
|
50%
|
|
15 Minuets
|
75%
|
|
20 Minuets
|
87.5%
|
|
25 Minuets
|
93.8%
|
|
30 Minuets
|
96.9%
|
|
35 Minuets
|
98.5%
|
Maximum Operating
Depth: MOD
(Calculation for MOD)
For CNS exposure of
1.5 max on EAN 32
Oxygen Partial
Pressure ÷ FO2 =
Pressure in ATA (Bar)
1.5 ÷ 0.32 = 4.6875 ata (bar)
Pressure in ATA (Bar)
1.5 ÷ 0.32 = 4.6875 ata (bar)
(Pressure in ATA - 1) × 33ft = MOD in
feet
(4.6875 - 1) × 33 = 122 fsw
(4.6875 - 1) × 33 = 122 fsw
(Pressure in Bar - 1) × 10m = MOD in
meters
(4.6875 - 1) × 10 = 37 msw
(4.6875 - 1) × 10 = 37 msw
