Visiting Our TDI Booth at ANY Event Should Come with a WARNING
At the opening bell at a recent consumer show, I hid behind a couple of closed-circuit rebreathers on the SDI/TDI booth so that I could finish the day’s first cup of coffee in peace; well, that plan failed miserably. Rather than dodging work, I was soon surrounded by people attracted by the display who peppered me with questions about CCRs.
Perhaps it was the phase of the moon, or more likely a rising tide of consumer interest in anything to do with rebreathers, but the whole day went very much along the same lines. It was exceptionally busy.
The questions asked ranged from: “what exactly is a rebreather?” and “do I need special training to use one of these?” to the more sublime.
The most challenging question of all, though, came more or less out of left-field while I was dealing with one of many queries about cost.
“I am not CCR certified, but I would like to know how much bailout gas you carry on a CCR dive?” Initially, I figured the gal asking the question was a stringer sent by TDI’s training department to deliver a pop-quiz. “Check on Lewis & shake him up a bit, why don’t ya!” But of course it was a genuine and earnest question from a diver who had already enrolled in a “recreational CCR program” (TDI’s air-diluent no-deco course), but was yet to take delivery of her training materials or sit down and chat with her instructor.
As with so many questions about diving, and especially diving that involves good risk-management practices, there is more than one correct answer. Perhaps it is worthwhile reviewing what that means right now.
The calculations for gas volume requirements on a CCR are remarkably simple.
Step One: begin with a known or a realistic guess for an oxygen metabolic rate for the diver. This is a fancy way of saying we need to know how many litres of oxygen a diver burns through in a minute). In a perfect world, we will work with an actual number, but for the sake of simple math, let’s assume a diver uses (metabolizes) two litres per minute. (For those still struggling with old imperial units, a couple of litres are equal to about half a gallon or about 1/14th of a cubic foot).
For most of us, two litres per minute is a little on the high side but it works as a base number. By the way, the depth of our dive has no effect on our metabolic rate. Unlike depth, workload does make a difference, but two litres has enough latitude that it is a pretty good estimate for most divers on most dives.
Step Two: calculate your starting volume of available oxygen. For example, if we start our dive with a three-litre oxygen cylinder filled to 200 bar, we have around 600 litres of gas. At a consumption rate of two litres per minute, that means we have enough oxygen for a maximum of 300 minutes! Some of that gas will be wasted but nevertheless, that’s one heck of a dive, isn’t it?
Of course there are a few more steps in the overall process, but we can state that on first analysis when diving a CCR, the limiting factors for gas management rarely have anything to do with how much gas we have “on-board.” With a CCR the limiting factor in the segment of our dive plan given over to gas management is the volume of off-board gas we carry for use should the CCR go completely off the rails.
Because of this, the TDI overall gas management guideline for CCR divers reads something like: Proper pre-dive planning must ensure that you have a sufficient volume of breathable bailout gas to allow a safe ascent on open circuit from any stage of the dive and for the type of dive being planned.
The emphasis on the type of dive is a reference to the different bailout requirements on, say, a cave dive which plans a 75-minute swim each way, to a dive on a buoyed wreck sitting at a depth that someone could free-dive to. Of course bailing out from a technical dive that requires staged decompression brings into play some multi-bailout bottle logistics as well… but more about that in a while. For the time-being, let’s think single gas bailout.
Some time ago, Lamar Hires wrote an excellent article for TDI on bailout planning for Cave Diving. One of the key points in his essay is summed up in the following paragraph.
“… I guess after years of starting a dive with about 270 cubic feet of compressed gas, I can’t get past the need to have at least 80 cubic feet of bailout gas. Even if practice gets you out on 30 cubic feet of gas, having at least 80 cubic feet gives you the extra gas to deal with the catalyst that got you off the loop. I think this is the one point training cannot emulate. During training you always know it’s a drill. You wait for the queue and respond. There aren’t any flashing lights or taste of a caustic cocktail. AND In the real world there are no “abort the drill” signals.”
The short — Coles Notes – version of Lamar’s take-home messages were: plan to use lots of gas during a real bailout from the CCR loop, AND plan to have a redundant gas source in the event of a Murphy Event hitting your OC kit in ADDITION to your CCR going AWOL. In other words, in an overhead environment, it is safer by far to have two independent gas sources EACH capable of getting the diver back to the cave’s entrance.
One of the important other issues when figuring out how much gas is ENOUGH during a bailout is captured in Lamar’s phrase, “the catalyst that got you off the loop.” The reality is that a diver’s breathing rate and therefore the volume of gas he will consume per minute will be hugely elevated from his usual needs. This is as true for open-water divers as it is for those in an overhead environment.
Many experienced CCR users suggest calculations for required gas volumes use a breathing rate up to three times the rate used for a non-stressful dive. This consumption rate may seem inordinately high but when a diver bails out from a scrubber failure – a worst-case scenario – breathing rates can be extraordinary.
For the record, here’s an example. Let’s use a consumption rate of 14 litres per min (about 0.5 cubic feet) as a baseline.
Now let’s factor in a dive to 30 metres (100 feet). The gas density at 30 metres is four times normal atmospheric density so a diver with a normal consumption rate of 14 litres a minute will now require four times that, or 56 L/min! Let’s also set a simple wreck-diving parameter with the diver approximately a five minute swim from the ascent line when things go pear-shaped. Five minutes at 56 litres per translates into 280 litres.
OK, but that figure assumes NORMAL breathing rates. What happens in a worst-case situation with the diver stressed, and his breathing influenced by carbon-dioxide build-up? We can use the 3X factor, which gives us a required volume of 840 litres (for the imperialist among you, this is about 30 cubic feet). This sounds like a whack of gas but please consider that this is the gas requirement just to get back to the ascent line! In addition, the diver will need gas for his controlled ascent at 9 metres per minute (a little more than three minutes) PLUS a safety stop at around 6 metres for three to five minutes (assuming a dive within no-decompression limits).
In a situation where the swim back to the surface is an exceptionally long one – for example during a cave dive – a diver’s gas consumption rate MAY drop after several minutes and advanced dive plans sometimes use a sliding scale for gas consumption that gradually falls back to a more normal rate after ten to fifteen minutes of swimming.
However, suffice it to say that the best answer to how much bailout is enough usually suggests “more than you’d expect!” Certainly a prudent diver executing “sport” dives within the NDL and within 40 metres depth, should consider carrying at least 1,800 litres or around 60 cubic feet.
For technical dives that require staged decompression, the gas requirements become more complex. A CCR diver must now carry a bailout gas suitable for use from the deepest portion of the dive, and at least one oxygen-rich gas to help optimize decompression.
An important guideline is that on this type of dive, each diver in the team carries sufficient volumes of gas to finish his dive on OC without compromising conservative decompression planning. (Of course, the decompression obligation of an OC diver is often quite different to that of a diver doing a similar dive on CCR, which complicates matters somewhat.) Often, in a team-based plan, each diver carries the same bottom gas while each has “overlapping” deco gases. For example, on a trimix dive to 60 metres or so (around 200 feet) one diver will carry bottom mix and an EAN50 while her buddy has bottom mix and 100 percent oxygen. In the event of separation and CCR failure, each diver can conduct a “safe” ascent independently; however, more rationally, the team sticks together and the diver with a failed unit can optimize his ascent by using his deco gas in addition to his buddy’s.
Among the many advantages offered by a CCR if things go wrong are many more ways to “fix” things and get home safely. Bailing out to OC is often the “no-brainer” option; certainly it is the option commonly taught as the default reaction to a problem for any new CCR divers. Because of this the correct answer to any questions regarding how much bailout gas is needed must never be a guess, an assumption or a ‘ball-park’ amount. It has to be a number based on actual real-world requirements. And in the final analysis, needs to allow a comfortable margin for unforeseen events… most notably, slower exits and higher consumption rates.
I probably spent way too much time explaining the facts and guidelines to the young woman who visited the SDI/TDI booth, but I believe she left with a realistic outlook!
By the way the new WARNING about visiting the booth is…ask questions at your own peril…we do not guarantee you will see the rest of the show and sorry no cash refunds BUT we guarantee you will leave with more than your money’s worth!
To learn more and arrange your next TDI course please visit https://www.tdisdi.com
About the author Steve Lewis is a well known Trainer and Marketing Consultant to many of the major industry participants. He is the very successful author of a book called “the Six Skills and Other Discussions.” This book is available at select dive stores and through onLine stores such as Amazon and Create Space eStore via: https://www.createspace.com/3726246.