DIVE TIPS: Buoyancy Skills

 

Skill #4 is Buoyancy: The force that opposes Gravity
 
By Steve Lewis
(Taken from lecture notes for a presentation made to instructor candidates, May 2009 and excepted from 12 Dialogues with Doppler, Steve’s book scheduled for publication Summer of 2010)
 

I fly because it releases my mind from the tyranny of petty things.  Antoine de Saint-Exupery, 29 June 1900 — 31 July 1944, French aviator and author.

 
Of all the basic skills, buoyancy is the most obvious, probably the most talked about, and certainly the one skill most sought after by even the newest diver. No surprise I guess, since fluency in this skill is what gives us that floating weightless in the water column, flying like Superman, Le Petit Prince or Wonder Woman feeling that is the cherry and whipped cream on top of scuba diving’s ice-cream sundae.
 
This skill, and all that’s related to it, is hugely important to everyone who dives, but not just because it the closest we can come to flying and once mastered releases us from the “tyranny of petty things.” It is important because without control of her buoyancy a diver will sink before she is ready to; or bob around on the surface while her buddies are swimming around on the bottom. Without control of her buoyancy, a diver will have to work enormously hard to maintain her position in the water column once she does get below the surface, and may inadvertently demonstrate a breaching whale impersonation when her dive is over, risking all kinds of health issues relating to rapid ascents. For technical divers who have a decompression obligation requiring timed mid-water stops, there are a whole series of even more compelling reasons to have this skill down pat.
 
Buoyancy control is initially talked up in an entry-level open water class and practiced in the swimming pool during the very first confined water sessions. Perhaps because of this, probably seven or eight out of ten divers will tell you it’s one skill they have squared away.
 
You may be one of the lucky majority and you may understand and have your buoyancy controlled as fluidly as an Atlantic Blue-Fin Tuna; however, just in case you feel there’s room for improvement, let’s spend some time going over a few bits of information and some simple tricks that help technical divers get a firmer grip on this particular skill.
 
OK, but first, a confession. I am a nerd and there are several terms and definitions in diving that do not jive with my nerdy, pocket-protector scientific sensibilities. Here’s one:  The dive industry teaches us that a diver needs to be positively buoyant to float on the ocean’s surface, negatively buoyant to sink to its depths, and neutrally buoyant to hang motionless in the water watching its residents swim by.
 
All that stuff is nonsense[1]. In the world beyond diving textbooks, buoyancy only comes in only one flavor and not in three.
 
Buoyancy is an opposite and opposing force to gravity, and we owe our understanding of it – and the mental picture of some old Greek dude jumping out of his bathtub screaming “Eureka!” – to Archimedes.
 
It is gravity that lends a diver and her gear their apparent weight, and it is gravity that will cause her to sink: not negative buoyancy. Throw that diver and her kit into a swimming pool and their total apparent weight will change; it will become less. The Force of Buoyancy — which is equal to the weight of fluid she and her dive gear displaces (and which is described by Archimedes Principle) – will counteract the force of gravity and have the effect of pushing her up towards the surface.
 
If the weight of displaced water adds up to more than her and her gear, she is going to float. Less and she will sink because gravity wins. If they are the same and there is perfect balance between the two forces, she will remain suspended weightless and blissful. It’s that simple: there is nothing complicated to think about; no half measures or different states to consider; and nor is there any need for off-the-wall terms to describe them.
 
Things float, things sink, and things maintain their position in the water column. Which of these states applies to a diver at any given time will depend on the balance between gravity and buoyancy. And it is playing these two forces one against the other that a diver must learn to control.
 
With that behind us, let’s start with some information that relates directly to buoyancy and buoyancy control; and this means diving what is referred to as a Balanced Rig.
 
For a novice open-water diver about to take that first giant step into the whole underwater adventure, diving a balanced rig essentially boils down to being correctly weighted. There’s not much more to it, and one of the primary exercises for novice divers is to conduct a weighting test before venturing into anything deeper than the shallow-end of the local swimming pool.
 
The same thing holds true for technical divers, but like everything else connected to technical diving, the simplest of issues somehow get tangled up in no end of complexity.
 
For instance, one very important difference is that most technical diving rigs do not include ditchable weights. The lead worn around a sport diver’s waist on a removable belt, has been replaced with weight of twin cylinders, lights, backplate and a whole catalog of bits and pieces. Technical divers do no, in reality cannot, ditch weight in the event of an in-water emergency, and so starting a dive dramatically over-weighted for a technical diver is extremely dangerous.
 
A second and also a very important difference is there are many more equipment options open to a technical diver, and sadly there is no norm or one-size-fits-all solution. The closest thing to it, and a kit configuration the majority of technical divers on open-circuit would recognize, approximates the “standard” North Florida Cave Divers Kit[2].
 
At its base level it consists of a pair of high-volume steel cylinders fitted with a DIN isolation manifold, screwed to a steel or aluminum backplate, a simple continuous webbing harness to hold it to the diver’s back, some D-Rings to hang things from, a steel belt buckle, and a Buoyancy Compensator in the shape of a back-mounted wing.
 
One huge variable in this so-called standard are the size and shape of the cylinders that make up the principle back-mounted doubles. Technical divers are spoiled for choice. One popular manufacturer currently offers six different models of steel cylinders suitable for use as doubles. My local dive shop displays and sells cylinders from three different manufacturers.
 
It would be simple to dictate that all technical divers should use specific cylinders regardless of the type of diving they do, body size, and so on. Certainly gas matching would be easier and so would balancing a rig. But that approach to gear selection leaves no wiggle room for the individual who does not fit into someone’s idea of average.
 
What we are safe in saying is that a good way to build a balanced rig is to start by deciding which cylinders are best for your needs. For the sake of illustration, let’s use a pair of mine. Out in my gear locker – a euphemism for what is a usurped double-car garage – are a pair of X7-120 Worthington High-Pressure cylinders joined by a Halcyon 300-bar manifold. According to the published specs, these doubles weigh about 35 kilos on land without any air in them. Since we know how much air these tanks can hold when full, we should be able to work out how much extra weight they will have when filled.
 
The nominal capacity of a Worthington X7-120 is 15.3 litres. The working pressure is 230 bars. Since there are a couple of them, we can calculate the volume of air by multiplying 15.3 (nominal capacity) x 2 (there are two of them) x 230 bar (final fill pressure) which equals 7,038 litres. That is a lot of gas. Air has a mass of about 1.2 grams per litre (write that down someplace and try to remember it because it is a value you will need sooner or later). Therefore, 7,038 litres has a mass of 7,038 x 0.0012 (the weight of one litre of air in kilos) which equals 8.4 kilos. 
 
These numbers give us some very interesting information. They tell us that the tanks are heavy when empty and even heavier when filled to capacity! They also give us something called the Potential Buoyancy Shift for these tanks. Buoyancy Shift is term to describe the difference between the starting weight of a tank or set of tanks and their weight at the end of a dive. This shift in weight is a function of the gas consumed on the dive and is of major concern to open-circuit divers.
 
In my double X7s the potential Buoyancy Shift is 8.4 kilos. However, it would be a poorly-planned dive that finished with ALL the primary gas exhausted. Usually a technical dive is planned to wrap up with about a third or more of the starting gas volume remaining or a maximum of two-thirds gone. So the actual Buoyancy Shift would be about 5.6 kilos. Armed with this little knowledge nugget I can plan my weighting / balance to be prepared for almost six kilos of extra buoyant force at the end of a dive.
 
At this point too, it would be reassuring to know the Buoyancy Characteristics of the cylinders. Buoyancy Characteristics essentially means what do they weigh in the water. Looking again at the spec sheet, a single Worthington X7-120 has an apparent in-water weight of 4.9 kilos when filled and 0.9 when empty.
 
That means two cylinders plus the manifold will start a dive weighing about 10 kilos in-water. I am not so concerned about their empty weight because they will not be empty until next time they are visually inspected; I am interested, however, in knowing their weight when I’ve sucked up two-thirds of my gas supply. Since I already know the buoyancy shift is 5.6 kilos, it is simple arithmetic to calculate that a pair of Worthington 120s will weigh about four and a half kilos in the water when I am trying to hold a 3 metre stop.
 

 
Worthington Galvanized X-SeriesH cylinders (data supplied by manufacturer)
Description
X-Series 65
X-Series 80
X-Series 100
X-Series 120
X-Series 119
X-Series 130
Model
X7-65
X7-80
X7-100
X7-120
X8-119
X8-130
Service

Pressure

230 bar
230 bar
230 bar
230 bar
230 bar
230 bar
Nominal

Capacity

8.2 liters
10.1 liters
12.2 liters
15.3 liters
14.8 liters
16.0 liters
Buoyancy*

FULL

– 3.9 kg
– 4.1 kg
– 4.5 kg
– 4.9 kg
– 4.9 kg
– 5.3 kg
Buoyancy*

EMPTY

– 1.8 kg
– 1.4 kg
– 1.1 kg
– 0.9 kg
– 0.9 kg
– 0.9 kg
Weight

EMPTY

11.4 kg
13.1 kg
15.0 kg
17.2 kg
19.0 kg
19.5 kg
Outside

Diameter

184 mm
184 mm
184 mm
184 mm
203 mm
203 mm
Cylinder

Length

42.4 cm
50.0 cm
60.1 cm
71.1 cm
60.1 cm
64.8 cm
*Approximate in sea water with valve
 
 

 
 
OK, let’s put away the scrap paper and calculators and return to the basics.
 
 At its simplest, balanced means using a buoyancy control device (wing) capable of providing adequate displacement (buoyancy) to float everything including the diver at the beginning of the dive while having sufficient apparent weight at the conclusion of the dive to comfortably hold a position in the water column at a depth of 1 metre (about a yard) without bobbing to the surface like a champagne cork. 
 
In addition, when the kit is at its heaviest – which is when the diver is carrying the maximum volume of gases and when no stage cylinders, deco bottles or other equipment has been dropped, tied-off or staged for later retrieval – the total package should not have a total in-water weight greater than the diver can swim back to the surface and maintain at the surface in the event of a failure of the primary buoyancy device.
 
The first step in finding out if one’s kit is balanced or will pin its user to the bottom of the pool like an upturned turtle is to know the apparent weight of the whole package when it is submerged in water.
 
The worst options in my opinion are to “ball-park” it and dive a set-up that one is tempted to trust because a good friend dives something similar; or to dive it because a guy at the local dive store said it would be fine; or to go ahead and dive it because it seems to fit together nicely and it is all from the same manufacturer… so it must work.
 
Your mileage may vary on this but I count three strikes for those options.
 
Strike one; all my buddies are different shapes and sizes and none are the same as me; so what works for them will not work for me necessarily. Strike two; for all I know, the well-meaning fellow at the dive shop was filling in for his cousin Bill and neither of them has ever been in the water in scuba gear. Strike three; most manufacturers make lots of variations on a basic idea when it comes to dive kit. Backplates for example come in various thicknesses of steel and aluminum. Which is the best model for a specific diver will depend on where they are going to dive – salt water or fresh – what size cylinder or cylinders they will use and a number of other factors. Most manufacturers design and market wings with different degrees of “lift,” the common term for the amount of buoyancy a wing can deliver when fully inflated. Which is the best model for a specific diver will depend on a whole heap of variables including whether the user will be diving dry or wet. And so it goes on.
 
You could of course do the long-hand calculations and weigh each piece of kit and find out its volume or research its buoyancy characteristics on the “Net” and put it into an Excel spread sheet to total it all up.
 
There is a much easier way and it is the most accurate. Put you proposed gear package together – harness, backplate, fully charged primary cylinders, and stage bottles; or if a rebreather, scrubber loaded, diluent and oxygen bottles filled, and bailout cylinder(s) attached – and lower the whole mess into a pool suspended from a fish weigh scale. A fish scale at its simplest is a large hook suspended from a spring with a calibrated scale marked in pounds or kilos beside it.  
 
More fun, more accurate and a definite step up in status is the 21st Century version: the digital fish scale. For the record, I own a Cabela’s Digital Fish Scale. This little gem retails for less than $25, reads SI and Imperial weights,  and is fully featured with all sorts of useful and time-saving options – some of which I have used once or twice. Mine has never seen a fish, but it was money well spent and has helped to demonstrate to more than one student that they either:
1.       REALLY do not need a wing delivering 100 pounds of lift
2.       Actually have to dive a steel backplate with their choice of doubles.
 
Three quick pointers when checking the in-water weight of dive gear. 1) The wing should be deflated fully. 2) Attach the scale to a D-Ring or some other suitable point, with the whole kit already partly or fully submerged since the dry-land weight of a typical tech diving package is enough to bust most scales! 3) Weighing the rig with a second time with all cylinders, including stages almost empty, will save you working out Buoyancy Shift manually.
 
(to be continued)
 


[1] Hey, it’s not life-threatening to use Diving-ese to describe buoyancy, but please remember that negative, positive and neutral buoyancy are terms from the same textbook that explains to its readers there are only three states of matter – gas, liquid and solid – and we all know that’s simply not the case… That’s right, what about plasma or Bose-Einstein Condensates?!
 
[2] The North Florida Cave Diver’s Kit goes under several popular names. Hogarthian, a term used to describe bare-bones, no-frills, minimalist approach to gear configuration named after pioneer cave diver William Hogarth Main, is probably the most well-known and most often misused.

 

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