At the surface, a properly weighted diver will float at eye level. This statement, or one like it, appears in virtually every diver training organization’s materials. What you won’t find agreement on is whether this is with a full lung of air or half a lung of air. It may also be unclear is whether this is done which a full tank or one that is closer to empty. It doesn’t matter. No matter what your Open Water Diver manual says, it’s probably wrong. At least much of the time. Why is that?
- Generalizations like “…will always float at eye level” fail to account for factors such as varying degrees of compression between exposure suits of different thicknesses.
- They also fail to account for cylinder size. The bigger your tank, the greater the change in overall buoyancy you may experience during the dive.
Saying that a properly weighted diver will always float at eye level is as bad as generalizations such as “…wear ten percent of your body weight in lead.” Like the broken clock that still gives the correct time twice a day, it may be true for some divers under some conditions…but not for many divers or even most divers.
What is especially problematic is that most diver training materials fail to define exactly what “proper weighting” is and why it is important.
So what exactly is proper weighting?
That’s easy. You are properly weighted when you are wearing the least weight you possibly can, without having to fight to stay down. Why is that important?
- Buoyancy Control: Your need to add or remove air from your BC is dictated by the compression and expansion of your wet suit or dry suit, and by the compression and expansion of any air you have previously added to your BC or dry suit. If you start your dive with, say, 2.0 kg/5 lbs of unnecessary weight, it will be offset by a 2.0 kg/5 lb air bubble that will compress and expand as much or more than your exposure suit does. This will require you to make buoyancy adjustments more frequently and make overall buoyancy control significantly more difficult.
- Trim: The more unnecessary air you need to add to your BC, the more it may tend to drive your torso up and your feet down. This can make maintaining horizontal trim difficult, if not impossible.
- Safety: Every pound or kilogram of unnecessary weight you carry increases your risk of drowning. An overweighted diver who is struggling to stay at the surface because he either forgot to inflate his BC or because BC won’t hold air, and who further forgets to jettison any ditchable weight will eventually lose that struggle and sink. In this respect, properly weighted divers stand a far better chance at survival.
Obviously, overweighting is a bad idea. Being underweighted, however, can be equally as bad, as it puts divers at greater risk or too-rapid or uncontrolled ascents.
Any way you cut it, there is no substitute for carry precisely the right amount of weight.
Is there any real value in doing pre-dive weight checks?
This depends on a variety of factors.
- Obviously, if you sink like a rock before you have fully vented your BC, you are most likely overweighted. Conversely, if you can’t sink after completely venting your BC and fully exhaling, you most likely have too little weight.
- If you are wearing a thin, relatively incompressible wet suit or dive skin (or a shell-type dry suit, in which you will be maintaining a constant volume throughout the dive), you may start the dive slightly negative, due the weight of the gas in your tank which you will consume during the dive. This will allow you to return to the surface at the end of the dive with the top of your head just touching the surface of the water when your lungs are half full. If you wear any less weight than is necessary to achieve this, you may have difficulty staying down towards the end of the dive.
- On the other hand, if you are wearing a thick, highly compressible wet suit, It’s possible that, with your BC fully vented, you may actually float somewhere between forehead and eye level at the beginning of the dive. This will require you to exhale fully after venting your BC in order to initiate a descent. Taking advantage of your suit’s initial compressibility in this fashion may help to reduce the need for and frequency of BC adjustments under water.
The bottom line is that some form of pre-dive weight check can help determine whether you are grossly overweighted or underweighted. It is not the best place, however, to determine precisely how much weight you will ultimately need.
By the way, if you hear or read anywhere that weight checks should be done with your lungs fully inflated, it’s dead wrong. Your overall buoyancy is determined by your mid-tidal volume, i.e., the halfway point between exhalation and inhalation. This means that, if there is any value to a pre-dive weight check, it is best done with your lungs half full.
The most ideal place and time for a weight check
If your ultimate goal is to always use the least weight possible, it begs the question, when would I need the most weight? Answering that question is actually fairly easy.
- In terms of time, it would be at the end of the dive, when you have the least gas in your cylinder and, as a consequence, are most buoyant.
- In terms of depth, it would be at the point in the dive where you are spending the greatest amount of time in shallow water, where exposure suit expansion would also tend to make you the most buoyant.
As you have no doubt guessed, we are talking about safety stops here. Safety stops are a vastly more accurate place and time to do final weight checks than anything you can do at the start of the dive. The procedure for doing so is idiot-simple and works the same regardless of whether you dive wet or dry, in fresh water or salt, or with a thick or thin exposure suit. Here it is:
- Ascend slowly to safety stop depth. Anything reasonably close to 5.0 m/15 ft is fine.
- Upon arriving at this depth, make sure your BC is completely vented.
At this point, if you are wearing the right amount of weight, you should be able to hover, motionless, at safety-stop depth with no air in your BC.
- If you find yourself fighting to stay down, make a note to add a small amount of weight prior to the next dive and repeat this procedure, as needed, until you get it right.
- Conversely, if you find yourself sinking and have to add air to your BC in order to maintain depth, it means you are overweighted and need to remove some weight before diving again.
Remember that, as you ascend from safety-stop depth, you may experience a slight increase in buoyancy. Depending on exposure suit thickness, you can generally offset this and keep your ascent rate under control just by breathing a little less deeply.
If you are diving a particularly thick wet suit, however, you may need to add a small amount of weight in order to keep your ascent rate manageable. This means you will have a slight amount of air in your BC at safety-stop depth.
The job isn’t finished until the paperwork is done
A fact that seems to elude many new divers is that proper weighting is entirely dependent on factors such as exposure suit thickness and compressibility, the salinity of the water, and cylinder size and weight. In other words, just because you determine you need a certain amount of weight when diving a 13-liter steel 100 in a 7 mm suit at the local rock quarry, doesn’t mean this will be the same amount of weight you will need when diving an 11-liter aluminum 80 in a 2 mm shorty in Cayman.
Don’t be one of those people who shows up on a dive boat asking the crew how much weight do I need? They have no way of knowing.
It is better to carefully document the right amount of weight for every combination of exposure suit, cylinder size and salinity in your dive log. Not only can you refer to it when diving in the same combination of equipment and conditions, you can also use it as a starting point when estimating how much weight you will need for a new situation. For example:
- Given the same equipment, you will generally need 2.0-2.5 kg/4-6 lbs more in salt water than in fresh, depending on body size and weight.
- If changing cylinder size, you can consult any number of online tables that document the differences in buoyancy between cylinders.
Unfortunately, there is no standard reference you can consult that will that lists, categorically, the differences in buoyancy between wet suits of varying thicknesses. This is too highly dependent on size and material. You can, however, assume that, the thicker the suit, the more weight you will need. This, at least, will give you a starting point.
Getting back to our original point, statements such as properly weighted divers will always float at eye level are neither accurate or helpful. They are also symptoms of a larger problem. That is, the people who work at far too many of the major training organizations really need to get out and both dive and teach more. Then we wouldn’t have to correct misinformation such as this.