The “P-Valve,” Avoiding a Wet Drysuit

The “P-Valve,” Avoiding a Wet Drysuit

By Steve Lewis

PValveOK, hands-up all of you who have taken a pass on that extra glass of water because you’re about to zip yourself into a drysuit and go diving? Better yet, how many of you have been wrapped in the comfort of your drysuit hanging on a safety-stop at the end of a nice long dive WISHING you had taken a pass on that extra glass of water?

Well, the solution to that particular challenge is to dive with a “P-valve,” and for those of you unfamiliar with the concept of punching a hole in a perfectly good drysuit to retrofit this little convenience, let’s say right up front, I cannot think of a better way to spend a few pennies. (Forgive the pun). Let’s explore some of the reasons why. There is a tenuous connection linking hydration and DCS. While most of us have “grown-up” listening to advice from trusted sources such as DAN (Diver’s Alert Network), Duke University Medical Center, and various hyperbaric and technical diving gurus telling us that being well-hydrated helps to off-set some of the risk of decompression stress, the scientific papers to back this advice up with solid data are thin on the ground. However, many of what passes for leading authorities in the technical diving community fly in the face of science and swear that good hydration is key to a successful outcome on any and all dives.

“Dive Hydrated” is the common mantra and most of us follow that guideline without question. And regardless of the position of the medical and scientific establishment with regards DCS and hydration, there are volumes of papers and reports in sports medical journals pointing to the importance of good hydration and the peak performance of mind and muscles in athletes. You may or may not feel this is entirely relevant to you and your diving, but I believe it is to mine. There have been occasions when I have felt sluggish and a little dimmer than usual and the physical problem has been dehydration. Therefore, my daily average intake of about three litres of water is unchanged by the diving calendar; and ample water intake pre, post and sometimes during a dive has become the norm. But this behavior does bring up the potentially embarrassing issue of bathroom breaks while zipped into one’s “thermal protection.” Essentially: how and where?

Let’s discuss the “How” first. There are of course, adult-sized diapers for both men and women, and while these may be a viable solution for some, diapers are not the most green of products and are certainly not the most reliable, hygienic or comfortable. It’s unclear who first decided that a better solution to the question HOW was to plumb a piece of hose to a simple-to-open valve on the diver’s leg, which when closed kept water out, but when opened allowed liquids to be dumped “off-board.” The connection between the outside of the suit and the male diver was achieved with a little medical device known as a condom catheter (WARNING: reader discretion advised > “The P-Valve Struggle – condom catheter“). My first encounter with a so-called “P-Valve” occurred while cave-diving in North Florida. Fixing a newly purchased model to what had been a perfectly dry drysuit, required some will-power, the empty brass cartridge from a .45 caliber pistol, a hammer, and a strategically placed piece of wood on the left thigh of the suit. Once the hole was made – clean and the perfect size – liberal amounts of sealant and exact positioning of bolts, nuts and washers, resulted in a watertight seal. As an aside, I have since owned more than 10 drysuits, all of which have been fitted – either by the suit manufacturer or as a post-sale upgrade – with a P-valve of some sort or another, and quite remarkably, all have functioned well. All that remained during my very first experiences with a P-Valve was the connection between it and me.

Enter the external male catheter. The ubiquitous Google search will turn up countless amusing and sometimes alarming stories focused on the… issues… divers around the world have suffered through coming to terms with the fitting, successful use of, removal and disposal of condom catheters. I am going to resist the urge to add my experiences over and above the following advice. While condom catheters are essentially a small latex or silicone tube with one end designed to connect it and its wearer to some sort of plumbing… including a drysuit’s P-valve there are subtle differences between Brand X and Brand Y. External male catheters are available in several different models, different materials, and yes, different sizes. They can be purchased in bulk online from several different medical supply houses and many dive centers carry a small stock for folks who only need to buy in smaller quantities.

Your mileage may vary, but the most popular and best-selling according to a spokesperson at one of the largest online suppliers, Lighthouse for the Blind, based in St. Louis, Missouri, is self-adhering and non-latex. This model has a wide-band of “glue” on the inside to help keep it in place, which is certainly an asset when diving, but that takes a brave soul to remove from its place of work from time to time. One more thing: “What you divers have to consider,” said the nice spokeswoman at Lighthouse. “Is that these items were designed to hold fast to someone either in a wheelchair or invalided in bed, and not swimming around in the ocean.” She insisted that getting the correct size is important to “staying connected” and I agree with her! We can leave this topic with a tip of the hat to the women responsible for the design and manufacture of the “She-P:” the corresponding piece of kit to an external male catheter for female divers.

PValve2While far from perfect – according to female dive buddies – this rather mystifying, and reasonably newly-launched contraption has revolutionized a day on the boat for hundreds of women divers… tech and sport. It allows them, like their male counterparts, to connect themselves to an outboard dump and conduct dives well-hydrated (ask DUI about She-P). Welcome to the club! It may also come as no surprise to a savvy observer with a grasp of basic physics that a simple piece of tubing connected to a “flow-through” bolt or twist-to-open valve on the outside of a drysuit would be an unbalanced system. That’s to say that the pressure in the suit, the ambient pressure outside the suit, and the pressure inside the valve’s plumbing are not the same. For the best results, and fewest unpleasant surprises, the plumbing in an unbalanced system is best primed before the diver subjects him or herself – and their equipment – to increased ambient pressure. If this step is neglected, the sudden “equalization” of pressure between the closed bolt, conjoining hose and apparatus on the inside of the suit to the water pressure outside, can be a shocker… to say the least. An option available very soon after the introduction of the “store-bought” unbalanced P-Valve was a balanced system. This type of dump is fitted with an additional little side-branch of hosing designed to equalize the whole of the valve with the pressure inside the diver’s suit, which in turn is supposed to do away with the need for pre-priming the system. Some divers swear by balanced valves and others swear at them because – they maintain – a balanced system is way more prone to leaks… and those leaks are often NOT the result of water that started life outside the suit getting inside the suit.

I think we can wrap this up with one final item: cleanliness. There are several suggestions for maintaining a clean and hygienic off-board dump. I know several rebreather divers who flush their P-valves with the same solution that they use to disinfect their counter lungs. Some other divers flush their P-valve system with a mix of white vinegar and rubbing alcohol and then rinse with water. Some use a dilute bleach solution. I use a 10 percent solution of Dettol™, a liquid antiseptic and disinfectant available over-the-counter in most drug stores. It smells of pine oil and the active ingredient is chloroxylenol – a pretty powerful germ killer. I first encountered the use of dilute Dettol as a topical disinfectant for open wounds and figured that if it’s OK for the Royal Hampstead Teaching Hospital, it’s probably OK to rinse out my P-Valve and the inside of my drysuit with it. Once again, your mileage may vary, but Dettol followed by a thorough rinse with fresh water has kept me safe from any infections for many, many dives.

Well, there really isn’t much to add except to restate that even though the jury MAY still be considering the verdict on the relationship between thorough hydration and DCS, many of us choose to drink lots of water when we dive, and feel better for being able to get rid of some of it during our dive! Oh, that brings us to the unanswered question of WHERE. Well, not to put too fine a point on things: wherever. Dive safe.

Contact SDI TDI and ERDI

If you would like more information, please contact our World Headquarters or your Regional Office.

Tel: 888.778.9073 | 207.729.4201
Email: Worldhq@tdisdi.com
Web: https://www.tdisdi.com
Facebook: www.facebook.com/TechnicalDivingInt

Air Management: More Than an Occasional Glance at an SPG!

By Steve Lewis

You may have sat-in on dive briefings that end with the advice: “Please come back on board with at least 50 bar of air in your tanks (500 psi from US divemasters)… have a good dive.” As far as it goes, not a terrible thing to tell the average sport diver, but does it go far enough?

Ask a technical diver that question, and my bet is you’d get a resounding NO!

There are plenty of ad-hoc definitions pointing out the differences between a sport dive and a technical one, but certainly one of the most telling signs is that the dive plan for a technical dive is a bit more complex than a simple: “Let’s come back with 50 bar in our tanks!” In truth, most technical divers would get sweaty palms when the needle on their SPG hovers close to that level.

From the very start of technical dive training – a TDI Intro-to-Tech program for example – there are two basic rules that instructors and textbooks hammer away at constantly. One is a slight modification of the first rule of scuba “Never hold your breath: keep breathing.” That advice is still valid but in tech diving it becomes: “Always have something appropriate to breathe, because running out of air/gas is NOT an option.”

The second rule deals with trouble in the water. It states: “If something goes wrong at depth, it’s best to fix it at depth, because bolting to the surface is not an option!”  Because of rule one and two, it becomes apparent that proper gas management is somewhat more detailed than taking an occasional glance at an SPG, and as students in their first tech class learn, it takes planning and a little work to stay safe, but the benefits are well worth the effort.

Let’s take a brief look at what’s involved.

Gas management, or more precisely gas volume management, starts with knowing your personal gas consumption rate. It does not matter if you work in cubic feet or litres, the important starting point is to have a figure based on your actual breathing rate.

There are several ways to collect this information. Some TDI instructors ask Intro-to-Tech students to average out actual consumption from previous dives, but most will have students do “data collection” at a set depth for a specific time, noting starting and ending pressure. Back on the surface, these numbers are manipulated a little and used to produce a Surface Air Consumption (SAC) rate. This is the baseline number that will be used in future dive plans.

Some divers (and some instructors… me included) will take the calculation for this baseline even further, and will actually monitor resting consumption rate on the surface while doing nothing more energetic than watching a video or reading a book.  This number has to be modified with some extra loading to take into account the fact that diving puts a bit more strain on the body than sitting reading a book, but in my opinion, gives cleaner starting data.

Whichever method is used to arrive at the baseline, that baseline becomes a constant and we can plug it into all future dive plans. For the record, an average baseline SAC for a relatively experienced diver is around 14-16 litres, or about 0.5 to 0.6 cubic feet, a minute, and in many textbooks, a figure within this range is used for most examples.

The next step is to include the parameters of the dive into the gas management plan. The effects of depth, workload, and other dive factors such as water temperature, visibility, and so on are also considered.

These can vary tremendously. For example, one dive to exactly the same depth as another may require twice as much gas because of stressors such as poor visibility, colder temperatures and current.

At the end of this set of calculations including these highly variable “Dive Factors,” we have converted our SAC rate into something we typically call our Required Minute Volume (RMV). In other words, for a moderately simple dive to 30 metres or 100 feet, our average 14 litres or half a cubic foot of gas needed on the surface per minute can easily become 124 litres or 4.5 cubic feet of gas needed per minute.  That’s a big jump, but perhaps not a surprise.

The final step in this part of the planning process is to multiply our RMV by the scheduled time at depth.  Let’s say we intent to spend 30 minutes on the bottom.  Armed with this knowledge, we can multiply our RMV by 30 to arrive at the required volume of gas needed for the dive… in our example this final figure would be around 3700 litres or 135 cubic feet.

However we slice it, that is a lot of gas, and let’s remember that’s only the gas needed for the bottom time; we have to also consider the gas required to get back to the surface. More calculations including knowing what decompression stops we have to make on the way up and for how long… and what type of decompression gas we are going to use!

On top of all this, we also follow the golden gas management rules of technical diving: The Rule of Thirds for back-gas and the Rule of Halves for decompression gas.

Essentially (and in its simplest form) the Rule of Thirds states that we use one third of our starting volume for the first half of our bottom time (the swim in), one third for the second half (the swim out), and the final third as contingency gas. In effect, that last third belongs to our buddy, and remains untouched at the end of the dive (barring emergencies).

The Rule of Halves says that we take at least twice as much decompression gas on the dive as the plan calls for. This way, we have lots to share and lots of spare air if we get a free-flow or if we have to spend a little longer than planned decompressing.

The final cherry on top of the ice-cream sundae is matching up gas consumption to waypoints on the dive itself. For example, using the case of our 30 metre/100 foot dive for 30 minutes, we would probably have several “Go or Go Home” checkpoints earmarked. At a minimum, these would include one for arriving at depth, one for arriving at the focus of our dive, one for the turn-around point, and one for arriving back at the ascent point.

These waypoints would give each team member an opportunity to say: “Yep, I am fine, let’s go ahead,” or not depending on how they’re feeling and how their kit is behaving. In addition, it gives each team member an opportunity to check his or her actual gas consumption against the expected or budgeted gas consumption used in the dive plan. Their SPG should be used to confirm what they expect to see… and to do this we must first have done some calculations to translate volume into a reading on that SPG. For example in a pair of 14 litre cylinders, a one bar drop in our SPG translates to 28 litres of gas consumed. The calculations for imperial units is less simple, but inevitably we have to be able to read a gauge and work out for every 100 psi drop, we have burned through X number of cubic feet.

If this is starting to sound rather complicated, it is, but it’s not possible to explain a whole TDI course module in a few paragraphs… you have to take the course! And let me assure you that gas volume planning is far easier to manage than an OOA situation at depth when you have a serious decompression obligation to fulfill!

Take care and dive safe.

Steve is an active cave and wreck diver and instructor-trainer with SDI/TDI. He writes extensively on technical diving, diver safety, and risk management for open-circuit and closed-circuit diving. He has served on TDI’s TAP (Training Advisor Panel), and works as a marketing and product development consultant in the dive industry.

Contact TDI SDI and ERDI

If you would like more information, please contact:


Tel: 888.778.9073  |  207.729.4201

Email: Worldhq@tdisdi.com

Web: https://www.tdisdi.com

Facebook: https://www.facebook.com/TechnicalDivingInt

Decompression Diving and the Technical Diver

By Steve Lewis

Decompression-DivingWhen the diving community attempts to set a definition for a technique, procedure or even a piece of kit… whoops, sorry I should say equipment… there is seldom complete agreement.

Our community is simply too diverse, too geographically dispersed, too individual to be easily categorized. And as the saying goes: ask a half-dozen divers to define technical diving and you’ll get 12 different answers.

You may have experienced this particular phenomenon for yourself. However, the chances are good that most, if not all, of those definitions will mention something about decompression diving. It is this that truly marks the boundary between sport diving and its young sibling: technical diving.

Somebody once wrote that decompression diving is the art of “staying down longer and coming up slower!” And for most of us, this serves as a workable cocktail party definition.

More precisely, decompression diving (or staged decompression diving since all dives really are decompression dives), exposes a diver to much greater risk than traditional sport dives carried out within the “No Decompression Limits” of his or her chosen Personal Dive Computer (or dive tables). Those increased risks include — in no particular order and not a definitive listing — decompression sickness, running out of gas, getting lost, getting separated from their buddy, getting cold, disorientation attributable to narcosis, lack of situational awareness due to task-loading, and loss of buoyancy control coupled with the potential for run-away ascents and possible embolism. We might also add, either Hypercapnia, Hypoxia, or Hyperoxia resulting in loss of consciousness and death.

Presented with that sobering list of party-poopers, is it any wonder that for the first thirty years of recreational dive instruction, staged decompression diving was not heavily promoted, and certainly was not part of the regular curriculum of any of the mainstream training agencies. If you conducted decompression dives, you were branded by most of the dive community as a foolhardy daredevil with a death-wish; or simply an idiot.

Ironically, when TDI began teaching its original decompression procedures program back in 1994, the concept was so outside the box that several of the long-established training agencies — which had built their reputations entirely on sport-diving certs and by totally ignoring gases other than air, solo diving, deep diving, and decompression diving — suggested that teaching divers deco diving would result in greater instances of DCS, and a rash of diver drownings. “It’s just too complicated'” they crowed.

The facts of course turned out to be somewhat different and according to DAN statistics, the incidents of decompression sickness per 1000 dives has dropped, not increased, since TDI (and a couple of other ground-breakers) started to share the basic skills that are needed to conduct staged decompression dives correctly. Coincidence? Perhaps, but we like to think that by presenting divers with a choice to complete structured training with a qualified and insured and experienced instructor — rather than winging it with advice from cyber-divers — we are making scuba diving a little safer.

Certainly the TDI course has evolved somewhat since it was first introduced. The skills have remained unchanged — with a couple of notable exceptions — but the way we deliver them and the data we have to back-up some of the suggested techniques and procedures have become, forgive the pun, deeper. The simple fact is that 20 years ago, decompression diving was totally behind the curtain, hidden, hardly mentioned; and now it’s mainstream and the experience we have to inform our curriculum and our suggestions is much broader and more comprehensive than it could possibly have been in 1994. Hence, the change and the need for TDI to have a constant process of updating materials (SEE LINK BELOW).

However, as much as TDI’s decompression program has evolved — and even split to include the use of helitrox (light trimix) — many of the basic skills have remained the same.

In some regard, and bringing us full circle, the skills required to conduct staged decompression dives, truly define technical diving and technical divers.

So let’s look briefly at them and let’s find out what benefits they deliver to us as technical divers.

Number one is risk management. This breaks down into risk identification: what can happen; risk assessment: how likely is it to happen; risk analysis: if it happens, what will the consequences be; risk avoidance: how can I arrange things to mitigate or avoid those risks. During this process, we get a pretty clear sense of our personal limits and the limits of our equipment, our experience and our training. This is often a rather humbling exercise, but is always informative. Different instructors use different techniques to teach this set of skills… but it is usually fun!

Second comes an upgrade on the basic skills that we learned as divers from day one: buoyancy, propulsion techniques, and staying in contact with and communications with our buddy or buddies. The obvious fallout from this is that we expend minimum effort moving through the water and don’t waste time and burn ourselves out worrying where the heck our buddy has got to. Bottom line: more fun!

The third set of skills sees us taking a careful look at our kit and selecting the right gear for the job. Taking what’s needed no more, and knowing how to use what we do have with us. At this stage, we also develop some good habits such as pre-dive inspections, critical analysis of kit performance and limits and a bunch of related techniques that help make our diving — I think you may recognize a pattern developing — less stressful and more fun.

Lastly, we begin the practice of real gas management. OOA is no longer an option. We learn to plan the gas volume we need AND we begin to plan to use the right gas for the job. Although gas management is a very simple skill with guidelines that anyone can grasp after a few minutes instruction, it’s amazing to watch the light bulb go on above the heads of even very experienced sport divers who realize that they have been pushing the limit on most of their dives! Now, that’s fun to see!

All tied together in a neat package, the real take-home skills for a technical diving learning staged decompression diving is better dive planning skills and more fun and less stress executing those dives.

Of course, the real story is more involved than all this and for the complete low-down, you should contact your local TDI decompression instructor and get his or her advice and mentoring via a full TDI technical diving program.

Decompression Procedures Diver >

Contact TDI SDI and ERDI

If you would like more information, please contact:

Tel: 888.778.9073 | 207.729.4201
Email: Worldhq@tdisdi.com
Web: https://www.tdisdi.com
Facebook: www.facebook.com/TechnicalDivingInt

Gas Management Techniques for Sport Divers

Some fun facts about the “pony bottle”

Gas Management
With the rising popularity of sidemount diving, semi-closed and fully closed-circuit rebreathers, and of course the ubiquitous popularity of traditional North Florida Cave diver’s kits (doubles, manifold, backplate and wing), it’s sometimes easy to forget that the majority of divers still manage to have plenty of fun underwater wearing a single cylinder! A single cylinder is simple, comparatively light-weight, easy to set-up and operate and is without a doubt the most common kit configuration among scuba divers around the globe. But as popular as it is, a single cylinder does have one huge drawback, and a growing number of recreational sport divers recognize the short-coming and have opted to do something about it.

Chances are good that if you are a graduate from an SDI Solo Diver program, or if you came up through the University of Hard-Knocks, you probably already know that one huge drawback is that the diver has very limited options when Murphy tags along as a dive buddy. For example, with only one regulator first stage, the only backup life-support system is your buddy’s octo. A massive free-flow really gives little alternative but to share air and get outta Dodge.

Options are even more limited if your buddy is way over there not paying attention to anything but the critter in his viewfinder. A free-flowing regulator can empty a freshly filled cylinder in minutes, and the deeper you go, the faster it drains. Swapping regs and heading to the surface may be the only course of action open to you… unless you count reaching BEHIND your head and feathering the valve on your single tank; turning off your air to fix a free-flow is definitely not something you’d want to try as an emergency ad hoc drill anyway. The truth is that without pool practice and, at the very least, a donated octopus (backup reg) attached to your buddy’s tank in your mouth, a sport diver should never turn off his gas.

The simple alternative is to carry a redundant gas source, and the most functional and practical for the average single-tank diver is a “pony bottle.”

Time for a not-so-simple definition. Just about everyone who has lounged around the aft-decks of dive boats for a season or two will have heard the term Pony Bottle to describe a variety of small scuba cylinders – all a sort of perfect copy of a full-sized cylinder but looking as though they were put through a hot wash and dry cycle and shrunk – and used for a variety of tasks.

Other names for these mini-cylinders include sling bottles, stage bottles, buddy bottles and a half-dozen or so more equally descriptive names. As with so much that has to do with scuba (for example, what IS the definitive definition of technical diving, these days?) there are few unbendable rules when it comes to words and phrases describing pieces of dive gear. A classic example is a pony bottle. I like to tell people that it can only be used for a small cylinder used as a backup air source… exactly what we are talking about here. Of course, that is not absolutely true, but between us, let’s make it so.

Now let’s assume that we have decided that having a backup source of gas is a good plan and that the most practical way for us to carry that gas is to use a pony bottle; there are three more questions we need to answer.

The first is, “how much backup gas is enough?“

Well, the short answer is, “enough to get us back to the surface.” But how many litres or cubic feet is enough? Let’s do some basic calculations using an average consumption rate and an ascent speed that will keep our personal dive computers happy as clams. Let’s also pick a depth that is on the fringe of recreational sport diving: 40 metres or about 130 feet.

We start with a gas consumption rate of 15 litres/0.5 cubic feet per minute. (By the way, the imperial and metric measures used in this example are NOT a direct or exact conversion. Close, but rounded for convenience). Let’s also say that if we have to “bailout” to our pony bottle at depth, we are going to be a bit freaked out – Murphy does that to divers – and therefore our consumption rate is going to be doubled. So we can use 30 litres or one cubic foot per minute.

Our depth has a direct relationship to the density of the gas we breathe so at 5 bar/ata (40 meters or 130 feet) we will use about 150 litres or five cubic feet per minute!

Also, let’s make some allowance for fiddling around at depth for a couple of minutes before we start heading back to the surface. How many minutes exactly is tough to guess, but it would be a mistake to think that we would start to head up immediately we detected a problem and bailed out to our pony. It would be nice to think that’s the way things would unfold but the truth is it takes time to get our buddy’s attention, get ourselves calmed down, sort out our gear and start the swim home. Initially, let’s calculate that we stay at depth for three minutes.

Three minutes at our depth and stress adjusted consumption rate requires 450 litres or 15 cubic feet of gas. (Wow that immediately rules out one of those Barbie-sized tanks. doesn’t it?)

Now we can look at the ascent itself. In an emergency, the hard-wired, natural response that kicks in is the aptly-named flight, fight or freeze response. In diving, we have to resist flight – forget about freeze and fight – to remain controlled and panic-free. As such, our ascent rate must be unhurried and moderate. My personal computer is a fourth generation model controlled by a later version of the VPM algorithm, and as such, the controlling ascent speed is about 9 metres or 30 feet per minute. Let’s use this speed to get ourselves from 40 metres / 130 feet up to six metres or 20 feet for a five-minute safety stop, which is once again a conservative choice. This gives us a smidge less than a four-minute travel time. We can round up again and make this a full four minutes. (In fact my computer would serve up a variable ascent speed causing us to slow down to about 3 metres or ten feet per minute for the last few metres approaching the safety stop. But we can ignore that in these calculations; I will explain why later.)

Gas ManagementTo establish how much gas we will get through during that four-minute swim from depth to the safety stop, we have to know our average depth. The halfway point between 40 metres/130 feet and six/20 feet is 23 metres/75 feet which gives us 3.3 bar/ata. From this we can calculate our gas needs as: 30 litres X 3.3 bar X four minutes; or 5 cubic feet X 3.3 ata X four minutes. That’s around 400 litres (396 rounded up) or 14 cubic feet (13.2 rounded up).

So far, we need 450 litres at depth and 400 litres to swim to the stop, which adds up to 850 litres. For the imperial crowd, the required gas volume is around 30 cubic feet (actually 15 + 14 for 29 cubic feet. A note: if you are doing actual calculations to translate from imperial to SI or metric on the fly, there is some slop in the numbers quoted here because of rounding errors and soft conversion values. The differences though are moot and the principle message remains the same).

Now we have to spend five minutes at the safety stop. Using our base consumption rate as a guideline, our diver will use around 240 litres or eight cubic feet, and we can round those numbers up to cover the slow ascent from the stop to the surface. (The numbers are 1.6 bars / ata X 5 minutes X 30 litres / 1 cubic foot.)

Looking at our total gas requirements from the bailout at maximum depth then, we have:
450 litres / 15 cubic feet on the bottom; 400 litres / 14 cubic feet for the swim up; 240 litres / 8 cubic feet for the safety stop. This adds up to 1090 litres (let’s call that 1200) or 37 cubic feet.

Before moving on to touch briefly on some issue that fallout from discovering just how much gas we should think about carrying, let’s make a couple of things clear.

In the calculations used here, we have been conservative with the baseline per minute consumption figure. At least half the divers reading this article would use less than 30 litres or one cubic foot per minute as a working surface rate. However, the other half would probably use more. (And by the way, these numbers do work better if you plug in your personal SAC (Surface Air Consumption) and a factor modifying that volume to account for stress based on your abilities and needs, but frankly, our conservative baseline is a REALISTIC average).

Also, we have maintained the “high” per minute consumption rate for the whole of the swim to the safety stop as well as for the safety stop itself. In all likelihood, a diver who has him or herself under control would begin to “breathe easier” as they arrived at a shallower spot in the water column with their circumstances starting to brighten. Using a stressed consumption rate throughout the dive has resulted in a high total gas volume requirement. However, we have not factored ANY gas for a swim back to an ascent line at depth; we have factored nothing in for holdups while ascending, and nothing for blimps in procedures.

We have also opted for a slow ascent, followed by a five-minute stop at six metres or 20 feet. We could just as easily have computed a faster ascent speed and a stop at three metres or 10 feet for three minutes. The resulting consumption figure would have been slightly less. However, I believe that a controlled normal ascent and a five-minute stop provides a better edge against decompression stress in this scenario.

Finally, we have worked out all these numbers based on a dive at the very fringe of sport diving. A 40-metre or 130 foot dive is the maximum sanctioned for a sport diver with special training. Not all sport dives go this deep. However, in more than 20 years teaching divers about the basics of dive planning, — and being downright lazy –I’ve discovered that using a pinnacle dive (one that’s at the far boundary of what’s best practice for your experience and the maximum for your training) to calculate contingency needs follows perfectly the axiom of calculate once, use many times. In other words, if we follow these guidelines and then bailout from a shallower dive, we should have more than enough gas, all else being equal.

Clearly, the default sized pony bottle would be something that can hold this much gas. A decent choice in my opinion is a 6 litres / 40 cubic foot aluminum bottle. There are a couple of companies making this sized tank and they are relatively easy to find in local shops. Also, this tank has pretty good buoyancy characteristics in the water, is easy to handle with a little pool practice behind you, and is simple to carry with you in the water. The important thing is that fully charged, it carries ample gas for the purpose it’s being used for. There is the whole issue about whether to have it piggybacked on one’s main cylinder, carried as a sling bottle (classic North Florida Cave Diver rig) or as a side mounted bottle (my personal favorite because it is out of the way but accessible), but let’s leave that debate for another article. Instead, let’s look at what type of gas would be the best to carry and why.

The simplest and most straightforward choice would be to always carry in your pony bottle EXACTLY the same gas that you have in your main cylinder, but this does require us to be wary of a potentially fatal mistake. For example, last week, hypothetical diver Jillian was diving a wreck on which an EAN38 was perfectly suitable, and she had her main cylinder and pony filled with a nitrox 38. Everything on her dive was perfect and the pony stayed unused. She does not bother to drain it. This weekend, she and her buddy are going to dive a reef and intend to take a photo of an Elephant Ear Sponge at around 40 metres or 130 feet. At that depth, her pony bottle mix is hot, delivering an oxygen partial pressure of 1.9 bars / ata. This is problematic.

A simple fix is to have the pony filled with a gas that CAN be breathed on a pinnacle dive. For Jillian, or for the rest of us non-hypothetical divers for whom the specter of oxygen CNS toxicity is a real one, this would be a mix containing 28 percent oxygen, which delivers a ppO2 of 1.4 bar / ata at depth.

The principle of diving with a bailout bottle or redundant gas source is a sound one. Many divers opt to follow the practice. It gives a diver – and that diver’s buddy – options when things go pear-shaped at depth, and allows for a controlled, independent ascent (by which I mean an assent where we are not tethered to our buddy by their octopus).

As with ANY procedure that’s outside the classic stuff taught in most open-water sport programs, there are a few “good to know” knowledge nuggets focused on pony bottles:

  1. Have the valve and hand wheel within reach, and practice breathing from the reg while feathering the valve.
  2. Fit the regulator with a full-sized SPG and check it before every dive.
  3. Pre-breathe the bailout regulator before every dive.
  4. Drill bailouts often until the process becomes natural and fluid.
  5. Mark the cylinder contents and check MOD before every dive.
  6. Have the hose for the pony bottle second stage long enough to reach your mouth (and your buddy’s) easily. A 40-inch hose is a good start, longer is usually better.
  7. At least a couple of times each season practice complete ascent breathing from your pony bottle.
  8. Splurge on a good quality regulator for your pony bottle. It has to perform when you may be under stress.
  9. Treat your pony bottle system as life-support. Get the components serviced and checked on exactly the same schedule as your main cylinder and reg.
  10. NEVER, NEVER, NEVER use the gas volume in your bailout bottle or pony in the gas calculations for a dive. In other words, do not plan your dives around the 1200 litres or 38 cubic feet you have in the pony. That gas is a RESERVE and should be ignored in one’s principle dive plans.

Although not the law, the best general advice for ANY single-tank diver who wants the assurance and personal “cushion” that comes from carrying a pony bottle is that they would do well to get some face-to-face time with a good mentor or instructor familiar with the kit and the procedures governing its use. An excellent certification course on this score is the SDI Solo Diver Certification.

A similar version of this article was first published in issue 24 of Underwater Journal, An Underwater Adventure Magazine, the official publication of SDI™ & TDI™. To learn more visit underwater journal magazine https://www.underwaterjournal.com/

Written by Steve Lewis

Steve Lewis (doppler@techidvertraining.org) is an active instructor-trainer with TDI/SDI and has written scores of articles on dive safety and skills development and is a regular contributor to several online magazines and discussion groups. He occasionally dives “open-circuit with a single aluminum 80” but never without a pony bottle by his side filled with a lean nitrox. His best-selling book called “The Six Skills and Other Discussions” is available at select dive stores and through onLine stores such as Amazon and Create Space eStore via: https://www.createspace.com/3726246.