B.C.'s Are Not Weight Belt Compensating Devices

by

Larry "Harris" Taylor, Ph.D.

This is an electronic reprint of an article that appeared in Underwater USA (Oct. 1993, p. 35). This material is copyrighted and all rights retained by the author. This article is made available as a service to the diving community by the author and may be distributed for any non-commercial or Not-For-Profit use.

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There is within our sport diving community a disturbing trend to overweight. We are propagating a generation of divers who actually believe the myth that it is necessary to grossly overweight in order to go diving. That myth is simply not true!  Dan Orr, Training Coordinator for D.A.N., has interviewed and photographed hundreds of divers in preparation for his slide presentation "Dressed To Kill." His findings indicate, "Over-weighting is a problem that knows no boundaries in experience or certification level." Dan's slide presentation contains numerous "candid" photos of divers who have gone to great (and often bizarre) measures to accomplish this over-weighting. Such extremes include, but are not limited to, strapping large hip weights on scuba cylinders with duct tape, using multiple ankle weights including chains, and use of duct tape to secure weights on the ankles. Although the problem is most easily recognized on the basic checkout, this problem with over-weighting appears throughout our diving community.

One of the genuine pleasures of diving is to glide weightlessly through the water environment.  With time a diver develops a sense of oneness of self and Planet Ocean. It is this freedom from gravity that is, in part, responsible for that very real "divers high." Over-weighted divers are not free from the constraints of gravity and thus deny themselves great joy. Diving weightless is fun! Crashing into (and destroying) our reefs and/or disturbing everyone's visibility by mucking around on the bottom because of over-weighting are not fun; it is work! Let's examine and hopefully destroy this myth "that over-weighting is O.K. because you can add air to your B.C." 

Simply put: Work is the movement of mass through distance. If you carry more weight, you move more mass through the water; thus you will do more work.  More work means more air consumed and less bottom time. If you compensate for the extra weight by adding air to the B.C. you increase your drag in the water; thus you do even more work.  The over-weighted diver generally adds a lot of air to the B.C. As the diver moves, the air shifts and such divers often feel like they are moving around out of control. (They, of course, are out of control.) The added air poses an additional problem on ascent. The air rapidly expands and increases a diver's ascent rate. An efficient diver tries to minimize work done by streamlining his body; thus an efficient diver moves horizontally through the water. An over-weighted diver generally moves through the water with head higher than feet. This position presents a larger cross-sectional area and creates a lot of drag.  Again, the diver must do more work. In this position most of the thrust from the fin is wasted trying to stay level in the water column and forward movement is not as much as it should be. Again, more needless energy is expended. Finally, the extra weight puts a strain on the back and increases risk of diver injury. Bottom line: If you overweight, then you will overwork.

The Greek philosopher Archimedes described the fundamental principle involved in weighting. He discovered that an object would be subject to a buoyant upward force equal to the weight of the water displaced by the object. This means that if an object immersed is denser than water (more mass than an equal volume of water), then the object will sink.  If the object immersed is lighter, the object will float and weight must be added for the object to sink.  Most human beings have a density than is nearly the same as water. (No big deal, we are composed primarily of water.) Lean muscle tissue is denser than water; it sinks. Fat floats. 

There are two primary sources of buoyant force that enter the water with the diver. They are fat tissue and air spaces. The fat problem can be addressed by proper diet, exercise and lifestyle modifications. The air space problem can be addressed by merely taking time to evaluate the situation.

The primary source of buoyancy lies in the wet suit. The foam rubber material uses trapped air for insulation. This volume of air must be compensated for by the addition of weight. Except for purchasing a dry suit that does not change buoyancy at depth, there is very little a diver can do to change this factor.  The diver becomes heavy at depth because the water pressure decreases the volume of the trapped air in the suit and this reduces buoyancy of the suit.

Divers are people. People do not breathe water. As you enter the water, your primitive brain reminds you that you do not breathe water and you tend to inhale and hold larger volumes of air. For this illustration consider the lungs to be like a Mae West jacket: as you inhale, you inflate your "life preserver" and descent becomes more difficult." This is particularly noticeable in new divers.  Doing surface dives and floundering around kicking with fins in the air does nothing but burn air, tire divers and increase frustration.  Jerry Ashe, NAUI instructor and Florida dive guide, states, "Most visiting divers have no concept of how much weight they should be wearing to achieve a hovering state of so-called “neutral buoyancy." " Too many divers," he says, "simply put extra weight on to get underwater, instead of learning to relax." The best way to descend is to simply relax.  This allows lung volume to be normal. If properly weighted (see below), descent is initiated by simply exhaling. Many early difficulties in descent can be overcome by relaxing a few minutes at the surface prior to submersion.  (I have seen divers remove more than eight lbs of lead when shown how to relax. 

Many divers who have descent problems have failed to drain all of the air out of their B.C.'s. Some inexpensive bladder stabilizing vests actually trap large volumes of air. This is not a problem with higher quality jackets. If you are having problems descending, it might be wise to have a buddy look at your jacket to determine if large air volumes are being trapped between the bladder and the outer covering. This is the type of situation that is easily solved: Throw money at the problem (buy a better quality jacket) and the problem disappears. Often divers do not actually vent all of the air from their B.C. Divers should examine their own individual gear and assume a descent position that places the B.C. vent at the highest possible point. It is helpful to have a buddy/instructor examine descent procedures to determine the optimum venting position. Descent in this position allows the water pressure to push all of the air out of the B.C. (I have seen divers remove 10 lbs of lead, or more, when they were shown how to properly vent their B.C.'s.)

For a 1/4" farmer john wet suit in fresh water, I use the following procedure to establish proper weighting.  Start with 10% of your body weight on your weight belt.  Inflate your B.C.  Enter water just slightly deeper than your height WITH A NEARLY EMPTY TANK! (~500 psig). Assume a vertical position in the water.  Take a few moments to relax.  Put the regulator in your mouth and breathe normally.  Vent your B.C. Your dive buddy/instructor can verify that your B.C. is empty. You should float at eye level. You should sink as you exhale and rise as you inhale. If you cannot sink below eye level, add weight in 2-pound increments until you can float at eye level. If you sink below eye level, then remove weight in 2-4 pound increments. Since your buoyancy is set with an empty tank, there will be no problem at the end of your dive in shallow water or holding a 10-foot stop because of buoyancy changes caused by the consumption of air. A full tank will be a few pounds heavier than an empty tank, thus you will initially enter the water slightly heavy.  Initial descent is then a matter of venting the B.C. and exhaling. This procedure should be done with each change in equipment configuration. The mark of a good diver is the ability to dive with a minimum of weight. 

Divers should realize that buoyancy is an individual matter that depends on a number of variables. Lee Somers, Ph.D., Diving Safety Officer for the University of Michigan, in his booklet, Buoyancy and the Scuba Diver, reminds divers that their buoyancy changes with size (weight gain/loss), body composition (%fat), lung capacity, breathing volume, psychological state (relaxed vs. tense), and exercise load. As these variables change, so will the amount of weight necessary to achieve a hovering state. Since buoyancy changes with time and experience, divers should get in the habit of routinely checking their personal buoyancy requirements. 

The weights on the belt should be balanced, i.e. left and right sides should be equal so that the diver does not roll to one side because of more weight on one side or the other. Have your dive buddy look at your belt to verify that the weight is evenly distributed. The weight should be as far forward as possible on the belt.  No weight should be on the back part of the belt underneath the scuba cylinder. (This minimizes strain on the back and shifts the center of buoyancy closer to the center of mass. Translation: You will be more stable in the water with lesser tendency to roll. You will be able to remain stationary on the surface without rolling to one side.) Finally, eliminating the weight underneath the scuba cylinder makes the belt much easier to ditch in an emergency.

The ability to control buoyancy is a fundamental skill. Divers who cannot master this skill will be working too hard to ever really enjoy the absolutely fantastic "wonderfulness" of gliding weightless through Planet Ocean. Lou Fead, the EASY DIVER, said it best, "Dive with your brains, not your back!" If you remember that B.C.'s are not weight belt compensating devices, then you will work less and enjoy diving more.

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About The Author:

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

  Copyright 2001-2004 by Larry "Harris" Taylor

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