Scuba Diving Problems With Gases and Pressure 101
From Scubadoc’s Diving Medicine
Quick Review of Diving Physics
In order to fully appreciate the physiological problems that can be encountered while diving, one must thoroughly understand the concepts of pressure, the physical characteristics of gases and the basic laws that govern gases and their effects on the body.
Terminology and Definitions
Compression-That part of a dive that increases pressure upon a diver. The deeper a diver goes-the more the pressure.
Decompression-That part of a dive when the diver ascends toward the surface, decreasing the pressure. In a chamber dive, that part of a dive when the pressure is being lowered.
Recompression-a return to compression after ascent to the surface on a water dive; a return to surface pressure from altitude; a term used to describe medical treatment of decompression sickness.
Hyperbaric-a word used to describe increased pressure over the pressure in one atmosphere .
Some objects float in water while others sink, and still others neither float nor sink. This is a function of buoyancy. Objects that float are called positively buoyant, those that sink are called negatively buoyant. Neutrally buoyant objects neither sink or float.
Archimedes, a Greek mathematician, stated what is known as Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
It is from this principle that we can see how a heavy ocean liner can float: it is because of the large amount of water that it displaces. Whether an object floats or sinks, is based on not only its weight, but also the amount of water it displaces.
As divers we are mainly concerned with two different liquids: fresh water, and salt water. Because their weights are not the same, they have to be calculated as different fluids when considering buoyancy. A cubic foot of fresh water weighs approximately 62.4 lbs, while a cubic foot of salt water weighs approximately 64 lbs., the difference is due to the dissolved minerals in salt water.
Let’s take a moment and look at an object in water and Archimedes Principle. An object weighing 63 pounds will sink when placed in fresh water as it weighs more than the water that it is displacing. It is negatively buoyant – it will sink. It is however being buoyed up with a force of 62.4 lbs, so if we weighed it in the water it would only weigh .6 lbs.
The same object in salt water would still weigh 63 lbs, but would be buoyed up by a force of 64 lbs, and it would float. It would be positively buoyant in salt water. By adding 1 pound we could make the object neutrally buoyant if we kept the size the same, (without changing it’s displacement).
Pressure-a force acting on a unit of area; Pressure = Force divided by Area
Atmospheric Pressure-pressure exerted by the weight of the atmosphere; this varies with the altitude.
Barometric pressure-a measurement of atmospheric pressure; one atmosphere of pressure is equal to 760 mm Hg or 1.03 kg/cm2 or 14.7 psi (pounds per square inch).
Hydrostatic pressure-the force of a column of water acting upon a body immersed in the water, equal in all directions at a specific depth. During descent, pressure increases 0.445 psi per foot of depth in salt water or one atmosphere per 33 feet of salt water (FSW).
Gauge pressure-the difference between absolute pressure and atmospheric pressure. This can be converted to absolute pressure by adding 14.7 psi or 1.03kg/cm2.
Absolute pressure-this is the sum of all pressures acting on an object. In diving this is the sum of the atmospheric pressure (33 + the hydrostatic pressure).
Some Characteristics of Gases
Oxygen, the gas that is capable of supporting life, exists in the atmosphere on the surface of the earth at a concentration of 21%. There are specific ranges of human tolerances and can cause toxicity when concentrations exceed 30%. Oxygen also supports combustion and creates chamber safety problems. the maximal allowable concentration in multiplace chambers is 23%.
Carbon dioxide is a direct product of metabolism and is found at a maximum level of 1.5% at the surface. It is the gas that determines our rate of ventilation and causes shallow water blackout with consequent drowning. The concentration of CO2 determines ventilation schedules in multiplace chambers.
Carbon monoxide is the product of incomplete combustion of fuel, usually caused by faulty compressors. Its’ toxicity is caused by its’ affinity for hemoglobin and the histotoxic effect on our cytochrome A3 system. The maximum allowable level is 10 ppm (0.001%), which is about what we get on a city street.
Nitrogen-Nitrogen is an inert gas consisting about 79% of the air we breathe. It produces nitrogen narcosis in humans at a depth of 100fsw, causes decompression sickness on ascent because of the bubbles that form on reduction of pressure. it is the gas that determines our decompression schedules.
Helium-this is an inert gas present in air in very small quantity (0.0005%) and is used to prevent nitrogen narcosis. It is used as an emergency breathing gas but results in body heat loss, creates communication difficulties and increases the chance of decompression sickness.
Gas Laws and their Physiological Significance
At a given temperature the volume of a given mass of gas will vary inversely with the absolute pressure.
Boyle’s Law-This law of physics determines the volume of gases and accounts for the major portion of diving medical problems. Stated simply-the volume of gases are reduced when pressure is increased (diver descending) and the volume is increased on reduction of pressure (diver ascending). Bubbles are reduced in size when chamber pressures are increased and air-containing spaces (lungs, middle ears, sinuses) expand when chamber pressures are decreased or when the diver surfaces.
Another example of this problem of size and volume is explained by George Safirowski.
“However, since most divers are introduced to the “one cubic foot of water” graphic, and are required to understand the weight of salt water, fresh water and air, it is much easier for them to visualize and understand an explanation using cubes instead of spheres since they
were already introduced to have a concept of cubes.
A cube 1 ft x 1 ft x 1 ft = Volume displacement 1 cuft
A cube twice the physical size 2 ft x 2ft x 2 ft = Volume displacement 8
The smaller cube displaces 1/8 volume of the larger cube.
From there is easy to explain, it takes 8 ata to have 1/8 volume
displacement. The numbers work out the same for spheres showing that a 12” diameter bubble will not become 6” diameter until it is under
pressure of 8 ata, but the formula is much more complex.
Java-based Volume/Depth/Temperature Calculator
At a constant pressure the volume of a mass of gas is proportional to the absolute temperature
T x P = V
Charles’ Law-This states that volume varies with the temperature, explaining why compressing gases increases heat and decompression cools. Tank fills need to be done under water in order to keep the tanks cool; release air from a scuba tank and you’ll notice that it becomes cold.
Total pressure exerted by a mixture of gases is the sum of the partial pressures that would be exerted by each gas alone as if it alone occupied the total volume.
Dalton’s Law-This is the law that explains oxygen toxicity, nitrogen narcosis and the danger of even minute quantities of contaminant gases. It’s the law of partial pressures, in which all the individual partial pressures of gases are totaled into one partial pressure.
Partial-pressure effects: The partial pressure of a gas is determined by the concentration of the gas and the ambient pressure, eg, the concentration of O2 in air is about 21%, and the partial pressure of O2 in air at surface (1 atm abs) is about 0.21 atm. The concentration of O2 in air remains the same at depth, but the partial pressure reflects the increasing pressure and compression of the gas. At 2 atm abs, the number of O2 molecules per unit volume is twice what it is at the surface, and the partial pressure is double.
The physiologic effects of gases are related to their partial pressure and change according to depth. Toxic effects appear as the partial pressure of O2 increases. Pulmonary oxygen toxicity can cause lung damage with extended exposure to a PO2 above 0.6 atm (equivalent to 60% O2 at surface or 30% O2 at 33 ft). Oxygen convulsions may occur, especially in working dives, if the PO2 approaches or exceeds 2 atm (eg, 100% O2 at 33 ft or 50% O2 at 99 ft).
Increased partial pressures of N2 produce nitrogen narcosis, a condition resembling alcohol intoxication. In divers who breathe air, this effect becomes noticeable at 100 ft or less. It is generally incapacitating at about 10 atm abs (300 ft), where it produces an anesthetic effect resembling that of 30% nitrous oxide at sea level. (Helium lacks this anesthetic property and is used in place of N2 as the diluent for O2 in deep diving.)
Partial pressures of O2 and CO2 in alveolar gas are modified by the pressure of depth in breath-hold diving and in underwater swimming without breathing apparatus. The impulse to return to the surface and resume breathing depends largely upon CO2 buildup in the body. A breath-holding diver may hyperventilate beforehand to extend time underwater; this blows off CO2 but adds little to stores of O2, and may then cause unconsciousness from hypoxia without warning before PCO2 rises enough to become an effective stimulus.
Diving to a significant depth during the breath-hold complicates the situation by elevating the PO2 and permitting extended O2 uptake at depth. A diver who has “pushed the limits” under those circumstances may lose consciousness when alveolar PO2 falls to a low level on ascent. This phenomenon is probably responsible for many unexplained drownings among spearfishing competitors and others who do extensive breath-hold diving. The term shallow-water blackout is sometimes applied, but it is best reserved for its original meaning: unconsciousness from CO2 buildup in rebreathing types of scuba. (Hypoxia is also a potential problem in rebreathing units if O2 is displaced by excess N2.)
Carbon dioxide poisoning: In normal individuals on land, hyperpnea or breathlessness usually provides ample warning of increased CO2 in inspired gas. Such a response may be more the exception than the rule under water, especially where high PO2 and exertion are also factors. Some individuals develop spontaneous CO2 retention through an inadequate increase in pulmonary ventilation during exertion. Whatever the source, abnormally high PCO2 per se can cause loss or impairment of consciousness at depth and can also increase the likelihood of O2 convulsions and augment the severity of nitrogen narcosis. The tendency to retain CO2 may be suspected in divers who frequently experience post-dive headaches or pride themselves on low air-use rates.
Graham’s Law-This states that gases flow to areas of lesser pressure and explains oxygenation between tissue compartments and the movement of inert gases through out the body.
In fluid, dissolved gas is directly proportional to the partial pressure of the gas to which the fluid is exposed.
Henry’s Law-This rule explains O2 transport, inert gas transport and the evolution of bubbles in a solution. It states that the volume of a gas is directly proportional to the pressure above a liquid.
P = FORCE/AREA
Pascal’s Law-Pressure at any point in a body or solution has that pressure transmitted equally throughout the solution. This is the reason that deep tissue compartments and internal bubbles experience the pressure changes that occur outside the body.
How gases move about
Perfusion-the flow of liquids in the body (blood or lymph) , through an organ or tissue during which gases and/or chemical substances are exchanged and/or redistributed.
Absorption-The process of moving a gas into the liquid phase in the body.
Solubility-This process determines the amount of gas that will dissolve at a given pressure.
Diffusion- The movement of dissolved substances from higher concentrations to lower concentrations.
Gradient-The amount of change of one quantity of substance with respect to another; a gas in solution will diffuse across a gradient, from an area of higher concentration to one of less concentration.
Pressure Conversion Factors for One Atmosphere of Pressure
10.08 meters sea water
33 feet sea water
101.3 kilopascals/square meter
1.033 kg/square cm
1034 cm H2O
1 ATM=The weight of all the air above the surface of the earth=33 feet sea water (fsw)=14.7 pounds per square inch(psi)=0.445psi/fsw=2.25 fsw/psi=0.0303 ATM/fsw
One pint=one pound