DECONSTRUCTING THE PROCESS OF LEARNING AND INSTRUCTION: Educational models and what they teach us

Modified from a handout given to sport and technical instructor candidates

“There are just two things in life: but I forget what they are…”
John Hiatt, American poet, musician, b: August 20, 1952

It’s nobody’s intention to throw cold water in your face, but please resist the temptation to call yourself a professional educator on the strength of graduating from a scuba instructor class: technical, sport or otherwise.

In the space of a couple of weeks — the duration of the average sport instructor program — or a couple of days — the average length of programs upgrading existing teaching credentials — there is little opportunity to make it otherwise. Your instructor’s certification card is only a ticket to ride… an invitation to start the process of learning how people learn; and by dint of hard work, teaching yourself how to teach.

With luck, experience, quite considerable additional effort, and some bloodshed — which I hope is entirely metaphoric — the best you can hope for is that you’ll become an empathetic, well-informed, process-driven and safety conscious lay educator. And in the greater scheme of things, and the absence of a PhD in education, that’s not an insignificant achievement.

Our industry’s goals for you are surprisingly modest… follow the supplied guidelines, deliver the prescribed curriculum with enthusiasm and accuracy, and the chances are very good that you will fulfill them. However, I am sure you have higher ambitions and want to do better than average, so let’s see what we can do t0 help you in that regard.

Coloring inside the lines… sometimes
An instructor’s purpose is to guide students through a set curriculum towards effective learning. Always, always effective learning. That’s a winding path. Or perhaps it would be more accurate to say that there are several paths leading to effective learning. Some are well beaten, pretty obvious and relatively safe. Others are more circuitous, overgrown with all sorts of interesting vegetation harboring countless temptations and distractions; but they also end up in the right spot.

And of course some paths look promising but go off on a complete tangent wasting everybody’s time and effort only to peter out someplace miles from the destination. The trick of course is to pick the pathway that best suits your students’ interests and your teaching style; and often that is not the most direct or well-worn route.

A decent map helps. A map helps everyone in the class avoid the tangents and points out which pathways lead in the right direction. With a map in hand, a motivated instructor can guide his charges around the obvious and accompany them along the more engaging route, and still arrive at effective learning changed but intact.

The map we hand you as a newly-minted instructor is rather like a page torn from Ptolemy’s Atlas with huge areas labeled Terra Incognita, and you are expect to fill in the blanks. The biggest help I can offer on that score, is to suggest you look over the shoulder of a professional educator — someone who understands the way learning works — and copy bits of their map. It’s not cheating if you mention your source.

The conditions of human learning: Gagne’s Nine Events of Instruction
I cannot think of a better first shoulder to look over than that of American educational physiologist, Robert Gagné.

Gagné had a profound influence on a broad spectrum of American education including military, institutional and industrial training. His theory on instructional design and what’s now called Task Analysis was detailed in The Conditions of Learning, originally published in 1965 by Holt, Rinehart and Winston. If you can find a copy, I recommend reading it cover to cover.
The theory detailed in The Conditions of Learning remains influential, and grew from Gagné‘s work as a training designer during WWII. His challenge then was to develop teaching materials that could be used by subject experts with little or no formal training as teachers. They needed to impart very specialized technical skills to thousands of raw Army Air Corps recruits in a ridiculously short time… about two years worth of on-the-job experience had to be crammed into about a month.

Based on his experience and research cracking that problem — and further developed until his death in 2002 — Gagné became convinced that in most training situations, effective and efficient learning takes place when the final task is first broken down into a set of component parts.

The analogies between the farm hands Gagne helped turn into aircraft mechanics in a month, and the challenges facing scuba instructors and instructor-trainers are simply too similar to be ignored.

Gagné identified the conditions necessary for effective learning based on the way mental events are triggered in adults by various stimuli — visual, audible, tactile etc. Gagné created a nine-step process that he called, the events of instruction. These events correlate to and address the conditions of learning.

The nine events of instruction are: Gain attention; Inform learners of objectives; Stimulate recall of prior learning; Present the content; Provide learning guidance; Elicit performance; Provide feedback; Assess performance; Enhance retention and transfer to the job.
With very little mental juggling and word substitution, that list should sound awfully familiar to anybody who’s sat through an Scuba Diving International™ Instructor Development Course. But let’s recap a little here and take the time to “enhance retention…”

Gain attention
Gagné tells us that in order for any learning to take place, an instructor’s first task is to capture the student’s attention. We can do this effectively in a number of ways and the more varied and creative those attention-getting actions are, the more attention they’ll attract.
One example could be the opening segment of a multimedia program that has wild animation accompanied by sound effects or music. This would wake up the senses with auditory and visual stimuli. But after a while, even that approach would become ho-hum and a change of tempo would be key.

Something to interleave with that type of approach is to ask a thought-provoking question or hit the audience with an out of the ordinary, lesser-known fact. Even better if its relationship with the topic about to be discussed is not immediately obvious but requires thrashing around in one of those leafy thickets of interesting vegetation beside the pathway to completely understand. Curiosity motivates students to learn. So does self-preservation and stressing the importance of a topic by marrying its importance to staying ‘safe’ is another good punctuation to a bunch of whiz-bang AV effects.

To anyone familiar with the physiology of sales, this is the benefit statement / value proposition. Effective learning can certainly begin by walking learners though a strong value proposition. And my experience as an instructor-trainer informs me that individuals with a sales background have a leg up on their instructor course classmates understanding how to gain a learner’s attention.

Although gaining attention is labeled the first task, it is also the most constantly called upon. Effective Learning will not take place if the learner’s attention is not coupled to the “lesson” from start to finish.

Inform learners of objectives
Tell them what you are going to tell them and explain what outcomes to expect! Does, “at the end of this presentation you’ll be able to…” sound at all familiar? It should because it’s an essential step integral to the success of an SDI™, TDI™ instructor training course. Early in each lesson students should be presented a list of learning objectives. This fires-up expectancy in their minds and helps motivate learners to buy into the lesson and complete it.

Perhaps more importantly, the list of lesson objectives forms the base scaffolding that assessment of performance and certification are built upon. Therefore it is essential that this instructional event be clearly presented and completely understood by everyone in the class. The phrasing of objectives, the way an instructor presents his or her expectations to the class, colors everything else that happens during the course. Instructor trainers evaluating the progress of instructor candidates, will heavily weight these actions in that evaluation.

Stimulate recall of prior learning
Associating new information with existing knowledge. This event can be triggered by the instructor, by the student or by some other source. The classic opening, “Have you ever experienced your ears popping or hurting when driving through the mountains or when flying?” is an example of this event in action. Establishing some comparison to past shared experience and lessons learned will facilitate the learning process for a new concept. It is easier for students to encode and store information in their long-term memory, when there are ties built between it and pervious personal experience and knowledge. The way this was hammered home to me was, “New concepts will not stick without old associations to hold them in place.” I thank long-time friend Bret Gilliam for that particular nugget.

Perhaps the most simple and straightforward way to stimulate recall is to ask students questions about previous experiences. Having them dig around in their memory and then build the associations with previous concepts themselves is the sort of rich-content experience that will have them thinking their instructor is a genius; when in fact their instructor sat back and watched… no more.

Present the content
Tell them what you told them you were going to tell them. This is where the meat, potatoes and crème brûlée of the new content is served to the student. The size of the portions and how they are arranged on the plate depends on the way the course work was designed. However, effective learning is best guaranteed when content is rationed out and organized meaningfully, and typically is explained and then demonstrated by the instructor or an assistant (divemaster for example).

To maximize appeal and broaden the impact and effectiveness of this event, a variety of presentation styles and media should be used. Text, graphics, audio narration, video, self-directed exploration, even chalk and talk all qualify. Also, with a well-designed course and rationally executed learning materials, the content message will be audience appropriate and presented in a logical progression. For example; a detailed discussion on Fick’s Laws of Diffusion and their influence on dual-gas phase decompression algorithms, with a group of newly-certified open water divers is likely to be a one-sided conversation. On the other hand; cut the topic down to its core essentials — several factors can drive bubble growth in a diver’s bloodstream — make it appropriate for beginning divers — ascent rates are important on all dives including those within the NDL — and effective learning may take place.

Provide “learning guidance”
An instructor’s role is to help students grasp the core essentials. Demonstrate, show examples, dispel myths, indicate erroneous examples, explain with diagrams, help with mnemonics and analogies. Ask questions. Different students respond to different stimuli in unique ways… and the same student may be motivated to learn by changing stimuli from one day to the next. An instructor’s challenge is to recognize these subtleties in the classroom, swimming pool, ocean or lake. The key traits for an instructor during this step are empathy and patience. The guidance offered during this event, will help learners encode information ready for storage in long-term memory.

Elicit performance (practice)
This is the event of instruction during which the student confirms for themselves that they have a correct understanding of what’s being taught. In the case of an in water skill such as clearing a mask of backward fining, they get to perform practice drills that test the new skill or demonstrate changed or new behavior. Repetition further increases the likelihood of retention and mastery of the subject/skill.

Provide feedback
Not to be confused with the following step, exercises within tutorials, presentations and dive-skills demonstrations demand the instructor discuss correct and incorrect solutions with students by providing specific and immediate feedback. If the performance is a physical skill demonstrated in-water, video is unbelievably helpful for doing this effectively. Additional guidance and answers provided by the instructor at this stage are called formative feedback.
Formative feedback is quiet special because it can and usually does come from several directions. During debriefing of skills dives, it’s not uncommon for other students to crack the nut for one of the class who is having trouble mastering a skill.

When I first began teaching technical diving programs, I had a hugely difficult time with a particular student who was unable to perform a compound but really quite simple skill the rest of his class mastered after just a few attempts. The skill was core to the course and it had to be mastered before we moved on. I demonstrated it to the student on dry land, in the water, and even had his two classmates run through it for him while I watched him watch them.
Back on the surface, it was one of his buddies that explained which part of the process he was missing. I did not see it.

A similar thing has happened to me in an academic setting. Trying to explain a “basic” maths process to a student who simply did not get it only to have their buddy use an analogy that placed the problem in terms they grasped immediately.

I think my point here is to encourage feedback from everyone involved in the class. Often, I sit back and listen while each member of a class does a step-by-step, blow-by-blow analysis. Direct the process; but don’t suffocate it.

Assess performance
Once instruction is finished and demonstrations are completed, students should be required to take a post-test, exam or final assessment. This assessment must be completed without the additional coaching, feedback, or hints from the instructor. Mastery of material or a pass certification is typically granted when the student attains a certain percentage of correct answers, or demonstrates a skill within the range of acceptable proficiency.

With a written test or exam, this assessment is relatively simple to accomplish but with underwater skills, subjectivity CAN become a muddling factor. Strive to be objective. With skills where no guidance has been provided, set the bar for a pass or fail based on 80 percent of your own performance. Never be afraid to fail a student who is unable to master a skill or retain and understand a concept. The fact is, not everyone can dive. Not everyone can cave dive, wreck dive, deep dive, do staged decompression dives! As instructors, we have a responsibility to get that message across.

Enhance retention and transfer to the job

Essentially, use it or lose it. In diving, there is a sort of built-in mechanism that aids students to apply skills learned from a training program “back on the job.” The environment and the divers themselves self-select, and instructors should make this clear to graduates. Often the imperative for enhanced retention and transfer of skills is survival. A bungled skill — let’s say valve shut down — or misremembered concept — nitrogen uptake for example — sooner of later will cause a serious problem, perhaps injury or death. The environment will test skills without pity and in the technical diving realm, where skills are more complex and numerous than in sport diving, skills need to be tested with detachment regularly. A diver who has learned effectively will realize their shortcomings and ask for help from other divers, will reread texts or research the answers… but most of all, practice.

Events of a Lesson
Applying Gagne’s nine-step model for instruction to a training program is the single best way I know to ensure effective learning. Above all else in education there is no substitute for sound instructional design. There is no substitute, even with the Niagara of information pouring out of our computer screen, for an instructor who can help students maximize the effectiveness of information processing.

Gagné believed that all lessons should include the key steps of motivating the student to learn; giving clear objectives; directing focus on pertinent information (this based on the instructor’s “read” of the materials and the student’s personal learning style); stimulating recall by tying new concepts to previously learned material; providing guidance with hints and illustrations that appeal to the student’s curiosity; enhancing retention by adding familiar examples; promoting the transfer of learning; allow the student to show off what they have learned and providing feedback.

Strive to be the best instructor you can. Use the guidance available from visionaries such as Gagne. Etc.

Daily Limits for CNS Oxygen Toxicity

A posting on one of the popular online scuba forums got me thinking about how we teach CNS 24-hour limits, because there was nothing but incorrect information posted. A conversation with one of the senior ITs for the agency I teach for, followed up and I realized we need to put more emphasis on this topic in the classroom… especially given a couple of recent incidents.

I dug out my teaching notes and figured posting them here was a reasonable thing to do. If you have comments or suggestions, please let me know.

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First of all, a definition of oxygen toxicity syndrome (also known as the “Paul Bert effect”)

This is severe hyperoxia caused by breathing oxygen at elevated partial pressures… usually a function of breathing something with oxygen in it at depth or breathing pure oxygen as part of a decompression strategy. The high concentration of oxygen damages cells within the diver’s body. The precise mechanism(s) of the damage is not known, but oxygen gas has a propensity to react with certain metals to form superoxides; and these may attack double bonds in many organic systems, including the unsaturated fatty acid that residues in cells. High concentrations of oxygen are known to increase the formation of free-radicals in biological systems – such as divers. The formation of these free-radicals may then begin a sort of cascade of events which may directly harm DNA and other structures. Normally, the body has many defense systems against such damage but with hyperbaric concentrations of oxygen, these systems are eventually overwhelmed over time, and the rate of damage to cell membranes exceeds the capacity of systems to control damage or repair it. Cell damage and cell death then results.

If any anyone feels that tracking oxygen exposure is a waste of effort, I feel this alone should convince them otherwise. I addition, there have been several recent incidents of CNS poisoning in divers where the dives were conducted within acceptable limits. This gives us pause for thought and reinforces the need for us to be conservative in our CNS oxygen toxicity tracking.

Before moving on to methodology for tracking NOAA Daily Limits – NOAA seems to be the most accepted scale or system – let’s recap.

The oxygen exposure time for a single dive is compared to the Single Dive Exposure Limits on the NOAA table (1.6 for 45 mins, 1.5 for 120 mins 1.4 for 150 mins and so on).

The suggested working limit for this type of exposure is 80 percent of the maximum shown in the NOAA table. (e.g. 1.4 for 120 mins, 1.5 for 96 mins or 1.6 for 36 mins). This 80 percent limit has been almost universally adopted by technical diving communities around the world. In ALL further documentation unless otherwise stated, this is what is meant when the oxygen limit is mentioned.

When tracking with a single gas (bottom mix) the exposure at depth is all that needs to be considered since the oxygen pressure during normal ascent and at the depth of a standard safety stop, must by definition be less than 0.5 bar. For all practical purposes this amount of oxygen is too low for consideration in CNS calculations for recreational diving (sport or technical).With multiple gases (the use of a decompression gas) the oxygen pressure for ALL PHASES of the dive MUST be calculated and added together to find the total single dive oxygen exposure.

If a diver reaches the limits of the Single Exposure Time on a single dive then he must take at least a two-hour interval on the surface, breathing normal air. This surface interval is thought to reduce the CNS loading by about half. Current thinking is that CNS loading is subject to a 90-minute half-time. This means that a diver who gets out of the water with a CNS “clock” at 40 percent on surfacing, will have that loading reduced to 20 percent, 90 minutes later. (This CNS 90-minute half-time is under scrutiny and may be adjusted at some point in the very near future… so stay tuned.)

If two dives are conducted with less than a two-hour surface interval, treat them as a single dive for the purposes of CNS tracking. In other words, the in water times are added together and compared against the Single Exposure Time. If one dive is at a greater oxygen partial pressure than the other, that pressure is the one used with the combined in-water times of the two dives, to calculate total CNS loading.

If two or more dives are conducted within a single 24 hour period with more than two hours at the surface between each dive, then the total in water times are added and compared against the Daily Limit to arrive at the diver’s CNS loading. We will cover this in a moment.

In more complex decompression diving, the total CNS loading for bottom time and each staged decompression stop is taken into account – including jumps in oxygen pressure when gases are switched. The total times in minutes for each oxygen pressure for the dive, the whole dive, are added together and expressed as a percentage of the allowable total single dive limit.

If a series of dives in a 24 hour period reaches the Daily Limits, then a 24 hour surface interval breathing air is the safest option to be taken before diving again.

Daily Oxygen Limits or tracking CNS on multiple dives
Daily limit tracking is essential when multiple dives are planned and is particularly important for divers doing Live-Aboard trips where the first dive of day two can easily be less than 12 hours after the last dive of day one!

I have heard it said that NOAA daily limits are a proxy for pulmonary toxicity management. They are not. This is complete nonsense. Pulmonary toxicity has nothing to do with these calculations or the need to be vigilant keeping tabs on CNS toxicity!

Examples to illustrate the efficacy and value of Daily Oxygen Pressure Time Limits

This topic is a required as part of the curriculum for both TDI Advanced Nitrox and Decompression Procedures courses. The examples with most relevance for students will be slightly different from one course to the other. For instance in a stand-alone Advanced Nitrox course, we can use the example of a photographer on open circuit scuba making several shallow nitrox dives using a mix that delivers a partial pressure of 1.4 bar at depth. Since the reef is shallow, he can pull bottom times of an hour. Here are three dives that seem plausible.

By the way, these profiles where derived using V-Planner version 3.81 software by Ross Hemingway, and the algorithm being used is VPM – B.

DIVE PLAN #1
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.

OTU’s this dive: 103
CNS Total: 40.7%

107.5 cu ft Nitrox 50
107.5 cu ft TOTAL

DIVE PLAN #2
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.

OTU’s this dive: 103
CNS Total: 56.8%

107.5 cu ft Nitrox 50
107.5 cu ft TOTAL

DIVE PLAN #3
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.

OTU’s this dive: 103
CNS Total: 63.2%

107.5 cu ft Nitrox 50
107.5 cu ft TOTAL

Each dive is ‘safe’ from the point of view of CNS because none approaches the 80 percent margin, and none brings the diver close to required decompression (26 foot EAD!). HOWEVER, at the end of these three dives, the diver has about 180 minutes at a PO2 of 1.4 bar which maxes out his allowable daily dose.

According to NOAA’s table, he has to stay out of the water for 24 hours. I teach that there is no allowance made on the daily limit for the supposed 90-minute half-time decay of CNS loading… with the jury still out on what exactly happens to trigger a CNS episode, this seems the most logical and conservative practice to adopt.

This becomes more compelling given the aging of the average diver and the widespread use of anti-nausea meds and various other pharmaceuticals and dietary supplements: none of which have been studied sufficiently to allow use to disregard their possible interactions during nitrox diving. (Note: at the finish of the examples cited above, the diver’s OTUs are at about 300 which is far less than the daily limit and consistent with levels to aim for on multi-day exposures… in other words, CNS toxicity is the issue, NOT Pulmonary)

Now, let’s look at multiple decompression dives. Many sources warn against the practice of executing more than one staged decompression dives in a day. Let’s see why that might be. It does seem odd since pulling off two or sometimes three deco dives a day is common practice in some regions, especially in warmer water.

To illustrate why this requires careful planning and CNS tracking, here are the figures for two identical decompression dives with a SIT of six hours.

DIVE PLAN #1
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen – 10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 64.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

DIVE PLAN #2
Surface interval = 0 day 6 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 68.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

Again, each is within the single-dive CNS limit of 80 percent or less on the clock. There is a six-hour surface interval and each dive seems to have a conservative ascent profile with the use of oxygen to optimize off-gassing. But once again, we need to consider daily CNS loading.

The total time at 1.4 bar of oxygen for these two dives is about 80 minutes… that’s equal to about 45 percent (80/180) of the NOAA limit. In addition, the total time at 1.6 is 32 minutes which is about 22 percent 32/150) of the NOAA limit. This adds up to 67 percent for the day. No worries.

But here is the issue. The NOAA daily limit is for a 24-hour period NOT a calendar day. If this diver – on a decompression course and anxious to get in the final dive before the weather turns nasty – gets an early start the next morning and – thinking all is clear because he has had a good sleep – plans a slightly deeper and longer dive, he may be pushing the limits. Here are the two dives on day one with the early morning dive on day two added.

DIVE PLAN #1
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 64.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

DIVE PLAN #2
Surface interval = 0 day 6 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 68.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

DIVE PLAN #3
Surface interval = 0 day 10 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 145ft (2) Nitrox 26 50ft/min descent.
Level 145ft 32:06 (35) Nitrox 26 1.40 ppO2, 134ft ead
Asc to 70ft (37) Nitrox 26 -30ft/min ascent.
Stop at 70ft 0:30 (38) Nitrox 26 0.81 ppO2, 63ft ead
Stop at 60ft 3:00 (41) Nitrox 26 0.73 ppO2, 54ft ead
Stop at 50ft 4:00 (45) Nitrox 26 0.65 ppO2, 45ft ead
Stop at 40ft 5:00 (50) Nitrox 26 0.57 ppO2, 35ft ead
Stop at 30ft 9:00 (59) Nitrox 26 0.50 ppO2, 26ft ead
Stop at 20ft 17:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 97.5ft

OTU’s this dive: 97
CNS Total: 61.8%

146.0 cu ft Nitrox 26
17.0 cu ft Oxygen
162.9 cu ft TOTAL

Are those examples plausible? Certainly and I’ve witnessed it or something like it many times. Are they safe? Maybe, and maybe not because the additional 32 minutes of bottom time at 1.4 bar on the third dive plus another 16 minutes at 1.6 bar to optimize deco, has brought the diver’s 24-hour CNS loading to about 95 percent of NOAA’s limits.

Accordingly, if someone did these three dives, there should be a 24 hour break before the next dive.

Are these fair examples? I think so. Do they illustrate why tracking of daily CNS limits is of use when using high-test nitrox? I believe they do. Of course there are strategies we can adopt to mitigate the risks but it is important to consider that only by taking notice of NOAA’s Daily Limits are we made aware of just how much risk we are faced with. In light of several tragic incidents with divers using nitrox and executing decompression dives over multiple days, it seems prudent for us to follow this guidance.

About Oxygen… part one

This is the second in a short series of pieces about gases and gas behavior.

This series of articles are excepts from my book, Twenty Lectures on Technical Diving, due this summer. This particular piece is based on a presentation prepared for an Advanced Nitrox / Decompression Instructor Program in 2008

“In the natural sciences, and particularly in chemistry, generalities must come after [earning] detailed knowledge of each fact and not before.”

Joseph Louis Gay-Lussac, French chemist whose work on gas behavior was visionary and is misquoted by dive instructors the world over, 1778 – 1850

For divers, certainly for technical divers, a little detailed knowledge of oxygen and its behavior is important. Why? Well, breathing too much of it can be fatal. Breathing too little of it can be fatal. And as though that’s not enough reason to pay attention, oxygen has to be stored, transported and delivered with some care otherwise it can cause real damage to property and people! In short, if we forget or neglect to follow the rules, oxygen can be a real menace; however, the rules are straightforward and easy to remember!

Let’s start off with some basic chemistry and character assessment.

Oxygen makes up approximately 21 percent of air by volume… this compared to nitrogen at roughly 79 percent. These figures are fudged because air has many other components including things like water vapor, carbon dioxide, and traces of several Noble Gases like neon, xenon, and so on. But regardless of these facts, divers and diving texts simplify matters and quote the 21 percent figure. In truth, we can make this approximation without causing a fuss or compromising our safety. But it is worth remembering that it is unlikely that the percentage of oxygen in the air around us right now is 21 percent… it’s certainly less and it varies under the influence of humidity, temperature, the season, location and the environment.

Oxygen is non-flammable — which strikes some people as counterintuitive — but it is highly reactive. This means that on its own at atmospheric pressure, oxygen behaves itself, but introduce another substance into an oxygen-rich environment or increase the pressure and you have a potentially dangerous situation because oxygen bonds eagerly with almost everything. With the slightest encouragement that “bonding” process can take the form of an aggressive, all consuming fire.

For example, high-pressure oxygen delivery systems — the vessels, valves and lines used to fill scuba cylinders — must be designed and built with no sharp corners in the hoses or sudden restrictions that might cause adiabatic compression, and thereby start a fire. Oxygen fires in these environments are notoriously difficult to extinguish and often burn until the oxygen runs out or there’s nothing left of the system to burn.

In oxygen delivery systems, needle valves are used rather than ball valves so that oxygen flow can be finely controlled and the likelihood of sudden pressure increases is lessened. All scuba gear used for mixing and delivering hyperoxic gases should be composed of materials suitable for use in a high-pressure oxygen environment. These components must be cleaned of hydrocarbons, lubricated sparingly with special lubricants, and be carefully stored and used specifically to prevent contamination with dirt and grease. So, don’t eat a sausage and bacon breakfast burrito while putting together your decompression cylinder!

In addition, decompression cylinders of high–test nitrox or pure oxygen must be filled slowly. I have seen the high-pressure seat inside a tank valve vaporized during a hurried fill. The cylinder looked fine from the outside but the gas it contained was pure oxygen contaminated with the gases formed as the nylon burned. (Two lessons learned that day. The second being always pre-breathe gases that are going to be used on a dive, before the dive begins.)

Oxygen is more compressible than nitrogen. Its molecules are so “friendly” that they cram up nice and tightly when being pushed into enclosed spaces. So for a given pressure inside a scuba cylinder, one is able to put a greater quantity of oxygen than say, air or most certainly helium. This is important information for those divers who blend their own gases. Without fudge factors taking into account variations in gas compressibility, or calculations modified via Van der Waals’ or Beattie-Bridgeman equations, their mixes will contain higher than planned levels of oxygen.

For those of you who like details, oxygen has a density of roughly 1.43 grams per litre at normal room temperature and pressure (20 degrees, one atmosphere).

OK, so that covers some basics about handling oxygen, now what about breathing it?

Of course oxygen is the “active” ingredient in air and necessary for our body to function. One part of our circulatory system’s job is to deliver oxygen to the tissues within our body, and over millions of years, that transport system and the rest of the human body it serves has evolved to function comfortably breathing a gas with an oxygen fraction of about 21 percent.

At sea-level an oxygen fraction of 21 percent translates into an oxygen partial pressure of 0.21 bar or 0.21 atmospheres. The wonderfully adaptable engine that it is, the human body is able to acclimatize to attitudes where there’s a significant drop in atmospheric pressure and therefore in the partial pressure of oxygen.

The communities of La Paz, Bolivia and Lhasa, Tibet are both above 3,600 metres or 11,800 feet. The air at that altitude is approximately two-thirds as dense as it is at sea level. Since the fraction of oxygen remains unchanged, we can use Dalton’s Law to calculate that the partial pressure of oxygen available to the folks walking along Avenue Camacho, in the Bolivian capital or the tourists at Jokhang Temple in Lhasa is about 0.15 bar.

Without doubt, if we could magically and instantly transport everyone in this room to either of those spots, most of us would pass out and risk death as a result of severe high altitude pulmonary or high altitude cerebral edema.

But what about the people who live there… and what about the tourists? The key of course is time. Time to acclimate to the lower partial pressure by ascending gradually, giving the body time to make adjustments to less available oxygen. Visitors also get the  help of anti-mountain sickness drugs.

Even with these precautions, a significant proportion of “sea-level” tourists never truly get used to being at altitude and every year, some have to be evacuated to lower altitudes. Attrition rates vary but up to half the folks on trekking holidays in Nepal and Peru fall foul of altitude sickness.

This is wonderfully interesting but somewhat misleading for divers. We have to be extremely careful to avoid low partial pressures of oxygen, because there is no acclimatizing to hypoxic mixes for us. If someone pumped a gas mix into this room containing 15 percent oxygen, we’d all fall asleep. If we breathed that same gas with 60 kilos of dive gear strapped to us, and we had to move through a medium 800 times denser than air, there might be a few of us for whom the sleep would be infinitely long and dreamless.

Recreational divers do not and can not adapt to hypoxic mixes. Divers have to be particularly careful to pay this heed. Our bodies need a partial pressure of at least 0.16 bar to sustain a base-level of activity… 0.18 if we hope to swim or make sense of the world. Less than that and the brain begins a slow samba towards siesta time.

This is bio-physics or physiology and so the variables of individual susceptibility come into play when we talk about hypoxia. Its effects may be more or less pronounced depending on the person and even with the same person at different times. My personal comfort with this aspect of dive execution is conservative. I’ve seen divers using trimixes with less than 14 percent oxygen, breathing them on the surface. Their practice is to get quickly to a depth where the oxygen partial pressure or their mix becomes normoxic (0.21 bar). In the case of a 14 percent mix, this would be at approximately 5 metres or 16 feet.

I’m not comfortable with that practice at all. For me it’s tantamount to playing Russian roulette. It only takes one instance where something goes slightly wrong… a very minor thing… that requires a little extra effort, and there’s Mr. Sleepy tapping you on the shoulder. That’s just not the way to  start a dive to a depth that requires hypoxic back mix.

I’m more comfortable breathing a decompression mix that’s hyperoxic on the surface and then switching to back mix at some convenient point before reaching that decompression mix’s Maximum Operating Depth (MOD).

Hyperoxic? A gas containing a greater fraction of oxygen than air. And that’s a good a transition as any into defining best practice when there’s lots of oxygen.

THE SIX BASIC SKILLS: Number One, Breathing

Part of a lecture given to trimix instructor candidates in September 2007

“Our breath is the bridge from our body to our mind: the element which reconciles out body and mind, and [thus] makes possible oneness of body and mind. Breath is aligned to both body and mind and it alone is the tool which can bring them both together, illuminating both and bringing both peace and calm.”

Thich Nhat Hanh, Zen Buddhist Monk, The Miracle of Mindfulness

If I had to limit my advice to prospective technical divers to just one tip, it would be for them to learn how to breathe correctly. Correctly being deep, controlled, abdominal breathing… exactly as taught in yoga or martial arts classes… but for reasons that escape me, rarely in scuba classes. Odd that because breathing and breath control is one of the Six Basic Skills associated with diving, and regardless of its absence from so many classic and otherwise useful diving texts — finding any reference at all is as rare as seeing a good haircut at a Star Trek convention — it seems that there’s a compelling argument suggesting that we invest some effort into learning proper breathing and breath control for an activity that takes place in water too deep to stand up in.

The long-term benefits to health and well-being aside — and these are considerable so that’s a lot to ignore — correct breathing will help divers to focus on the tasks at hand immediately prior to their dive. Done during a dive, it will increase their energy levels while decreasing their CO2 levels. And at the end of a dive will be a useful part of a structured plan to optimize decompression prior to surfacing. But the most compelling argument is surely that practicing correct breathing techniques is the simplest and possibly most important thing that divers can do to improve their overall chances of survival in a situation that‘s gone completely pear-shaped… because controlled breathing helps to control and prevent panic.

Learning to Breathe Correctly
Let’s start at the very beginning with a simple exercise designed to teach the basic technique. Sit on a comfortable chair or if you prefer, cross-legged on the floor. Sit straight backed and erect, with your hands in your lap. (You may also lie on the floor for this exercise but the likelihood of you falling asleep after a couple of minutes is greatly increased!) Now close your eyes, relax and imagine you are getting ready to drift off to sleep. Let your concentration focus on breathing and let your breathing become deeper and slower than normal.

Be aware of nothing but your breathing and try to ignore any thoughts that drift into your mind except those about breathing in and out. Count the number of seconds (or heartbeats) it takes for you to fill and empty your lungs.

Think of these as two distinct halves of a complete cycle and make each last the same number of seconds. At this point, when you have some control over your breathing and the length of your breath cycle, concentrate on deep breathing.

Visualize filling the lower part of the lungs first, then the middle and upper portions. When exhaling, reverse the process and begin by emptying the upper part of the lungs, then the middle, and last of all the lower part. Each inhalation and exhalation should be an uninterrupted, smooth action, each phase flowing into the next without pause. Breath slowly and with no effort or strain. It is very important not to force anything. Also important is to keep your mouth closed.

Inhaling
OK, now to refine the mechanics of breathing. The goal here is to involve fully your diaphragm and not just your chest muscles. Start by pushing your stomach out as you breathe in and “engage” your diaphragm.

During this action imagine the air filling the lower portion of your lungs. Next, push your ribs sideways and continue breathing in. The stomach will automatically go inwards slightly. Visualize air now rushing into the middle portion of your lungs. Lastly continue to inhale as you lift the top of your chest and collar bone while you visualize air filling the very top portion of your lungs.

Exhaling
Reverse the steps, starting with the top of your chest and collar bone and end by drawing the stomach in. You may find a temptation to pull your stomach muscles in rapidly… avoid doing this. Every movement, every action and thought must flow into the next.

Work at keeping the transitions from one step to the next smooth and seamless with no jerkiness.

Your Goal
What I have outlined above is based on the Taoist therapeutic breathing exercises taught to me in my first martial arts class more than 30 years ago. I’ve probably misremembered bits and added my own interpretation — such is human nature — and it is only the first and most basic form of breathing exercise. But it’s a good foundation, and will serve you well.

Your aim is to learn the technique well enough to slip into deep breathing whenever you wish. You can add your own visualizations to the basic technique.

One visualization is to direct the energy created by each inhalation to a different area of your body… your hand, a foot, or shoulder joint. And during the exhalation, complete the visualization by imagining the outgoing air carrying away toxins. I imagine bubbles of gas being washed out during deco by doing this. Of course it’s all fantasy but it helps pass time!

Put aside ten or 15 minutes twice a day to practice. Do the exercises on an empty stomach, and wait at least two to three hours after a heavy meal, and about one hour after a light snack.

There are a couple of reasons for this. The first is that a full stomach makes it physically harder to actually do the exercise and the second is that a full stomach makes it harder to concentrate… something about blood (and oxygen) demands to aid digestion.

As with learning any new skill, it will seem somewhat artificial and may be difficult for some people to get on to at first. Persevere. What you are working towards is a process that requires no real effort and that puts zero strain on your body. This type of breathing will get you “into the habit” of filling and emptying your lungs properly… something regular shallow breathing does not do. Keep your chest passive during the entire cycle of inhalation and exhalation. Do not strain or exert yourself and keep things smooth.

Deep correct breathing is the foundation of good health and is required for full concentration. There are several intermediate and advanced steps that build on the basic technique outlined here and you can research these for yourself. Yoga and Tai Chi books will probably have a chapter or two devoted to meditation, breathing and its benefits. I suggest ongoing study. It’s worth it.

But for now, let’s work on what’s outlined here. Once your body has built up some muscle memory, you’ll be able to turn on deep rhythmic breathing anytime… sitting in your car, walking through a shopping mall, and while scuba diving.

It will help make you a better, happier open circuit diver and is — in my opinion — 100 percent necessary for diving closed circuit rebreathers since the breathing gas in these systems has to be driven through the scrubber bed by force of a diver’s breath.

Why it’s useful
Apprehension before a dive pushes divers off-routine and makes them forget or rush pre-dive checks. This always has serious repercussions. At very least, it greatly increases the likelihood of a crappy dive where nothing gels and the diver is constantly playing catch-up with his gear and the dive.

Panic kills divers. Things go wrong underwater. A diver reacts poorly and there is a domino effect as that reaction and its fallout pulls him further and further outside his comfort zone until he loses control and his fate is in the hands of a most unforgiving environment.

Carbon dioxide kills divers. This is certainly the case with CCR divers but all divers over breathing their equipment run a greatly heightened risk of what C.W. Shilling in his 1984 book, The Physician’s Guide to Diving Medicine describes perfectly as: “… overexertion, fatigue, exhaustion, respiratory embarrassment, panic and resultant accident is the repeated sequence of events leading to a fatality.”

Deep controlled breathing is the closest thing to a magic bullet. The research of Thomas J. Griffith, Arthur J. Bachrach and Glen H. Egstrom, David Colvard and other scientists studying human behavior and stress underscores the effectiveness of what Griffith calls “The Calming Breath Response.” In that work, he states that breathing and breath control are critical elements in controlling diver stress and panic. “Erratic respiration greatly increases the probability of panic and a dangerous situation.” Now in all fairness, Glen H. Egstrom, co-author with Bachrach of the authoritative study Stress and Performance in Diving, and professor emeritus of kinesiology at the University of California, Los Angeles suggests that “…relaxation and other techniques aimed at reducing over stimulation appear much better suited to the pre-dive condition than to handling stress underwater under high arousal.” But in conversation agreed that the application of practiced deep breathing during a dive “coupled with mental visualization and cognitive rehearsal would be an appropriate response to sudden stress.”

So armed with that encouragement, I suggest a few minutes of deep breathing anytime you feel stress. Do it before kitting up for a dive. Do it for a few minutes immediately before jumping into the water (pre-breathing the loop on CCR is the perfect time). Do it at depth, not just when something stressful occurs but anytime. And do it during decompression. I find this last helps me to put the dive into perspective and order ready for the debrief.

Remember, the number one rule of diving is don’t hold your breath and the codicil to that rule (#1b) is breathe correctly!

Thanks for your attention.