Pressure & Diving

Pressure is any force applied to an object. When you press on an object, you’re applying pressure to it. Your car’s tires are kept firm by the pressure of compressed air pushing out on the inside. Scales measure weight by reading the increase of pressure on top of it. As divers, we measure pressure to determine how full our cylinders are and how deep we are.

Pressure is also the cause of many diving injuries. Ear squeezes, lung overexpansion, and decompression sickness are just a few examples of injuries caused by pressure changes. For this reason, it’s important to understand pressure and how it affects air spaces.

In this lesson you’ll learn about atmospheric pressure, pressure underwater, and how pressure changes affect the volume and density of air.

scuba tank air pressure

Atmospheric Pressure

The weight of the air above us exerts pressure, and this is called atmospheric pressure.

Exactly how much pressure is exerted on us depends on altitude. Pressure is lowest at high altitudes because there is less air above you. And at lower altitudes, there is more air above you, so pressure increases.

In the United States, we use pounds per square inch (psi) to measure pressure. PSI refers to the amount of pressure exerted on 1 square inch of a surface.

Atmospheric pressure is equal to the weight of a 1 square inch column of air extending all the way to the edge of the earth’s atmosphere. At sea level, this column weighs 14.7 pounds, so the atmospheric pressure is 14.7 psi. To make calculations easier, a unit of measurement called “atmospheres” or “ata” is used. 1 atmosphere of pressure is equal to 14.7 psi.

atmospheric pressure

Gauge Pressure

There are times when you are only concerned about pressure beyond atmospheric pressure. For these measurements, we use gauge pressure, which ignores the 14.7 psi of atmospheric pressure that always exists at sea level.

Your submersible pressure gauge is an example of a device that reads gauge pressure. An “empty” cylinder still contains 14.7 psi of pressure at sea level. But your submersible pressure gauge ignores this 14.7 psi of pressure and reads “0” instead.

Your depth gauge is another example. As you’ll learn later in this lesson, the pressure you’re exposed to while diving is a combination of both atmospheric and water pressure. Your depth gauge is calibrated to read “0” at sea level, therefore ignoring atmospheric pressure and only measuring changes in water pressure.

Gauge Pressure

Absolute Pressure

While diving, both the atmosphere and the water above you exert pressure. The combination of the atmospheric and water pressure is called absolute pressure. And since water is about 800 times denser than air, pressure changes at a faster rate than it does on land.

The rate of pressure increase depends on whether you’re diving in salt water or fresh water. Because salt water has more density than fresh water, pressure increases faster as you descend in salt water.

Calculating changes in atmospheric pressure is challenging because air density decreases as altitude increases. Fortunately for us, water doesn’t compress like air, so pressure increases at a constant rate with depth.

Absolute Pressure in Salt Water

A 33-foot square inch column of salt water weights exactly 14.7 pounds, and exerts 1 atmosphere of pressure. Therefore, absolute pressure increases by 1 atmosphere every 33 feet.

As the graph to the left illustrates, the pressure at 0 feet is 1 atmosphere. Because 33 feet of salt water exerts 1 atmosphere, the absolute pressure at 33 feet is 2 atmospheres. Pressure continues to increase by 1 atmosphere every 33 feet.

A common mistake divers make when calculating absolute pressure is forgetting the 1 atmosphere of pressure at the surface. Remember, absolute pressure is the combination of pressure from both water AND air.

Absolute Pressure in salt water

Pressure Increases in Salt Water

Absolute Pressure in Fresh Water

Fresh water is less dense than salt water, so the rate of pressure increase is slightly different. Pressure increases by 1 atmosphere every 34 feet, as opposed to 33 feet for salt water.

As the graph to the left illustrates, the pressure at 34 feet of fresh water is 2 atmospheres, 3 atmospheres at 68 feet, and 4 atmospheres at 102 feet. The pressure continues to increase by 1 atmosphere every 34 feet.

Absolute Pressure in Fresh Water

Pressure Increases in Fresh Water

Boyle’s Law

The volume of an air space changes as the pressure surrounding it changes. The relationship between pressure and volume is best described by Boyle’s Law, which states:

“The volume of any gas is inversely proportional to the pressure.”

This means that volume decreases as pressure increases, and volume increases as pressure decreases. This law also states that the relationship is proportional. For example, if the pressure doubles, air volume decreases by one half.

Open Air Space Volume, Descent

In the example to the left, an open air space is filled with air and pulled to increasing depths of water. This container is open, so water enters as air volume decreases. This allows the container to retain its original shape and size.

When the bucket is pulled down to a pressure of 2 atmospheres (a depth of 33 feet in salt water) the pressure doubles, so the volume is one half of its volume at the surface.

At 3 atmospheres of pressure (66 feet in salt water) the pressure is 3 times that at the surface, so the air volume is one third of its volume at the surface.

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At a pressure of 4 atmospheres, the volume reduces to one quarter of the surface volume. This pattern continues with descent. For example, the volume decreases to one tenth of the surface volume when pulled to 10 atmospheres of pressure.

open air space volume on descent

Volume of an Open Air Space on Descent

Open Air Space Volume on Ascent

In this example, the bucket is filled with air at depth, then released to the surface. As it ascends the pressure decreases, which allows the volume to increase.

The bucket is filled at a pressure of 4 atmospheres, or 99 feet of salt water. When the bucket rises to a pressure of 3 atmospheres, the volume increases by 1 third. At 2 atmospheres, the volume is double what it was at 4 atmospheres. And at the surface, the volume is 4 times its original volume.

Since this is an open system, air escapes as it expands. This means the shape and size of the bucket is not affected by the expanding volume of the air on ascent.

open air space volume on descent

Volume of an Open Air Space on Descent

Pressure and Closed Air Spaces

Divers are more concerned about the effects of pressure on closed air spaces. These spaces can change in volume or even become damaged as pressure changes.

An example is your wetsuit, which compresses as you descend and expands during ascent. Your body also contains air spaces that can become closed if you are unhealthy or fail to take safety precautions. The next chapter covers these air spaces and how to protect them from pressure-related injuries.

The most severe rate of change in volume occurs from the surface to a depth of about 33 feet. This is because the pressure doubles with just 33 feet of depth. For this reason, you need to be particularly careful with air spaces in shallow water.

closed air space volume on ascent

Calculating Air Volume Changes

You can calculate exact changes in air volume by using a simple calculation. It is:

boyle

In the example to the left, a balloon contains 8 cubic inches of air at 33 feet. It is then pulled down to 99 feet, and we want to know the new volume.

First, we need to determine the original and final pressures. Since this is salt water, the pressure at 33 feet is 2 ata, and at 99 feet the pressure is 4 ata. Next, we determine the original volume, which is 8 cubic inches. So our formula will look like this:

boyle

The ratio equals 1/2, and 1/2 of 8 equals 4. So the balloon’s new volume at 99 feet would be 4 cubic inches.

boyle

Pressure & Air Density

Changes in absolute pressure also affect air density. For example, if the pressure doubles, the volume decreases to one half. Since the same amount of air now occupies half the space, the density is doubled.

Air density is directly proportional to absolute pressure. So if pressure doubles, density doubles as well. The chart to the left illustrates the relationship between pressure, volume, and density.

As the air becomes denser, it doesn’t flow as easily through your regulator and body’s air passages. This increases breathing resistance, which means you have to work harder to breathe than you do on land. While you’ll notice the change in breathing resistance, for most divers it’s not a problem unless they exert themselves.

pressure and air density

Depth & Air Consumption

There are several factors that influence your air consumption. Physical activity, temperature, psychological comfort, and physical condition are just a few examples. But the most significant factor is your depth, because the air’s density increases as your depth increases.

The density of the air determines how long your supply will last. Your air will last half as long at 2 atmospheres of pressure than it does at the surface. Continue down to 3 atmospheres of pressure, and your supply is one third of what it would be at the surface.

For example, if a diver consumes 30 psi per minute at 33 feet of salt water, that same diver will consume 45 psi/min at 66 feet, and 60 psi/min at 99 feet. At the surface, or 1 atmosphere of pressure, the diver would consume 15 psi/min. This is called your surface air consumption rate, and is valuable for predicting your air consumption at planed depths.

diver air consumption

Depth & Air Consumption

How Does Boyle’s Law Apply to Scuba Diving?

Natalie Gibb owns a dive shop in Mexico and is a PADI-certified open water scuba instructor and TDI-certified full cave diving instructor.

Diver Viewing Green Sea Turtle, Galapagos Islands

One of the fantastic consequences of enrolling in a recreational scuba diving course is being able to learn some basic physics concepts and apply them to the underwater environment. Boyle’s law is one of these concepts.

Boyle’s Law explains how the volume of a gas varies with the surrounding pressure. Many aspects of scuba diving physics and dive theory become clear once you understand this simple gas law.

In this equation, “P” represents pressure, “V” signifies volume and “c” represents a constant (fixed) number.

If you are not a math person, this may sound confusing. But, don’t despair. This equation states that for a given gas—such as air in a scuba diver’s buoyancy compensator device (BCD)—if you multiply the pressure surrounding gas by the volume of gas you will always end up with the same number.

Because the answer to the equation can not change (that’s why it is called a constant), we know that if we increase the pressure surrounding a gas (P), the volume of the gas (V) must get smaller. Conversely, if we decrease the pressure surrounding gas, the volume of the gas will become greater. That’s it! That’s Boyle’s entire law.

Almost. The only other aspect of Boyle’s Law that you need to know is that the law only applies at a constant temperature. If you increase or decrease the temperature of a gas, the equation doesn’t work anymore.

Applying Boyle’s Law

Boyle’s Law describes the role of water pressure in the dive environment. It applies and affects many aspects of scuba diving. Consider the following examples:

  • Descent – As a diver descends, the water pressure around him increases, causing air in his scuba equipment and body to occupy a smaller volume (compress).
  • Ascent – As a diver ascends, water pressure decreases, so Boyle’s Law states that the air in his gear and body expand to occupy a greater volume.
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Many of the safety rules and protocols in scuba diving were created to help a diver compensate for the compression and expansion of air due to changes in water pressure. For example, the compression and expansion of gas lead to the need to equalize your ears, adjust your BCD, and make safety stops.

Examples of Boyle’s Law in the Dive Environment

Those who have been scuba diving have experienced Boyle’s Law first hand. For example:

  • Ascent – As a diver ascends, water pressure around him decreases, and the air in his BCD expands. This is why he has to release excess air from his BCD as he ascends—otherwise, the expanding air will cause him to lose control of his buoyancy.
  • Descent – As a diver descends, the water pressure around him increases, compressing the air in his ears. He must equalize the pressure in his ears to avoid pain and a possible ear injury called ear barotrauma.

Scuba Diving Safety Rules Derived From Boyle’s Law

Boyle’s law explains some of the most important safety rules in scuba diving.

Here are two examples:

  1. Don’t Hold Your Breath Underwater – According to dive training organizations, a diver should never hold his breath underwater because if he ascends (even a few feet) to an area of lesser water pressure, the air trapped in his lungs will expand according to Boyle’s Law. The expanding air can stretch the diver’s lungs and lead to pulmonary barotrauma. Of course, this only occurs if you ascend while holding your breath, and many technical diving organizations modify this rule to “Don’t hold your breath and go up.”
  2. Ascend Slowly – A diver’s body absorbs compressed nitrogen gas while he dives. As he ascends to a depth with less water pressure, this nitrogen gas expands according to Boyle’s Law. If a diver does not ascend slowly enough for his body to eliminate this expanding nitrogen gas, it can form tiny bubbles in his blood and tissue and cause decompression sickness.

Why a Constant Temperature Is Necessary to Use Boyle’s

As mentioned above, Boyle’s Law only applies to gases at a constant temperature. Heating a gas causes it to expand, and cooling a gas causes it to compress.

A diver can witness this phenomenon when they submerge a warm scuba tank into colder water. The pressure gauge reading of a warm tank will drop when the tank is submerged in cool water as the gas inside the tank compresses.

Gasses that are undergoing a temperature change, as well as a depth change, will have to have the change in gas volume due to the temperature change accounted for, and Boyle’s simple law must be modified to account for temperature.

Boyle’s law enables divers to anticipate how air will behave during a dive. This law helps divers to understand the reasons behind many of scuba diving’s safety guidelines.

Under Pressure – Scuba Diving Risks

Natalie Gibb owns a dive shop in Mexico and is a PADI-certified open water scuba instructor and TDI-certified full cave diving instructor.

Scuba diver near the surface

Olga Melhiser Photography / Getty Images

How does pressure change underwater and how do pressure changes affect aspects of scuba diving such as equalization, buoyancy, bottom time, and the risk of decompression sickness? Review the fundamentals of pressure and scuba diving, and discover a concept no one told us during our open water course: that pressure changes more rapidly the closer a diver is to the surface.

The Basics

Air Has Weight

Yes, air actually has weight. The weight of air exerts pressure on your body—about 14.7 psi (pounds per a square inch). This amount of pressure is called one atmosphere of pressure because it is the amount of pressure the earth’s atmosphere exerts. Most pressure measurements in scuba diving are given in units of atmospheres or ATA.

Pressure Increases With Depth

The weight of the water above a diver exerts pressure on their body. The deeper a diver descends, the more water they have above them, and the more pressure it exerts on their body. The pressure a diver experiences at a certain depth is the sum of all the pressures above them, both from the water and the air.

Every 33 feet of salt water = 1 ATA of pressure

Pressure a diver experiences = water pressure + 1 ATA (from the atmosphere)

Total Pressure at Standard Depths*

Depth / Atmospheric Pressure + Water Pressure = Total Pressure

0 feet / 1 ATA + 0 ATA = 1 ATA

15 feet / 1 ATA + 0.45 ATA = 1 .45 ATA

33 feet / 1 ATA + 1 ATA = 2 ATA

40 feet / 1 ATA + 1.21 ATA = 2.2 ATA

66 feet / 1 ATA + 2 ATA = 3 ATA

99 feet / 1 ATA + 3 ATA = 4 ATA

*this is only for saltwater at sea level

Water Pressure Compresses Air

Air in a diver’s body air spaces and dive gear will compress as pressure increases (and expand as pressure decreases). Air compresses according to Boyle’s Law.

Not a math person? This means that the deeper you go, the more air compresses. To find out how much, make a fraction of 1 over the pressure. If the pressure is 2 ATA, then the volume of the compressed air is ½ of its original size at the surface.

Pressure Affects Many Aspects of Diving

Now that you understand the basics, let’s look at how pressure affects four basic aspects of diving.

Equalization

As a diver descends, the pressure increase causes the air in their body’s air spaces to compress. The air spaces in their ears, mask, and lungs become like vacuums as the compressing air creates a negative pressure. Delicate membranes, like the ear drum, can get sucked into theses air spaces, causing pain and injury. This is one of the reasons that a diver must equalize their ears for scuba diving.

On ascent, the reverse happens. Decreasing pressure causes the air in a diver’s air spaces to expand. The air spaces in their ears and lungs experience a positive pressure as they become overfull of air, leading to pulmonary barotrauma or a reverse block. In a worst-case scenario, this could burst a diver’s lungs or eardrums.

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To avoid a pressure-related injury (such as an ear barotrauma) a diver must equalize the pressure in their body’s air spaces with the pressure around them.

To equalize their air spaces on descent a diver adds air to their body airspaces to counteract the “vacuum” effect by

  • breathing normally, this adds air to their lungs every time they inhale
  • adding air to their mask by breathing out their nose
  • adding air to their ears and sinuses by using one of several ear equalization techniques

To equalize their air spaces on ascent a diver releases air from their body air spaces so that they do not become overfull by

  • breathing normally, this releases extra air from their lungs every time they exhale
  • ascending slowly and allowing the extra air in their ears, sinuses and mask to bubble out on its own

Buoyancy

Divers control their buoyancy (whether they sink, float up, or remain “neutrally buoyant” without floating or sinking) by adjusting their lung volume and buoyancy compensator (BCD).

As a diver descends, the increased pressure causes the air in their BCD and wetsuit (there are small bubbles trapped in neoprene) to compress. They become negatively buoyant (sinks). As they sink, the air in their dive gear compresses more and they sink more quickly. If they do not add air to his BCD to compensate for their increasingly negative buoyancy, a diver can quickly find themselves fighting an uncontrolled descent.

In the opposite scenario, as a diver ascends, the air in their BCD and wetsuit expands. The expanding air makes the diver positively buoyant, and they begin to float up. As they float towards the surface, the ambient pressure decreases and the air in their dive gear continues to expand. A diver must continuously vent air from their BCD during ascent or they risk an uncontrolled, rapid ascent (one of the most dangerous things a diver can do).

A diver must add air to their BCD as they descend and release air from their BCD as they ascend. This may seem counterintuitive until a diver understands how pressure changes affect buoyancy.

Bottom Times

Bottom time refers to the amount of time a diver can stay underwater before beginning their ascent. Ambient pressure affects bottom time in two important ways.

Increased Air Consumption Reduces Bottom Times

The air that a diver breathes is compressed by the surrounding pressure. If a diver descends to 33 feet, or 2 ATA of pressure, the air they breathe is compressed to half of its original volume. Each time the diver inhales, it takes twice as much air to fill their lungs than it does at the surface. This diver will use their air up twice as quickly (or in half the time) as they would at the surface. A diver will use up their available air more quickly the deeper they go.

Increased Nitrogen Absorption Reduces Bottom Times

The greater the ambient pressure, the more rapidly a diver’s body tissues will absorb nitrogen. Without getting into specifics, a diver can only allow their tissues a certain amount of nitrogen absorption before they begin their ascent, or they run an unacceptable risk of decompression illness without mandatory decompression stops. The deeper a diver goes, the less time they have before their tissues absorb the maximum allowable amount of nitrogen.

Because pressure becomes greater with depth, both air consumption rates and nitrogen absorption increase the deeper a diver goes. One of these two factors will limit a diver’s bottom time.

Rapid Pressure Changes Can Cause Decompression Sickness (the Bends)

Increased pressure underwater causes a diver’s body tissues to absorb more nitrogen gas than they would normally contain at the surface. If a diver ascends slowly, this nitrogen gas expands bit by bit and the excess nitrogen is safely eliminated from the diver’s tissues and blood and released from their body when they exhale.

However, the body can only eliminate nitrogen so quickly. The faster a diver ascends, the faster nitrogen expands and must be removed from their tissues. If a diver goes through too great of pressure change too quickly, their body cannot eliminate all of the expanding nitrogen and the excess nitrogen forms bubbles in their tissues and blood.

These nitrogen bubbles can cause decompression sickness (DCS) by blocking blood flow to various parts of the body, causing strokes, paralysis, and other life-threatening problems. Rapid pressure changes are one of the most common causes of DCS.

The Greatest Pressure Changes Are Closest to the Surface.

The closer a diver is to the surface, the more rapidly the pressure changes.

Depth Change / Pressure Change / Pressure Increase

66 to 99 feet / 3 ATA to 4 ATA / x 1.33

33 to 66 feet / 2 ATA to 3 ATA / x 1.5

0 to 33 feet / 1 ATA to 2 ATA / x 2.0

Look at what happens really close to the surface:

10 to 15 feet / 1.30 ATA to 1.45 ATA / x 1.12

5 to 10 feet / 1.15 ATA to 1.30 ATA / x 1.13

0 to 5 feet / 1.00 ATA to 1.15 ATA / x 1.15

A diver must compensate for the changing pressure more frequently the closer they are to the surface. The more shallow their depth:

Divers must take special care during the last portion of the ascent. Never, never, shoot straight to the surface after a safety stop. The last 15 feet are the greatest pressure change and need to be taken more slowly than the rest of the ascent.

Most beginner dives are conducted in the first 40 feet of water for safety purposes and to minimize nitrogen absorption and the risk of DCS. This is as it should be. However, keep in mind that it is more difficult for a diver to control their buoyancy and equalize in shallow water than in deeper water because the pressure changes are more extreme!

Source http://scuba-tutor.com/dive-physics/pressure/

Source https://www.tripsavvy.com/boyles-law-and-scuba-diving-2962935

Source https://www.tripsavvy.com/depth-and-pressure-scuba-diving-2963200

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