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Adiabatic Compression, as it applies to scuba divers.

Why does my instructor get excited? What exactly is adiabatic compression and why do I need to know about it?

Your instructor gets excited because of the possibility of fire or explosion! Adiabatic compression is defined as a process where high-pressure gas flows at or near sonic velocity toward the low-pressure end of a piping system with a major restriction, such as a dead-end, a down sizing, or an abrupt turn. The likelihood of a reaction (ignition) increases with increases in the rate of gas compression and gas temperature.¹

So let us break that down into terms that are understandable and apply to scuba diving and let you the diver understand why it is important. We all use compressed gas, usually air, to scuba dive with. Some divers use enriched mixes called nitrox. This is a compressed gas with a higher concentration of oxygen than air or a PO2 greater than 21%. There are also divers who use 100% oxygen (O2).

As you know, when a gas expands (decompresses) it cools. When a gas is compressed, it heats (also known as heat of compression). In a closed system, scuba unit, filling bank, etc… you are allowing the gas to move from a compressed state to a expanded state then back to a compressed state. Most people have seen and felt this when their scuba cylinder is filled rapidly the sides heat up. This is compression of the gas. If you allow the gas to flow freely from the cylinder valve, it expands rapidly as the pressure reduces and there is a decrease in temperature at the valve. It can be so great as to have frost form on the valve. This is expansion of the gas.²

When compressed gas flows from high to low pressure, it can quickly reach a very high velocity, especially at constrictions, valve seats, regulator poppits and angular fittings. In these instances, flow velocities exceeding 148 feet / sec. (45 m / sec.) are considered to be dangerous. There is even greater danger when the speed of the gas flows at or near sonic velocity (speed of sound is approximately 1,125 feet per second at 68o F). When speeding gas strikes an obstruction, it is instantly recompressed and the ambient temperature instantly skyrockets because no heat is lost to the surrounding materials (instantly in this case is not instantaneous but rather very, very fast). The higher the initial pressure, the greater the temperature generated.³ High temperatues can cause auto-ignition of containment material.

With 2,000 psi oxygen (PO2 100%), temperatures well over I,500°F can be momentarily produced. This is sufficient to ignite even such inert substances as Teflon and stainless steel. To state it simply, an uncontrollable fire can ignite and burn, hoses, metal and YOU.

This phenomenon, similar to diesel ignition, is called ADIABATIC COMPRESSION. Great care must be taken to control adiabatic compression.

Scuba cylinders are frequently filled, pressurized and depressurized by the diver. The potential for danger exists with concentrations of oxygen greater than that found in air (PO2 21%) and incomplete understanding of Adiabatic Compression.

How do you prevent mishaps? With careful design and operation of gas flow and filling operations and specific education about gas pressurization and depressurization. Several O2 fires have occurred during the preparation of diving gas mixtures due to lack of understanding and a inappropriate attitude toward adiabatic compression. Mishaps include oxygen hose fires where Teflon-lined hoses with stainless steel over-braid have detonated and burned , throwing molten particles about. Other accidents have included the neck of a scuba cylinder being blasted open and launching the valve.⁴

To prevent accidental detonations, always open O2 cylinder valves very slowly and pressurize systemic components as “gently” as possible. Design your system using slow-opening globe or needle valves that contain seats made from oxygen-compatible polymers. Transfer oxygen gas slowly to prevent excessive heating. The maximum transfer rate for pure O2 is 200 psi / minute and rates during mixing are usually 50 – 70 psi / minute. And lastly, open your scuba cylinders slowly, gently pressurizing the system.

Charles law states: “For any gas at a constant pressure, the volume of the gas is directly proportional to its absolute temperature”⁵ or more simply put “when a gas is compressed, temperature is raised”.

So remember, when working with compressed gases, always open the valves slowly. Gently pressurize systems. Transfer pure oxygen at 200 psi per minute or slower. And if you are involved with mixing gases, do not exceed rates greater than 50 – 70 psi per minute.

It can save your life!

For more information on Adiabatic compression and gas mixing consider taking a Mixed Gas Blender or Oxygen Service Technician course.

Some additional definitions of Adiabatic compression.

Adiabatic compression is a process where there is not heat transfer to or from the system, and all supplied work is added to the internal energy of the gas, resulting in increases of temperature and pressure. Adiabatic compression or expansion is favored by good insulation, a large gas volume, or a short time scale (high power level). In practice there will always be a certain amount of heat flow, as to make a perfect adiabatic system would require perfect heat insulation of all parts of a machine.⁶

Adiabatic compression is a chemical or mechanical process in which there is no exchange of heat with the surroundings (no heat entering or leaving the system). It is applicable to enclosed systems.⁷ The relationship of pressure and volume when a gas or other fluid is compressed or expanded (with no loss or gain of heat) is that compression causes an increase in temperature and expansion a decrease in temperature.

¹ Neal, Jan and Morrissette, D.. (1998). Mixed Gas Blender And Oxygen Service Technician. p. 11. Daytona Beach, Florida: Underwater Dynamics, Inc.

² Taylor, Larry “Harris”, Ph.D. Adiabatic Expansion. A Gas Laws Primer. 9 Mar 2010. http://www-personal.umich.edu/~lpt/primer.htm .

³ Boyd, Richard, Dr. and Kent, Greg. Inherent Hazards Of Pressurized Oxygen. Engineering Nitrox Partial Pressure Blending Systems © 2002 . GMC Library. Global Mfg. Corporation. 9 Mar 2010. http://www.gmcscuba.com/pdf/engineering%20nitrox%20partial%20pressure%20blending%20systems.pdf .

⁴ Boyd, Richard, Dr. and Kent, Greg. Inherent Hazards Of Pressurized Oxygen. Engineering Nitrox Partial Pressure Blending Systems © 2002 . GMC Library. Global Mfg. Corporation. 9 Mar 2010. http://www.gmcscuba.com/pdf/engineering%20nitrox%20partial%20pressure%20blending%20systems.pdf .

⁵ Heine, John, Bookspan, J. and Oliver, P.. (2000). Diving Physics. NAUI Master Scuba Diver. p.89. United States of America: NAUI.

⁶ Adiabatic. Temperature. Gas Compressor. Wikipedia. 20 May 2007. http://en.wikipedia.org/wiki/Gas_compressor.

⁷ Adiabatic. Chemical Molecule Library- Glossary. 9 Mar 2010. http://www.ch.ic.ac.uk/vchemlib/mol/glossary/ .

Dive Safe!

Dean

I am often asked (again just today) “How old does my child need to be to dive?”.   NAUI has a minimum age requirement for open water scuba diver, it is 15 years.

NAUI does have a Junior certification for ages 12 – 14 years.  Some courses (open water) allow a conditional Junior Certification. This Junior certification can be issued to persons age 12 – 14 years.  Junior Certification carries several restrictions.

* The parent or guardian and Junior diver agree that the Junior diver will dive with a certified adult (18 years of age) diver of the parent’s or guardian selection.

* The parent or guardian and Junior diver agree that the Junior diver will limit diving depth to 60 feet or less.

* By signing the Junior certification card, the parent or guardian agrees to be responsible for the selection of the certified adult buddy and the limitation of the diving depth.

* To remove the Junior cards conditional requirements, a new card must be requested by an active-status NAUI Instructor, parent or legal guardian one the Junior attains the age required for full certification. “Junior” divers may directly request from NAUI a full certification in the event that they have reached the age of majority.¹

Teaching children is not for everyone. Just as scuba diving is not for everyone. All parties involved have to be in agreement. For me, the joy is seeing that young person safely and enjoyably scuba dive. Nothing can replace the wonder seen in a young persons eyes, when they are exploring the beauty of the undersea world.

I teach at Scuba Steve’s Aquatic Adventures (http://scubastevesadventures.com/) in Tyler, Texas.

Dean Pennington II
NAUI Instructor Trainer
16 November 2009

¹NAUI Standards and Policies Manual (Tampa, Florida: NAUI, 1996), page 2.9.

So why is it important to be able to swim underwater while holding your breath? I think we all can think of several reasons. Some of them would include comfort underwater without scuba, more comfort when skin diving and while snorkeling. There is also the added benefit of preparing the diver in the event of an out of air emergency while on scuba. Granted, the diver will not hold their breath while doing an emergency swimming ascent, but the diver does become accustomed to not breathing while underwater.

The first time swimming the length of the pool underwater can be a daunting task. It is uncomfortable and you think you are going to die or drown. Here I will insert a WORD OF CAUTION.

Any breath hold training must be done with supervision. The supervisor must be aware of the inherent dangers of breath hold diving. All breath hold diving Should Not be done with hyperventilation due to the risk of shallow water blackout. Strict attention must be payed to the diver while swimming underwater.

Underwater swimming while holding your breath is easy, say for ten seconds. Almost anyone healthy can hold their breath for thirty seconds. It is when you hold your breath and begin exertion that the problem arises. It is exactly that, a rise in the level of  CO₂(carbon dioxide) in your blood stream that triggers the desire to breath. Your bodies sensors are very sensitive to change in the levels of CO₂. These sensors are called carotid bodies.

The good news is that like many things associated with your body, you can train it. Training your body to gradually accept greater levels of CO₂ takes practice.  Ideally this will be done in the water with supervision. In addition to training your body you must train your mind. As a reader here on Scuba Jedi, certainly you will understand the need to train your mind! I will not delve to deeply here but instead will say that you must calm your mind and your body. Excitement stimulates your heart to beat faster.

If your heart beats faster, then your body uses more oxygen. If your body uses more oxygen it in turn produces more carbon dioxide. When CO₂ reaches that critical value, you have a incredible urge to breath.

So four steps to help enable you to successfully swim 25 yards underwater in one breath include:

1.  Supervised practice
2.  Gradual increase in time underwater
3.  Calm mind and calm body
4.  No Hyperventilation!

Let us not forget neutral buoyancy. If you float or are wearing thermal protection you will need to offset this with a minimal amount of weight. Another consideration when swimming underwater is streamlining and remaining hydrodynamic. Stretch out and remain level while swimming and do not forget to glide!

Swimming underwater while holding your breath is exciting, especially if you can see that you are making progress in your distance. If you follow the simple steps outlined you will be touching the wall at 25 yards in no time!

Dean Pennington II

NAUI Instructor Trainer

8 November 2009

¹NAUI Standards and Policies Manual (Tampa, Florida: NAUI, 1996), page 2.27.
²NAUI Standards and Policies Manual (Tampa, Florida: NAUI, 1996), page 2.73.

NAUI courses are very thorough and routinely exceed minimum industry standards. The basic course is Scuba Diver, which takes the non-diver to open water certification. This is followed by Advanced Scuba Diver which improves your overall knowledge and skill in the water. It is designed for newly certified divers and introduces them to many different types of activities available to certified divers. The Master Scuba Diver course helps you acquire leadership-level academic knowledge and enables you to participate in advanced diving activities .²

Other NAUI courses include Rescue Diving and Nitrox Diver. Leadership level courses with NAUI include Divemaster and Scuba Diving Instructor.

The NAUI ideal is “Dive Safety Through Education”. The NAUI slogan is “The Quality Difference”. The corporate mission statement is “to enable people to enjoy underwater activities as safely as possible by providing the highest quality practical education, and to actively promote the preservation and protection of the world’s underwater environments”. The NAUI credo states that businesses and individuals are entitled to do business with whomever they choose and we (NAUI) do not demand that they be exclusive to NAUI. They do however expect NAUI members to represent and support the NAUI organization to at least the same extent they support any other organization.³

To become a NAUI leader, it takes more than desire. The step up to leadership levels of Divemaster and Instructor is a challenging process. It entails performing and committing to the limits of your ability.

The primary goal of NAUI leadership training is to help the candidate become as successful a diving leader as they can be. The training has four aspects:

1. Acquiring diving knowledge and developing the ability to perform and demonstrate specific essential water skills.

2. Learning leadership skills and developing an effective personal leadership style through study and the observation of other leaders.

3. Benefiting from evaluation and coaching on all aspects of leadership responsibilities.

4. Gaining experience through exercising leadership skills under the supervision and guidance of a NAUI Instructor. ⁴

As a NAUI leader you will develop an attitude that places the highest concern for student and diver safety. Published minimum standards of achievement are treated as goals to be acquired and exceeded. The instructor candidate will embrace the tenets of over-learning, maximizing individual potential, public speaking, developing personal best aquatic skills, minimum and no impact environmental activism, and learning by doing.⁵

NAUI Leadership and Instructor training is founded on the “Loved One” concept. That is, it is only those we would trust to train our loved ones to dive that we graduate at any level and especially so for leadership and instructor certification.

Ultimately, when you become a NAUI Instructor, you will have academic freedom. That is the freedom to meet the students academic needs. You are not required to follow a specific order of training , you are allowed to explore and innovate ways to teach and help individual students and divers meet certification requirements. ⁶

Becoming a NAUI instructor is about embracing a set of values that transcend the mundane pursuit of a vocation. It is about becoming filled with a passion to achieve one’s personal best and helping others to achieve the same .

¹ Barocas, I. (2003). NAUI Leadership and Instruction. p. 4. United States of America: NAUI.
² Carroll, S. (2000,2004). NAUI Scuba Diver. p. 7. United States 0f America: NAUI.
³ Barocas, I. (2003). NAUI Leadership and Instruction. p. 8. United States of America: NAUI.
⁴ Barocas, I. (2003). NAUI Leadership and Instruction. p. 5. United States of America: NAUI.
⁵Barocas, I. (2003). NAUI Leadership and Instruction. p. 6. United States of America: NAUI.
⁶Barocas, I. (2003). NAUI Leadership and Instruction. p. 7. United States of America: NAUI.