Notes about things regarding scuba diving that I have come to know.

Your Dive Weights

Weighting Concepts * Checking Your Weight * Getting Into Trim * Finding a Starting Point
Getting Down to Begin With * Transitioning Between Salt & fresh Water * Types of Weights

The topic of how much weight to wear is one that very few new divers have a good understanding of. All too often in the confusion of class with varying gear and for many the transition from a fresh water pool to the ocean a good configuration is never arrived at. Divers get over weighted to ensure that they will get down to complete the certification dives in spite of lungs that are expanded with excitement or apprehension. It's also common for certified divers to not be wearing the optimal weight. I will try to provide you with an understanding of the objective and methods for determining what you should be wearing. Getting this right can vastly improve your safety, proficiency and enjoyment as a diver. This is all about physics and your unique gear and body make-up. It can be objectively determined and for all intents and purposes everyone's best configuration is unique. The good news is that while its about physics not much math at all is needed, just an understanding of some relationships. I'll need to touch on some related topics along the way so hang in there as I try to bring it all together.

Weighting Concepts
I want to start by reviewing buoyancy as a concept. Buoyancy is the tendency to rise of fall in the water column. When we are neutrally buoyant we do not move vertically. When we are in the water we will take the place of (displace) a specific volume of water that has a weight. When our weight is equal to that of this displaced water we are neutral. If we weigh less we rise and if heavier we shall sink. Except for a few details that I will mention latter we pretty much weigh what we weigh and displace what we displace. A fit muscular person will be more dense and will be less buoyant than a person with a higher percentage of body fat. We are all unique.

Before we continue with that thought lets talk about why you have a BC. The name is self explanatory. It lets you compensate for variations in buoyancy so you can remain neutrally buoyant in all situations or let you descend or ascend as desired. As a secondary function (some will say primary) it provides surface flotation. Buoyancy is an ever changing state. The air in your 80 cubic foot scuba cylinder actually weighs about 6.4 pounds. You will end your dive about 5 pounds lighter just by consuming that air. Your wetsuit gets compressed as you dive deeper. Since compression reduces your displacement, buoyancy is reduced and you sink. At the beginning of your dive there will be air trapped in your wetsuit or BC materials that will escape while you dive. At the start of the dive and as a new diver you will tend to have more air held in your lungs. As you gain experience and settle into the dive you will relax and your lungs will be less puffed up. These are all changes that will happen and we compensate for them by adding or removing air from the BC bladder. If you are wearing any significant amount of neoprene you WILL be adding air to your BC as you descend. There is nothing evil or wrong about this.

Why is the right amount of weight is so important? Since you have a buoyancy compensator (BC) it's a common misconception that you can just add air to balance out for any extra weight. At first glance this is true but that practice comes at a high price. First of all nobody wants to be walking around or diving with 5, 10 or even more pounds of unneeded lead strapped to their body. It's just plain wasted energy. The real detrimental and dangerous part of being over weighted is apparent when you are diving. A gallon of water weighs about 8.35 pounds. For the sake of visualization I want you to see every pound of lead as a pint of air in your BC. That is, for every extra pound of lead you have you need a pint of air volume in the bladder. If you have 5 pounds more than needed that is over half a gallon of air. Can you picture the milk jugs?

So now lets look at that evil extra weight. Remember the pint per pound relationship? Let's say you have 4 pounds of extra weight on you. On top of whatever may be appropriate you now have half a gallon of extra air in the bladder. Now go back to your open water training and remember Boyle's Law. They may not have mentioned Boyle by name. He is the reason you never hold your breath, especially on ascent. As the pressure (depth) decreases the volume of the air will want to increase. Here you are swimming along at some depth. You have fiddled with your inflator and you are dialed in and neutrally buoyant. Up ahead there is a boulder and you want to rise over it to see what's on the other side. So you take a deep breath and exhale slowly as you give a few kicks and you glide upward. You begin to rise and as you do the air in your BC starts to expand thanks to Boyle. As the volume (displacement) increases so does the lift and you begin to rise faster. As you rise faster the volume continues to increase. If you don't intervene and vent soon you will be ascending out of control. Likewise whenever you swim up or down you will feel greater swings in your buoyancy when there is needless volume in your BC bladder. The result is perpetually using the inflator to add and subtract air in search of neutral buoyancy. The closer your BC bladder can be to empty the more 3 dimensional freedom you will have.

The BC air volume used to offset needless weight will amplify all of your buoyancy challenges.

Before you can determine the correct weight to wear you need too understand the objective. At some point, usually the very end of the dive you will be at your most buoyant having released 5 pounds of air from your cylinder. You will also be approaching the surface and your wetsuit will be returning to maximum buoyancy. This is the defining moment of deciding how much weight you need. You need enough to keep you from ever being forced to the surface. If for instance there is a boat passing overhead you never want to be uncontrollably ascending.

The Weight Check
So go make a dive with "enough" weight. If you don't even have a starting point we will cover this latter. In any case include a number of weights, preferably smaller weights, like 2 pounders. Place them in accessible pockets or use clip-ons. If you have some extra bolt snaps and zip ties you can make some temporary clip-ons. Use 2 ties per weight for safety and hang them from D-Rings.

Now go make that dive and plan to be back in 8-10 feet of water with about 500 PSI left in your cylinder. When you get back in the shallows make sure your BC is entirely vented. Have your buddy check for puffy chambers. This should get you negative on the bottom, unless you are correctly weighted or under weighted to begin with. While down and horizontal begin to hand off weights. Breathe normally. Eventually you will be light on the bottom and then able to hover effortlessly about 2 feet off the bottom. As you hand off weight you will feel a stage where you are uncomfortably light and at risk of going buoyant. Take that last weight back and make a note of what you have removed.

Adjustments:

This exercise should have given you the optimum amount of weight to wear. You will also need to decide where to wear it so you effortlessly dive in good trim in a horizontal orientation. That's next.

You want to be repeating the check periodically. As you get more comfortable you will tend to need less weight as your lungs will be relaxed. As your neoprene ages the bubbles will lose gas and become less buoyant. And of course any changes in your gear require a recheck.

If you have not actively checked your weighting then you do not know that it is correct.

Getting in Trim:
Trim in diving refers to being in a true horizontal prone posture. This will be a superficial treatment of the topic mainly to provide awareness and starting options. Be self aware and ask your buddy for objective feedback regarding your orientation as you dive. By being in good trim you will use less energy to move forward and your buoyancy will be more stable. Burning less energy equates to using less air and having a longer more pleasurable dive.

Picture your body as a see-saw with an imaginary pivot point. If you find that your feet tend to drop then move (don't add) some weight closer to your head. If you swim in a head up orientation your body will be punching a bigger hole thorough the water and waste energy. Your body will also have a lift effect, much like an airplane wing. You may be swimming along just fine. When you stop to look at something you will settle to the bottom. This is because the slope of your body is no longer driving water down to hold you up. If you are diving feet up the exact opposite will happen and you will tend to rise when at rest. Here is a list of things you can do to modify your weight placement. As a starting point I will assume that all of your weight is around your waist on a common weight belt.

With a little experimentation you can be hanging like a skydiver in free fall. Remember that you are looking for a happy medium. Your cylinder will become lighter during the dive. As you become more self aware this will be noticeable but manageable. Any changes in gear may have an effect so keep that in mind when you add items like a big light for a night dive. Since you are now diving in an optimal configuration you can afford some discrepancies from time to time. You don't need to obsess over every item you bring on a dive Just be sure to account for major or permanent changes.

Finding a Starting Point
There are some formulas floating around that suggest what to wear for weight. At best these are a rough starting point. At worst they are accepted as fact and divers go off diving incorrectly weighted.

I will assume that as a student or experienced diver you have access to a pool or some body of water. This will be a more explicit explanation of a common in the water weight check method. Just as we described how to be neutral at 8-10 feet we will now talk about being slightly positive on the surface. The human head weighs about 10 pounds. You want about 1/2 of that to be out of the water. In other words you want to be about 5 pounds positive on the surface. Since your lung volume can swing your displacement by about 5 pounds you can see how careful breathing can keep you in control to the end.

So get in the water with all of your gear. The water wants to be fairly calm so you are not disturbed with surge or upwelling. Preferably be in the water as long as possible to let bubbles of air escape from your wetsuit, BC and other gear items. Now do the following check:

Getting Down to Begin With.
There is a chance to that due to gear or technique the correct weight may not let you make an easy descent. A common reason for this is air trapped in suits and BC padding. For the first few minutes of a dive your BC padding may represent 3 or more pounds of temporary buoyancy. Once you are down for a few minutes the material saturates with water and becomes fairly neutral. Since 2 wrongs don't make a right adding weight back is not the answer. Lets' look at some options.
  • Is your BC fully vented? Have a buddy check. Some require patience or a counter intuitive tilt to let the last few pints of air escape. Be sure to hold the inflator high if using it to vent.
  • If making a vertical descent are your fins still? You may inadvertently be kicking upward. Cross your fins.
  • Consider a horizontal descent. Laying flat on the surface lift your inflator hose or your buttocks on the side of your back dump if so equipped.
  • As you let the air out of your BC take in a deep satisfying breath. Just as the BC goes empty exhale all of your breath. When done, exhale some more! You should begin to drop. When needed take a shallow breath and blow it right out to limit your average buoyancy. After a few feet your neoprene will start to compress, bubbles will escape and you will be on your way.
  • If that does not do the trick you want to make a duck dive. After fully venting kick your legs up in the air and jackknife down head first. When your legs are in the air they go from being fairly neutral to being all weight. This will drive your body down into the water column. As your fins submerge give a few good downward kicks.
  • Whatever you do relax and have fun. If you get all exasperated your lungs will be puffed up and it will get even harder to sink.
  • Once the drop is established add short bursts of air to the BC to control your velocity such that you can comfortably equalize on each breath.

Getting your weight and trim right are 2 things that will catapult your diving skills in a hurry.

Fresh Water/Salt Water
When going between salt and fresh water you will need to make some adjustments. Salt water is denser than fresh water. Earlier I mentioned that to be neutral we must weigh the same as the water we displace. Well, it so happens that seawater is denser that fresh. This means that for our given size we need to weigh more as well.

Specific gravity is a ratio of density of different materials with freshwater being the baseline of 1.0. In general the specific gravity of seawater is somewhere between 1.025 - 1.028. There are some dramatic exceptions, like the Dead Sea but for easy math lets go with 1.025. This means that the weight of the water you displace will be 2.5% greater in the ocean. I said I'd keep the math easy, here is another way of looking at this. 2.5% goes into 1 40 times. This means that for every 40 pounds of diver weight you need to add 1 pound when going to the ocean.

What you need to do is figure out what you weigh just as you will be diving. Ideally you will get all geared up and get on a scale. This means your exposure protection, tank, fresh water weights the whole kit and caboodle, just add water. Lacking the logistics to use the scale some quick calculations will get you close enough.
125# Diver
40# Tank filled with air
25# Weights
15# Exposure protection
15# Regulator & BC
240# Total Dry Diver Weight

This diver weighs 240 pounds. Dividing by 40 tells us that 6 pounds extra will be needed in the ocean. This is an entirely objective conclusion. You will usually end up with some sort of fraction, unless you are barely over an even number round up. If you are going from the ocean to fresh water you can use the same method to decide what to remove, however in this case round down the amount you will remove.

Remember that this also assumes the identical gear. When you start changing gear you need to revisit the whole process. Also remember that there are rounding errors in all of these decisions and you may find you can leave a little more weight behind with experience.

Types of Weight
While we are beating weights to death here are some thoughts about styles of weights. If you are going to be a cold water diver you will probably be looking at 20 pounds or more of dive weights. Over time you will have a need to reconfigure for assorted reasons and will probably come to own an assortment of weights that you mix and match. The cost of dive gear adds up fast and lead weights are no exception. before you part with your cash consider the following.

Hard or Soft?
Hard weights are one piece castings made from melted lead. They are essentially indestructible and will last as long as you do as a diver. Worn right they should not be uncomfortable but they are solids. They are the most compact weights in terms of space. This can be important when packing them into places like integrated weight pockets. They are most commonly laced onto a weight belt.

The alternative is to use soft weights. these are mesh sacks filled with a variation of lead buckshot. They get packed into BC weight pockets or pocket format weight belts. Being non solid they do conform more to the body of the diver. Being mesh sacks there is a wear factor and eventually they will rupture spilling the lead contents. You will also find that with each rinsing there will be clouds of lead residue that comes from the pellets chaffing against each other. With all of the concerns we have about lead pollution and poisoning I suggest avoiding these weights. This same released dust will tend to stain gear that they are worn in or against. Soft weights come in a range of sizes

Lace Through vs. Pass Through/ Bullet
There are 2 basic shapes of hard weights. the difference lies in how the webbing is laced through them and how they relate to your body.

Lace through weights have 2 slots that run verticaly though the weight running from the diver outward. The weight belt webbing runs along the divers body and enters the weight through the first slot. It the weaves back into the second slot returning to the divers body. From there it runs along the divers body to the next weight or an endpoint. These weights tend to hang away from the diver and I find them more comfortable. Since webbing weaves in and out they will cause more variation in weight belt fit as you add and remove weights.

The second style is called the pass through or bullet. These have a single slot that passes through the weight. This slot runs along the divers body and the webbing just runs straight through from weight to weight. In some areas it is popular to rig an entire belt with 2 pound versions of these that are shaped like cylindrical bullets. This can make for a very simple and adjustable weight system. Novelty weights can also be found in the shape of hand grenades and hearts.

Coated or Plain
Solid weights are lead castings and as such aren't much to look at. You can get solid cast weights that have been dipped in a durable vinyl coating. This makes them colorful, easy to spot and cleaner to handle. While a solid weigh leaches very very little lead the coating does encapsulate the material and prevent gear staining. The coating is close to neutral in buoyancy so it has no effect.

Factory or Home Made
The casting of weights is a time honored tradition for divers and molds are readily available. Some shops have locally cast weights for sale. I have seen some nice bullet weights but in general the hand cast block weights suffer from the following issues. Weights generally vary a lot compared to a factory cast weight. This can amount to pounds of error when rigging a system. Most are made with open back molds. This puts the raw cast face which usually has rough edges against the diver. This can be hard on the webbing and exposure protection worn. The melting process probably strips away most contamination but I always wonder what sort of toxic stuff may be mixed into the salvaged metal.

Etceteras
Whatever you get for weights I suggest marking them with a paint pen. Make the weight and your initials readily visible. This makes things a lot simpler when rigging for a dive or after sharing some lead.

Consider adding a clip-on weight or 2 to your dive bag. They can be handy when doing weight checks and other odd tasks. These weights will have a bolt snap or carabiner cast into the weight. They can also double as a pass though weight.

Be sure to have weight keepers to hold your weights in place. I like to use 4 per belt so that the weights on each side stay in place and can never fall off. I like to reserve at least 8 inches of clear space on my back. This lowers my center of gravity for steadier swimming and leaves a nice place for the air cylinder to lay if it hangs that low.

This page created 5/31/10