Water and the Physical Laws That Affect All Divers
What Are The Important Differences Between Air And Water?
1) Water is much heavier than air. A cubic foot of air weighs 1/12
pound (lb). A cubic foot of fresh water weighs 62.4 lbs and a cubic
foot of seawater weighs 64 lbs.
2) Water molecules are made up of hydrogen and oxygen, chemical
symbol H2O. Each water molecule contains two atoms of hydrogen
and one atom of oxygen. Salt water contains salt (sodium chloride,
chemical symbol NaCl) and other minerals in solution (i.e., dissolved
into it, not chemically combined with the water).
Salt adds to the weight of water, and for this reason sea water has
slightly greater weight - and hence pressure - than fresh water (sea
water contains approximately 35 pounds of salt for every 1000 pounds
of water).
It takes a depth of 34 feet of fresh water to equal one atmosphere of
pressure, as opposed to 33 feet of seawater.
3) Air is a mixture of gases, principally oxygen (O2, 21% of the air by
volume) and nitrogen (N2, 78% by volume). Each gas exerts its own
independent pressure, the sum of which equals the total air pressure
(Dalton's law). Unlike water, air (and any other gas or mixture of
gases) is compressible; the greater the pressure exerted, the more
tightly packed together are the individual gas molecules. Regardless of
the air pressure, however, water molecules are much more tightly
packed together than air molecules. Compared to air at sea level
pressure (1 atm.), water is about 800 times denser.
4) Just as air has weight and exerts pressure on all sides of an object in
the atmosphere, water exerts pressure around any object immersed in
it. We can push water out of the way because its weight is distributed
on all sides and the molecules can be easily moved. The resistance we
feel under water reflects the extreme density of water (compared to
air), and the fact that it takes time for water molecules to move out of
the way.
5) Water pressure, like air pressure, is a function of weight; the deeper
one goes the greater the surrounding water pressure. The marked
increase in water pressure with depth affects every scuba and
non-scuba diver, indeed anyone who goes under water (unless inside a
heavy vessel with walls that resist pressure, such as a submarine).
6) Since water is not compressible, unlike air it does not become
denser as pressure increases. A cubic foot of water at 130 feet depth has
the same weight and density as a cubic foot of water at 33 feet. In
contrast, a cubic foot of air at sea level weighs more than a cubic foot
of air in the Rocky Mountains or the Himalayas.
7) Differences between air and water, in both weight and density,
predict a radical difference in pressure with changes in altitude/depth.
Seawater pressure changes one whole atmosphere every 33 feet (every
34 feet for fresh water). Provided they are of the same circumference, a
column of seawater 33 feet high weighs the same as a column of
earth's entire atmosphere.
8) Water freezes, whereas air does not freeze at any temperature
occurring in nature. Seawater freezes at a much lower temperature
than fresh water because of the dissolved salt, which slows down the
formation of water crystals.
WHAT DOES BOYLE'S LAW PREDICT ABOUT CHANGES IN
PRESSURE WITH DEPTH?
It is always convenient to illustrate Boyle's law with balloon models.
Balloons are round and easily compressible. By taking an air-filled
balloon under water, Boyle's law can be nicely demonstrated
Assume a balloon is filled with 12 liters of air at the surface, and then
taken under water. The balloon shrinks and its air becomes denser with
increasing depth; this is because the pressure increases with depth and
the balloon is compressible. On ascent the opposite happens; the
balloon re-expands and the air density returns to baseline.
WHAT DOES BOYLE'S LAW PREDICT ABOUT BREATH-HOLD
DIVING?
So much for balloons. What about the body? Underwater, we can view
the body as made up of two groups of organs that differ in how they
respond to water pressure. One group of organs is compressible by the
water pressure, and the other group is non-compressible.
Bone, muscle, blood and solid organs such as the kidney, heart, and
liver are all non-compressible and therefore unaffected by water
pressure; these organs and tissues have the same (or higher) density
than water and can withstand intense water pressure without any
problem. Were our bodies made up of only non-compressible
structures, going under water would be a lot simpler (though still not
hazard-free).
The compressible areas contain some air and include the lungs, middle
ears, sinuses, nasal passages, interior of hollow organs (stomach and
intestines), and any air pockets you may not know about (e.g., a
tooth cavity). If these parts of the body did not contain air they would
not be compressible. Conversely, any part of the body that is
compressible under water must contain air or some other gas.
At the moment a diver holds his breath the mass of air in the body is
fixed; no new air can enter or leave. Given that the mass of air is fixed
at that point, Boyle's law predicts that the compressible spaces will be
affected by changes in pressure. A fixed mass of gas (the diver's
air-containing spaces) subjected to an increase in pressure underwater
will compress (shrink in size). But by how much?
Assume a typical breath-hold diver's lungs contain 8 liters of air at
full inhalation (one liter equals about one quart). At 33 feet his lungs
could theoretically shrink to half the sea level volume, or 4 liters, if
they behave just like a balloon. The same percent shrinkage could
occur in the sinuses, middle ears, and all other compressible body
spaces, if they too behave like balloons.
However, our air-containing spaces don't behave exactly like balloons;
they are not as easily and evenly compressible. The lungs are supported
by a bony rib cage, the sinuses are embedded in the skull, only part of
the middle ear is exposed to water pressure (the tympanic membrane),
etc. Also, trained breath-hold divers can transfer some air from the
throat to the middle ear (via the Eustachian tube), retarding the
shrinkage of that small space.
So, on the one hand, our compressible air spaces are much more
complex than the simple balloon models used to illustrate Boyle's law.
On the other hand, we do have compressible air spaces and Boyle's law
predicts that they must shrink in size if the outside pressure is greater
than the inside pressure. It is just impossible to predict exactly how
much they will shrink, or how such shrinkage will affect the individual
diver.
If the breath-hold diver goes too deep, the spaces will shrink so much
that blood starts leaving the capillaries to fill in the spaces. The result
can be lung, sinus or nasal hemorrhage, or a ruptured tympanic
membrane. With ascent, all the breath-hold diver's shrunken spaces
will expand back to their original size. Because there is no over
expansion, there is no risk of blowing out a lung or other air space.
Breath-hold diving, practiced for centuries, is still employed for both
recreation and commerce (ama divers of Japan and Korea, pearl divers
of Hawaii, resort divers of Mexico, sport divers of California). It
requires special skills beyond the ability to hold one's breath, such as
withstanding the squeeze of descent and quickly accomplishing a goal
when the desired depth is reached. Scuba requires none of these skills
but others, of a different sort, that are easy to learn. For most people,
scuba is much easier to master than breath-hold diving.
WHAT DOES BOYLE'S LAW PREDICT FOR THE DIVER BREATHING
COMPRESSED AIR?
Water contains oxygen (both dissolved and as part of the water
molecule, H2O). Unlike fish, land animals are not equipped to extract
the dissolved or free oxygen and use it for breathing. Like all other
mammals (including the aquatic variety such as dolphins, porpoises
and whales), humans under water are cut off from life-supporting
oxygen.
For centuries the only underwater option was breath-hold diving, an
activity practiced only by the most daring and hardy. For most people
even 20 seconds under water can seem like eternity. The advent of
scuba made it possible for just about anyone with healthy lungs and
heart to stay under water for long periods. (This advance has not come
without a price. Compressed air diving presents two physiologic
problems breath-hold divers don't worry much about: decompression
sickness and air embolism.)
Boyle's law predicts that the compressible spaces of a breath-hold diver
will compress because the amount of air in these spaces (i.e., the total
number of air molecules) is fixed at the point of breath-hold; hence,
as pressure increases the volume occupied by the air must decrease.
Does Boyle's law also apply for scuba divers? Absolutely; after all, a law
is a law. But the consequences are different because scuba divers
breathe compressed air under water. First, the amount of air (the
number of air molecules) in each of the scuba diver's compressible
spaces increases along with the water pressure. Second, the volume of
each compressible space remains fairly constant throughout the dive.
Thus even though a space is compressible it should not be compressed
during scuba diving. While diving, the extra molecules of air that
enter these spaces (lungs, sinuses, middle ears) allow them to
maintain the same pressure as the surrounding water pressure. At a
given depth, though the ambient pressure is increased the volume in
any compressible space remains constant because the gas density
increases.
HOW MUCH AIR IS IN A SCUBA TANK?
Scuba dives can remain under water because they carry a supply of air.
The amount of air carried in a tank depends on its size and filling
pressure. Most tanks used for recreational diving are designed to carry
anywhere from about 60 to 100 cu. ft. of air; the typical tank found in
most resorts carries 80 cubic feet (cu. ft.) when filled to 3000 psi. If
the tank is filled to a lower pressure the volume of air it contains will
be less.
HOW DOES AIR PRESSURE CHANGE FROM THE SCUBA TANK TO
THE DIVER'S LUNGS?
Sea level pressure is 14.7 psi. An 80 cu. ft. tank at 3000 psi contains
80 cu. ft. of air that has been compressed 204 times (3000 psi/14.7
psi) higher than sea level pressure. If you tried to breathe air at 3000
psi it would blow you away; you couldn't do it. The scuba diver is able
to breathe tank air by virtue of a two-stage regulator system that 1)
steps the pressure down to a level slightly above ambient, and then 2)
delivers the air at ambient pressure.
Table 3 shows examples of regulator and airway pressures for various
depths. The first stage regulator brings the pressure down to ambient +
a predetermined pressure. The pre-determined pressure is set by the
regulator's design, but is generally 120-140 psi. Thus the pressure in
the hose between first and second stages is 120 to 140 psi higher than
the ambient pressure.
The second stage regulator contains a demand valve that requires only
a slight inspiratory effort to open; when the diver inhales on the
mouthpiece attached to the second stage, the demand valve opens and
air enters the lungs at ambient pressure. Note that the air is at the
pre-determined pressure immediately upon leaving the second stage
(i.e., 120-140 psi), but it rapidly reaches ambient by the time it is
inhaled (Figure 5). The second stage regulator is also designed so that
airflow ceases when the diver exhales. (When airflow doesn't cease on
exhalation it is said to "free-flow," a problem that can usually be
corrected by adjusting the regulator or briefly occluding the
mouthpiece.)
Ambient pressure is determined solely by depth, and is the pressure
inside the diver's lungs when breathing with scuba equipment. Tank
pressures for a given depth will vary depending on the rate of air
consumption and duration of the dive; at each depth in this table a
tank psi of 1500 is shown as example. The first stage regulator lowers
tank pressure to ambient plus some intermediate pressure determined
by the regulator's design, in these examples 140 psi.
Recreational divers use "open circuit" scuba, which means all exhaled
air, enters the surrounding water; none is re-breathed. The value of
1500 psi for tank pressure in the table, for each depth, is shown as
example only.
Obviously, as the dive progresses the amount and
pressure of air in the tank will decrease. The rate at which the tank's
air volume and pressure decrease is a function of the diver's
ventilation rate (how much air is breathed per minute), depth, and
length of time under water.
Note that regardless of the tank's psi, as
long as it is above some minimum value the first and second stage
regulators will deliver air at the ambient pressure. (Not all regulators
work the same at very low tank pressures.)
What Are The Important Differences Between Air And Water?
1) Water is much heavier than air. A cubic foot of air weighs 1/12
pound (lb). A cubic foot of fresh water weighs 62.4 lbs and a cubic
foot of seawater weighs 64 lbs.
2) Water molecules are made up of hydrogen and oxygen, chemical
symbol H2O. Each water molecule contains two atoms of hydrogen
and one atom of oxygen. Salt water contains salt (sodium chloride,
chemical symbol NaCl) and other minerals in solution (i.e., dissolved
into it, not chemically combined with the water).
Salt adds to the weight of water, and for this reason sea water has
slightly greater weight - and hence pressure - than fresh water (sea
water contains approximately 35 pounds of salt for every 1000 pounds
of water).
It takes a depth of 34 feet of fresh water to equal one atmosphere of
pressure, as opposed to 33 feet of seawater.
3) Air is a mixture of gases, principally oxygen (O2, 21% of the air by
volume) and nitrogen (N2, 78% by volume). Each gas exerts its own
independent pressure, the sum of which equals the total air pressure
(Dalton's law). Unlike water, air (and any other gas or mixture of
gases) is compressible; the greater the pressure exerted, the more
tightly packed together are the individual gas molecules. Regardless of
the air pressure, however, water molecules are much more tightly
packed together than air molecules. Compared to air at sea level
pressure (1 atm.), water is about 800 times denser.
4) Just as air has weight and exerts pressure on all sides of an object in
the atmosphere, water exerts pressure around any object immersed in
it. We can push water out of the way because its weight is distributed
on all sides and the molecules can be easily moved. The resistance we
feel under water reflects the extreme density of water (compared to
air), and the fact that it takes time for water molecules to move out of
the way.
5) Water pressure, like air pressure, is a function of weight; the deeper
one goes the greater the surrounding water pressure. The marked
increase in water pressure with depth affects every scuba and
non-scuba diver, indeed anyone who goes under water (unless inside a
heavy vessel with walls that resist pressure, such as a submarine).
6) Since water is not compressible, unlike air it does not become
denser as pressure increases. A cubic foot of water at 130 feet depth has
the same weight and density as a cubic foot of water at 33 feet. In
contrast, a cubic foot of air at sea level weighs more than a cubic foot
of air in the Rocky Mountains or the Himalayas.
7) Differences between air and water, in both weight and density,
predict a radical difference in pressure with changes in altitude/depth.
Seawater pressure changes one whole atmosphere every 33 feet (every
34 feet for fresh water). Provided they are of the same circumference, a
column of seawater 33 feet high weighs the same as a column of
earth's entire atmosphere.
8) Water freezes, whereas air does not freeze at any temperature
occurring in nature. Seawater freezes at a much lower temperature
than fresh water because of the dissolved salt, which slows down the
formation of water crystals.
WHAT DOES BOYLE'S LAW PREDICT ABOUT CHANGES IN
PRESSURE WITH DEPTH?
It is always convenient to illustrate Boyle's law with balloon models.
Balloons are round and easily compressible. By taking an air-filled
balloon under water, Boyle's law can be nicely demonstrated
Assume a balloon is filled with 12 liters of air at the surface, and then
taken under water. The balloon shrinks and its air becomes denser with
increasing depth; this is because the pressure increases with depth and
the balloon is compressible. On ascent the opposite happens; the
balloon re-expands and the air density returns to baseline.
WHAT DOES BOYLE'S LAW PREDICT ABOUT BREATH-HOLD
DIVING?
So much for balloons. What about the body? Underwater, we can view
the body as made up of two groups of organs that differ in how they
respond to water pressure. One group of organs is compressible by the
water pressure, and the other group is non-compressible.
Bone, muscle, blood and solid organs such as the kidney, heart, and
liver are all non-compressible and therefore unaffected by water
pressure; these organs and tissues have the same (or higher) density
than water and can withstand intense water pressure without any
problem. Were our bodies made up of only non-compressible
structures, going under water would be a lot simpler (though still not
hazard-free).
The compressible areas contain some air and include the lungs, middle
ears, sinuses, nasal passages, interior of hollow organs (stomach and
intestines), and any air pockets you may not know about (e.g., a
tooth cavity). If these parts of the body did not contain air they would
not be compressible. Conversely, any part of the body that is
compressible under water must contain air or some other gas.
At the moment a diver holds his breath the mass of air in the body is
fixed; no new air can enter or leave. Given that the mass of air is fixed
at that point, Boyle's law predicts that the compressible spaces will be
affected by changes in pressure. A fixed mass of gas (the diver's
air-containing spaces) subjected to an increase in pressure underwater
will compress (shrink in size). But by how much?
Assume a typical breath-hold diver's lungs contain 8 liters of air at
full inhalation (one liter equals about one quart). At 33 feet his lungs
could theoretically shrink to half the sea level volume, or 4 liters, if
they behave just like a balloon. The same percent shrinkage could
occur in the sinuses, middle ears, and all other compressible body
spaces, if they too behave like balloons.
However, our air-containing spaces don't behave exactly like balloons;
they are not as easily and evenly compressible. The lungs are supported
by a bony rib cage, the sinuses are embedded in the skull, only part of
the middle ear is exposed to water pressure (the tympanic membrane),
etc. Also, trained breath-hold divers can transfer some air from the
throat to the middle ear (via the Eustachian tube), retarding the
shrinkage of that small space.
So, on the one hand, our compressible air spaces are much more
complex than the simple balloon models used to illustrate Boyle's law.
On the other hand, we do have compressible air spaces and Boyle's law
predicts that they must shrink in size if the outside pressure is greater
than the inside pressure. It is just impossible to predict exactly how
much they will shrink, or how such shrinkage will affect the individual
diver.
If the breath-hold diver goes too deep, the spaces will shrink so much
that blood starts leaving the capillaries to fill in the spaces. The result
can be lung, sinus or nasal hemorrhage, or a ruptured tympanic
membrane. With ascent, all the breath-hold diver's shrunken spaces
will expand back to their original size. Because there is no over
expansion, there is no risk of blowing out a lung or other air space.
Breath-hold diving, practiced for centuries, is still employed for both
recreation and commerce (ama divers of Japan and Korea, pearl divers
of Hawaii, resort divers of Mexico, sport divers of California). It
requires special skills beyond the ability to hold one's breath, such as
withstanding the squeeze of descent and quickly accomplishing a goal
when the desired depth is reached. Scuba requires none of these skills
but others, of a different sort, that are easy to learn. For most people,
scuba is much easier to master than breath-hold diving.
WHAT DOES BOYLE'S LAW PREDICT FOR THE DIVER BREATHING
COMPRESSED AIR?
Water contains oxygen (both dissolved and as part of the water
molecule, H2O). Unlike fish, land animals are not equipped to extract
the dissolved or free oxygen and use it for breathing. Like all other
mammals (including the aquatic variety such as dolphins, porpoises
and whales), humans under water are cut off from life-supporting
oxygen.
For centuries the only underwater option was breath-hold diving, an
activity practiced only by the most daring and hardy. For most people
even 20 seconds under water can seem like eternity. The advent of
scuba made it possible for just about anyone with healthy lungs and
heart to stay under water for long periods. (This advance has not come
without a price. Compressed air diving presents two physiologic
problems breath-hold divers don't worry much about: decompression
sickness and air embolism.)
Boyle's law predicts that the compressible spaces of a breath-hold diver
will compress because the amount of air in these spaces (i.e., the total
number of air molecules) is fixed at the point of breath-hold; hence,
as pressure increases the volume occupied by the air must decrease.
Does Boyle's law also apply for scuba divers? Absolutely; after all, a law
is a law. But the consequences are different because scuba divers
breathe compressed air under water. First, the amount of air (the
number of air molecules) in each of the scuba diver's compressible
spaces increases along with the water pressure. Second, the volume of
each compressible space remains fairly constant throughout the dive.
Thus even though a space is compressible it should not be compressed
during scuba diving. While diving, the extra molecules of air that
enter these spaces (lungs, sinuses, middle ears) allow them to
maintain the same pressure as the surrounding water pressure. At a
given depth, though the ambient pressure is increased the volume in
any compressible space remains constant because the gas density
increases.
HOW MUCH AIR IS IN A SCUBA TANK?
Scuba dives can remain under water because they carry a supply of air.
The amount of air carried in a tank depends on its size and filling
pressure. Most tanks used for recreational diving are designed to carry
anywhere from about 60 to 100 cu. ft. of air; the typical tank found in
most resorts carries 80 cubic feet (cu. ft.) when filled to 3000 psi. If
the tank is filled to a lower pressure the volume of air it contains will
be less.
HOW DOES AIR PRESSURE CHANGE FROM THE SCUBA TANK TO
THE DIVER'S LUNGS?
Sea level pressure is 14.7 psi. An 80 cu. ft. tank at 3000 psi contains
80 cu. ft. of air that has been compressed 204 times (3000 psi/14.7
psi) higher than sea level pressure. If you tried to breathe air at 3000
psi it would blow you away; you couldn't do it. The scuba diver is able
to breathe tank air by virtue of a two-stage regulator system that 1)
steps the pressure down to a level slightly above ambient, and then 2)
delivers the air at ambient pressure.
Table 3 shows examples of regulator and airway pressures for various
depths. The first stage regulator brings the pressure down to ambient +
a predetermined pressure. The pre-determined pressure is set by the
regulator's design, but is generally 120-140 psi. Thus the pressure in
the hose between first and second stages is 120 to 140 psi higher than
the ambient pressure.
The second stage regulator contains a demand valve that requires only
a slight inspiratory effort to open; when the diver inhales on the
mouthpiece attached to the second stage, the demand valve opens and
air enters the lungs at ambient pressure. Note that the air is at the
pre-determined pressure immediately upon leaving the second stage
(i.e., 120-140 psi), but it rapidly reaches ambient by the time it is
inhaled (Figure 5). The second stage regulator is also designed so that
airflow ceases when the diver exhales. (When airflow doesn't cease on
exhalation it is said to "free-flow," a problem that can usually be
corrected by adjusting the regulator or briefly occluding the
mouthpiece.)
Ambient pressure is determined solely by depth, and is the pressure
inside the diver's lungs when breathing with scuba equipment. Tank
pressures for a given depth will vary depending on the rate of air
consumption and duration of the dive; at each depth in this table a
tank psi of 1500 is shown as example. The first stage regulator lowers
tank pressure to ambient plus some intermediate pressure determined
by the regulator's design, in these examples 140 psi.
Recreational divers use "open circuit" scuba, which means all exhaled
air, enters the surrounding water; none is re-breathed. The value of
1500 psi for tank pressure in the table, for each depth, is shown as
example only.
Obviously, as the dive progresses the amount and
pressure of air in the tank will decrease. The rate at which the tank's
air volume and pressure decrease is a function of the diver's
ventilation rate (how much air is breathed per minute), depth, and
length of time under water.
Note that regardless of the tank's psi, as
long as it is above some minimum value the first and second stage
regulators will deliver air at the ambient pressure. (Not all regulators
work the same at very low tank pressures.)
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