Gases & Gas Laws


Knowledge of how gases behave, or their characteristics requires absolute comprehension by the rescue diver. Keep in mind that the most common gas is air. Compressed air (air under pressure) is utilized by divers to breathe underwater. Oxygen is almost never utilized at depth. There are diving systems that use pure oxygen called CLOSED CIRCUIT SCUBA or O-2 RIGS. These systems are mostly operated by the military and research divers and are not intended to be used below 33 feet.

Oxygen (O2) is a most extraordinary gas for it sustains life and supports combustion, furthermore, it is one of the most abundant of the gases. Atmospheric air is approximately 21% oxygen and water is about 89% oxygen by weight. Application of 100% oxygen to patients is frequently employed in medical and hyperbaric treatments.

Nitrogen (N2) is also a diatomic gas that cannot be ascertained by any of the senses. Unlike oxygen nitrogen does not support life or combustion. It is an inert gas often employed to blanket many flammable liquids. In the air it is the carrier of oxygen. It is this acting as a carrier gas that most affects the diver. At high partial pressure nitrogen can have a narcotic effect on the person who breathes it. For this reason in deep diving operations as done by the military and commercial divers, the carrier gas is changed.

Carbon Dioxide (CO2) is a colorless, odorless, tasteless gas, but it may have an acid taste and odor in high concentrations. It is an active gas that is frequently used for carbonation either naturally or artificially, and found in beer and soft drinks. Furthermore it is used as a fire extinguishing agent. Carbon dioxide is the natural by product of respiration of animals and the gas all plants live on. At high concentrations the gas is highly toxic. Divers who employ full face masks or closed circuit systems can succumb to it. Free divers may black out due to high blood levels of carbon dioxide brought about by holding their breath. .

Carbon Monoxide (CO-) does not occur naturally being a product of combustion. It is lethal gas in low quantities, and it will kill. It is difficult to detect without a CO METER (a device to determine the amount of carbon monoxide in air). Fortunately this gas bonds with other elements and dissipates rapidly. It will not maintain its identity unless in high concentrations. Divers should never come in contact with carbon monoxide, but sometimes it happens. When the compressor filling the tanks have a malfunction and introduces CO into the Scuba tanks, however this is rare. Most compressors are regularly checked to maintain pure diving air. Any diver who has breathing problems underwater such as shortness of breath or feels dizzy, must abort the dive. If he surfaces this way, give high concentration oxygen by demand valve. Carbon monoxide may not be the problem, but the oxygen wont hurt if it is not.

The Gas Laws

Man lives in an envelope of air that exerts pressure on his body. During normal day to day existent this pressure goes unobserved. Most people never become conscious of pressure until they go up in an airplane or drive up steep hills. Divers on the other hand notice pressure changes as soon as they enter the water. Understanding how these changes affect the body is vital to his safety and well being. There are general gas laws that dictate how gases function, especially during changes in temperature and pressure. These laws must be fully understood to deal with decompression problems and over pressure injures, they additionally affect lift operations and air consumption, along with every activity any diver performs.

Air has weight, weight is pressure, and pressure is stated in pounds per square inch (psi). At sea level the weight of air is 14.7 psi. This is the weight of a column of air one inch by one inch extending all the way to the top of the atmosphere. When referring to this pressure it is stated as one ATMOSPHERE, or one ATM.

When comparing air to water we find that air is compressible, while water is considered incompressible. At sea level, air is denser than at higher terrains, however water maintains an equal density no matter what the depth. When pressure is applied to air that cannot escape it will compress. When pressure is applied to water which cannot escape the pressure is transmitted in all directions throughout the fluid.

Seawater Weighs 64 Pounds Per Cubic Foot.

To determine its pressure in pounds per square inch, as air is stated, we take 64 (the weight of seawater) and divide it by 144 (the number of inches in a cubic foot):

64.0/144 = 0.445 psi seawater

62.4/144 = 0.433 psi fresh water

Now that it is known that 0.445 psi is the pressure per foot of seawater, we can now determine the pressure at any depth by multiplying 0.445 times the depth. This is known as the HYDROSTATIC PRESSURE or the weight of the water acting upon the divers body.

0.445 X Depth = psi for stated depth

Example: the hydrostatic pressure of 15ft.
0.445 X 15 = 6.7psi.

Now take 14.7 psi (One ATM) and divide it by 0.445
This number represents one ATM in relation to seawater that equals 33 feet.
14.7 ÷ 0.445 = 33 ft. so that: 1 ATM = 33 ft. seawater

Since water transmits pressure freely, and pressure increases at a constant rate, it can be affirmed if 33 feet equals 1 ATM, then 66 feet equals 2 ATM and 99 feet equals 3 ATM so onto the bottom of the ocean. However, we know there is one atmosphere above the surface of the water that must be added in. At 33 feet the diver is under 2 ATM and at 66 feet 3 ATM.

To Determine Atmospheres: Take The Depth Divided By 33, And Add One.

(Depth / 33 [1 ATM]) + 1 = ATM
Example: 66ft./33 = 2 + 1 = 3 ATM

To Determine Depth From Stated Atmospheres: Take the stated ATMs, Subtract One, and Multiply By 33

(No. of ATMs - 1) X 33 = Depth

Example: What is the depth of 4 ATM? 4-1 = 3 X 33 = 99ft.

Since atmospheres increase every 33 feet it can be stated pressure increases 14.7 psi every 33 feet.

To Determine The Pressure A Diver Is Under At Any Given ATM: Take the ATM and Multiply by 14.7

At 33 feet or 2 ATM the pressure would be: 2 X 14.7 = 29.4 psi.

This is known as the ABSOLUTE PRESSURE. Absolute pressure is the hydrostatic pressure, plus the one atmosphere above, and stated in pounds per square inch absolute (psia).

To Determine Absolute Pressure At Any Given Depth:

The Depth Times 0.445 plus 14.7 psi.

Gauges are set to read ''0'' pressure, without the 1 ATM or 14.7 psi. They measure the difference between the atmosphere and the air in the container being measured.

GAUGE PRESSURE is stated as psig and is therefore the difference between the absolute pressure and the pressure being measured. .

If a tank pressure gauge reads 3000 psi, it refers to the tank being 3000 psi over atmospheric pressure.
To get absolute pressure add 14.7 to the what the dial reads (psiga).
Mechanics of Pressure
Pressure is weight: 1 ATM = 33 ft(sw) = 14.7 psi
Weight of air: 1 ATM = 14.7 psi = 760 tor = 33 ft(sw)
Pressure increases with depth; every 33 ft = 14.7 psi
Pressure decreasing with ascent at 14.7 psi per 33 ft ascent

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