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.
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