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Article by Steve Lewis:
Photo by Tamara Thomsen
Here’s a skill-testing question for all the open-circuit trimix divers out there, at least, for the ones who are awake and up for some simple math. What is the inspired partial pressure of helium at a depth of 60 metres/200 feet for a diver breathing a trimix gas containing 45% helium? Actually, I’m pretty sure any reader with a TDI nitrox card or above could easily do the calculation after applying a little thought and Dalton’s Gas Law. It goes something like this: 0.45 (fraction of helium in the breathing gas) X 7 (depth in bar or absolute atmospheres) = 3.15 (the helium partial pressure… that’s what we’d be breathing in).
Great, now, what’s the answer for a closed-circuit rebreather diver using the same gas in her diluent bottle (one with 45 percent helium in it) at the same depth? If you haven’t a clue but suspect the answer is NOT gonna be 3.15 ata, or you need more information before you’ll commit, you’re either smart or awake enough to be suspicious.
In a world in which most of us have to cost-justify our discretionary purchases to ourselves and/or to a higher power, the natural questions to ask are: What’s in it for me? What are the personal benefits? Is this the best way to spend money and time? Will it impress the heck out of my friends? Etc.
I’m pretty sure these are the types of questions that roll around the back of our minds when we consider buying new dive gear or investing in a higher level of dive training. The money we spend on fun, toys, and pastimes, is limited and all of us need to make sure that we get a “bang for our buck.”
The Next Class
So, we have asked, or have been asked if we teach scuba of any description, “Why should I bother taking the ’next’ class?” That’s an excellent question, and the right one. Unfortunately, given human nature and the state of our industry, it’s likely that a few of you have heard that “next class” referred to as a money grab.
For example, I recently overheard a diver telling his buddy (a freshly certified CCR air diluent diver it turns out), “Taking a CCR trimix class is a waste of your time… you’re already trimix certified on open-circuit… there’s nothing more to learn!”
Anyone who knows me will tell you that’s the type of bait I am going to have a tough time ignoring. I rose to it.
There are no reliable stats that we can pull out of the “student database” at TDI HQ telling us how many open-circuit trimix divers take an entry-level CCR program, and a couple of weeks/months later are diving with trimix diluent in their rebreathers. However, I know there are at least a few. The most apt descriptor that I can use in public to describe that action is: foolhardy.
The best argument in favor of following the progression laid out in TDI’s curriculum, at least the argument that has the strongest logic behind it, concerns increasing depth and its relationship to something called the Safety Equation.
The Safety Equation
There are probably several definitions capable of explaining what the Safety Equation is, but for diving it simply means that when assessing personal safety and our “fitness” to do a particular dive, we factor all the risks on one side, taking into account how likely they are to occur, and the resultant grief/trouble resulting should they occur; and on the other side, how well-prepped we are to deal with/mitigate/avoid them. Essentially, it’s a risk/benefit assessment.
Depth brings a whole different order of magnitude to the risk side of the equation. I believe we sometimes take that for granted and forget just how quickly a change of depth changes everything else: and none of it positively. In a way, who can blame us?
As a community, we do a spotty job at self-policing. Newly certified open-water students, we start our underwater adventures with a suggested depth limit of 18 metres/60 feet. This is considered the maximum depth from which a new diver can make it to the surface if, when all else fails, they have to totally bailout. It’s also a depth at which most divers will only take on a moderate inert gas load and have a manageable decompression stress. Also, the narcotic effect at 18 metres is slight for most folks.
Unfortunately, and this is when the wrong messages begin to be sent, this suggestion often gets quietly ignored and we can probably all tell stories about seeing newly certified open-water divers venturing much deeper than that recommended max on their first dives out of class. We could spend hours debating the pros and cons of that practice, but let’s take it as common sense that the progression from open-water to, let’s say 30 metres/100 feet, intensifies the risks associated with sport diving, and to conduct that type of dive, compared to a basic open-water one, requires more experience, increased skills, perhaps different gear, and a background that includes skills practice at very least.
For example, give an average occasional vacation diver with a handful of dives under their belt a free-flowing regulator to deal with at 6 metres/20 feet, and dollars to donuts, most of the time, they’ll make it to the surface with nothing more than a bruised ego. Do the same thing from two or three times the depth (the suggested maximum depth), and the bruises might be larger, but still manageable in most cases. Try from five times deeper, and there is a very real risk of serious injury or worse. We all know this is the case, and agree I suspect. Certainly, a free-flow for an experienced diver – sport or tech – is an inconvenience rather than an emergency. The key differentiator here is experience. The maximum depth suggestion is made for a good reason.
As one plans to dive a little deeper, let’s say to the sport-diving limit suggested by SDI of 40 metres/132 feet, many other factors must be added to the safety equation. It’s not just a case of can I swim to the surface without drowning, but what shape pretzel will I be when I get there. At this depth – which I believe is right on the cusp of “technical” diving – one has to consider seriously: decompression stress, thermal stress, increased gas consumption, and the need for redundant gas since a quick bolt for the surface is totally ill advised. Deeper means more risk, more complications, more preparedness, more knowledge and wisdom, more gear.
In Technical Diving
In technical diving, we focus a little more thoroughly and thoughtfully on limits, and what they mean. Any experienced technical diver knows that there is an order of magnitude difference separating a dive to 40 metres/130 feet, and one to 50 metres (about 160 feet). It’s only another atmosphere, but now we will be 16 stories below the surface. Decompression obligation is real; there is no bolting for the surface, no cutting a dive short because of a difficulty. Any problem has to be solved at depth, contingencies must be planned for.
As we venture progressively deeper, one learns about the limits of one’s gear, and oneself.
Beyond that 50 metre depth, things escalate exponentially. A dive to 60 metres/200 feet requires lots of gas, lots of decompression, more gear. Go beyond that, and the challenges increase alarmingly.
For most divers, these are self-evident truths. What we sometimes need a reminder about is that there is a real difference between open and closed-circuit; experience using the former, does not necessarily translate into any useful skills diving with the latter.
And essentially, when I heard the “you don’t need a trimix CCR class” advice, this was the reasoning behind my opening gambit. I explained that the only open-circuit trimix experience that would be any help on a CCR trimix dive was if your response to any challenge would be to bailout to open-circuit. “That,” I explained, “is not what being a CCR trimix diver is all about. That’s being an open-circuit diver using a very expensive gas mixing machine at trimix depth. And that is NOT the same thing.”
Now, allow me to explain. Nobody, least of all a TDI CCR instructor, is going to fault you for bailing out to open circuit. There is a simple phrase we all promote: there’s no shame in bailing out. However, put in the most diplomatic terms possible, a student who bails out for every contingency in any advanced level CCR program is going to be doing a fair amount of explaining during dive debriefs. I would bet on a better than even chance that type of behavior would fail to earn them a CCR Trimix diver cert. For advanced CCR certs, the student is expected to demonstrate more “CCR-oriented” methods of dealing with failures.
At issue is awareness, familiarization with the unit, and total comfort with its strengths, its weaknesses, the opportunities it offers, and the threats it presents. Each unit is slightly different (one reason TDI among all tech agencies requires its instructors to have more than a passing familiarity with the units his or her students are using). But critical is the presence of mind to deal with, recover from, and manage in the best way possible, various system failures and scenarios – real and simulated – such as leaking hoses, busted hoses, stuck solenoids, strange cell readings, low gas, a partially flooded loop, etc. In many cases, a student will be expected to switch to Plan B or Plan C during simulated failures.
And, I might add, they are expected to be able to communicate their challenges, the options chosen to meet those challenges, and next steps to their dive team. So, it probably goes without saying that freaking out and bailing on your buddies is a shortcut to spending the rest of the course in the hotel room.
It’s very easy to read a set of standards for a class, and look at the required skills required to earn a pass without completely understanding their application or each skill’s nuances, hence the role of an instructor to interpret those standards. Also, each instructor adds his or her personal touch on the execution of a skill. For example, one requirement is to plan an ascent with one diver on open circuit, with other team members on CCR. Sounds simple, but are there ways to carry contingency gas so that the OC diver is running the best possible ascent schedule… somewhere close to her buddies?
Of course there is
After explaining to the guy, to whom CCR trimix is a waste of time, about the bailout “thing” and the deco gas “thing” I asked the question poised in paragraph two above. Oddly, neither he nor his buddy understood that 45 percent helium in the diluent bottle did not automatically translate into 45 percent in the loop gas. (You guessed it – the actual percentage will vary depending on the oxygen setpoint, how much oxygen is actually in the diluent, and, particularly on ascent, the contents of the exhaled gas, including water vapor).
Perhaps some “next classes” are a waste of time, but looking down the SDI/TDI/ERDI curriculum, I do not see the proverbial underwater basket weaving, and certainly there is no fluff when it comes to CCR diving. Each step has value and should be followed as prescribed. And that’s not just my opinion; it’s the best, safest and most secure way. The stats prove it!