Whether you are just starting to scuba dive, considering signing up for a scuba course, or need a quick refresher, this post will provide you with a comprehensive list of terms and acronyms used in relation to the physics of scuba diving.
One of the most important considerations in diving is related to safety. Understanding the physics of diving is key to safety.
You can use this post as a reference guide before starting a scuba diving course or as a refresher before going on a scuba holiday.
If you are studying for your PADI Open Water Instructor Development Course, you can use the list below as flashcards. It contains all the terms you need to know for your Instructor Development Course and PADI Instructor Exam.
You might also want to check out the full list of terms and abbreviations on scuba diving equipment.
If you are studying for your PADI Open Water Diver Course, you might also be interested in the following:
Atmospheric pressure
Atmospheric pressure is defined as the pressure exerted by the Earth’s atmosphere. At sea level, atmospheric pressure is equal to 14.7 psi or 1 atmosphere (atm).
Atmospheric pressure decreases as altitude increases vice versa. Pressure inside a diver’s lungs is the same as the pressure outside.
Hydrostatic Pressure
Hydrostatic pressure is the pressure exerted by the weight of the water. Hydrostatic pressure is cumulative as the more water you have on top of you, the higher the pressure.
Hydrostatic pressure increases at a rate of 0.445 psi per foot of seawater. Therefore, at a depth of 33ft (or 10 meters) the hydrostatic pressure will be 14.7 psi or one atmosphere.
In freshwater, hydrostatic pressure is slightly lower. At a depth of 34 feet (or 10.4 meters), you will have a hydrostatic pressure of 14.7 psi (one atmosphere).
In salt water hydrostatic pressure increases at a rate of 0.445 psi per foot, whereas in freshwater, hydrostatic pressure increases at a rate of 0.432 psi per foot.
Absolute Pressure
Absolute pressure equals atmospheric pressure plus hydrostatic pressure and is most commonly expressed in pounds per square inch absolute (psia) or in atmospheres absolute (ata).
Ambient Pressure
Ambient pressure of an object is defined as the pressure of the medium that surrounds it.
The ambient pressure in water with a free surface is a combination of the hydrostatic pressure of water and atmospheric pressure on top of the water.
Gauge Pressure
Gauge pressure is defined as the difference between atmospheric pressure and the pressure being measured.
To better visualize this concept, consider the pressure showing on your SPG at the surface before you open your cylinder valve.
If your SPG is working correctly, this should be 0. This is because at surface, the pressure you are measuring (if your scuba tank is closed) equals atmospheric pressure.
Partial Pressure
Partial pressure is defined as the proportion of total pressure contributed by each single gas in a gas mixture.
Density
Density is defined as weight per unit volume. It is expressed in pounds per cubic foot or grams per cubic centimeter.
Gas density increases as absolute pressure increases. Therefore, as depth increases, gas density increases, and it makes it more difficult for a diver to breathe.
Different mediums have different density. For example, water is denser than air. Water is therefore able to conduct heat far more efficiently because it is denser.
Note that the denser and more elastic the medium, the better sound is transmitted.
Henry’s Law
Henry’s law states that the amount of gas that will dissolve into a liquid is almost directly proportional to the partial pressure of that gas.
Freshwater
Water or H2O is a major constituent of all living matter. In scuba diving, Freshwater denotes lakes, rivers and any body of water that is not salty.
Freshwater is less dense than seawater. Therefore, an object that is neutrally buoyant in seawater will sink in freshwater.
Seawater
Seawater is defined as water whereby sodium chloride is the most abundant chemical. Because of its properties, seawater is denser than freshwater.
Atmospheric Air
In scuba diving, atmospheric air is often approximated as 21% of oxygen and 79% of nitrogen.
As you might expect, atmospheric air is however composed of many other gasses such as argon, carbon dioxide and a series of rare gasses.
Atmospheric air might also be contaminated. One of the most common and dangerous sources of contamination is carbon monoxide.
Oxygen
Oxygen is a colorless, odorless, tasteless, and active gas that combines with other elements.
From atmospheric air, only oxygen is actually used by the body. The other 79% of the air serves to dilute the oxygen.
Pure oxygen (100% oxygen) is often used for breathing in hospitals, aircraft, and hyperbaric medical treatment facilities.
Sometimes, mostly in technical diving, 100% oxygen is used in shallow diving and in certain phases of the dive.
Breathing oxygen in high percentages at depth can be dangerous and lead to oxygen toxicity.
Nitrogen
Nitrogen forms the largest proportion of the air we breathe, often approximated to 79%. In scuba diving, nitrogen plays a fundamental role as it is the major source of decompression theory.
In fact, in scuba diving, Nitrogen is often used to dilute oxygen although it has several disadvantages.
For instance, when breathed at increased partial pressures (usually in excess of 100ft. or 31 meters), nitrogen has some anaesthetic properties.
These properties commonly manifest in a state characterized by loss of judgment and disorientation which can be fatal for divers. This phenomenon is called Nitrogen Narcosis.
Helium
Helium is often used by technical divers to dilute oxygen. It has a lower density than nitrogen and does not cause problems similar to Nitrogen Narcosis.
Given that it is a lower density gas, it reduces breathing effort at higher depths.
It however has some disadvantages. First disadvantage is that it impairs voice communication and second major disadvantage is that it helium has high thermal conductivity resulting in higher heat loss via the respiratory system.
Carbon Dioxide
Carbon dioxide is the gas expelled by our respiratory system while we breathe out. In general, it is not considered poisonous, but it may cause unconsciousness which can be fatal for scuba divers.
Carbon Monoxide
Carbon monoxide is a poisonous gas interfering with our blood’s ability to carry oxygen. It is colorless, odorless, and tasteless.
For divers, the most common source of carbon monoxide is contaminated air supply stemming from a malfunctioning compressor or from a compressor located next to electricity generators exhausts.
Argon
Argon is a narcotic gas with high density, making it inappropriate to breathe. It is however often used in diving to inflate dry suits.
Due to its high density, it reduces the conduction of heat.
Neon
Neon could be used as a substitute for helium as it causes less voice distortion and has lower thermal conductivity.
It is however almost never used as it is highly expensive and causes increased breathing resistance at moderate or heavy workloads.
Hydrogen
Hydrogen could be used as breathing gas as it is readily available and produces less breathing resistance at depth than other gases.
It is however almost never used due to its explosive properties.
Refraction
Refraction is caused by light traveling at different speeds as it passes through different substances.
Refraction makes objects underwater look 33% bigger than they actually are. For instance, when viewed underwater, objects normally appear closer by a ratio of 4:3.
Visual Reversal
Visual reversal refers to an object’s tendency to appear further away than its actual distance. Water turbidity is the single most important factor affecting visual reversal.
Conduction
Conduction is the process by which heat or electricity is transferred through the material of a substance. It is also the process by which sound waves travel through a medium.
For example, a diver is affected by conduction as the body heat is transferred from our body to the water inside our wetsuit.
For instance, water is able to conduct heat 20 times faster than air.
