Partial Pressure or PP in Scuba Diving

Aug 1, 2022

“Partial Pressure” or “PP” in 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.

Partial pressure refers to the pressure exerted by an individual gas in a mixture of gasses. Here’s how it works!

Partial Pressure Applied to Scuba Diving

An easy way to think of partial pressure in scuba diving is to consider it a measurement of the concentration of a particular gas in a diver’s mixture of breathing gasses. As the concentration of a particular gas in a diver’s breathing gas mixture increases, the physiological and psychological effects of that gas may increase or change. For example, extremely high partial pressures of oxygen may be toxic (oxygen toxicity) and very high concentrations of some gases, such as nitrogen, may cause narcosis.

Two factors determine the partial pressure of a gas in scuba diving—the percentage (or fraction) of the gas in the breathing mixture and the depth (and therefore the ambient pressure) at which a diver breathes the gas. The higher the percentage of a gas and the deeper a diver descends, the greater the partial pressure of the gas.

Calculating the Partial Pressure of a Gas

It’s easy! Simply multiply the percentage of the gas in the breathing gas mixture by the ambient pressure of the dive. For example, if a diver is breathing air (21 percent oxygen) at a depth of 66 feet of sea water, the partial pressure of oxygen is:

0.21 the percentage of oxygen as a decimal fraction
x 3 ata/bar*
the ambient pressure of the dive in units of either atmospheres or bar
= 0.63 ata/bar
the partial pressure of oxygen in air at 66 feet of sea water

Partial pressures of a gas are given in units of either atmospheres or bar. While these units are technically different, they are close enough to be used interchangeably in all but the most picky of calculations.

Abbreviations

Divers use the abbreviations “P” and “pp” when referring to the partial pressure of a gas. For example, in reference to the partial pressure of oxygen (O2), a diver may encounter the following abbreviations: PO2, pp O2, and O2 pp.

Don’t understand why the diver is at 3 ATA of ambient pressure? Then it is time to review the basics of pressure and scuba diving.

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.

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

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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.

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.

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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.

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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!

Can I scuba dive if I have high blood pressure?

Scuba diving can be a great way to keep fit and provide a workout for your heart.

However, diving can have significant effects on the body, including increasing blood pressure, which could pose a risk when diving, or a risk to your health in general.

There are a number of medical conditions that could put you at risk when diving. So if you have a health condition, it’s a good idea to get checked out by your doctor before you go diving.

There are a number of medical conditions that could put you at risk when diving

Mike Knapton
Associate medical director

The British Sub-aqua Club is the governing body for sub-aqua diving and snorkelling in the UK. It has a useful website with all the information you will need to take up scuba diving. This includes a self-declaration form which is a short questionnaire asking you about any significant medical conditions. If you have any of these conditions you will need a medical reference and potentially a medical examination.

These include diseases of the heart and circulation including high blood pressure, angina, chest pains and palpitations, diabetes and stroke.

You would also be recommended to have a medical if you are on any medication (other than the oral contraceptive pill).

Subject to a satisfactory medical – which can be undertaken by your GP, but will incur a private fee – you should be able to dive safely.

Meet the expert

Dr Mike Knapton is Associate Medical Director (Prevention and Care) at the BHF, overseeing the strategic role in helping patients and the public reduce their risk of heart disease. He remains a GP and works one day a week at a practice in Cambridge.

More useful information

British Heart Foundation

British Heart Foundation is a registered Charity No. 225971. Registered as a Company limited by guarantee in England & Wales No. 699547. Registered office at Greater London House, 180 Hampstead Road, London NW1 7AW. Registered as a Charity in Scotland No. SC039426

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