I find myself answering such questions often on other forums. This post consists of an article I wrote a few years ago, plus a compilation of replies I have made recently to various weighting questions. The questions themselves have been stripped, so I hope this makes sense - try and put it in context.
-Sean
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This article was originally written for a technical diving audience, but is equally applicable to recreational divers. Some comments may not apply, but the same concepts are involved with respect to weighting and establishing ideal buoyancy characteristics of your rig.
The proper approach is to dive with a balanced rig, in such fashion as to enable a gentle kick to get you to surface in the event of a loss of buoyancy. The idea here is simple, but requires the correct gear in the first place, to work. This means not using drysuits made of neoprene or other material where the buoyancy varies with depth, and using the correct tanks for the application, so you don't require massive amounts of inflation to offset the excessive negative buoyancy (use only aluminum stages, and never use steel tanks when wetsuit diving).
To develop a balanced rig, start by determining the amount of integral weight that you need. This will be the amount of weight that you require to enable you to sink, with full tank(s). This weight essentially just offests the buoyancy of the suit and insulation, and includes your tanks, backplate and V or P weights in some cases. As the dive progresses, you will consume the gas in your tanks, and get lighter. You need to be able to maintain your depth at the deco stops when your tanks are empty, so you add droppable weight to compensate for the weight decrease due to gas consumption. Droppable weight is in the form of a negatively buoyant canister light, or a weight belt. The only purpose of the wing is to compensate for the weight of gas in your tanks. Thus, barring any problems, you will start the dive with a slightly inflated wing, and end the dive with an empty wing. This means that the majority of the lift capacity of your wing is used only for emergencies, or keeping the rig afloat when you take it off on the surface!
With a rig that is balanced in this fashion, you will always be able to get up when you have to. The worst case scenario is that you have a complete wing failure at the beginning of a dive, when you are heaviest, and your drysuit fails too. In this case you lose the droppable weight to bring you back to neutral and kick up. You can also move some of the integral weight to droppable, which will result in positive buoyancy if you lose the droppable weight, but has the advantage for the ocean divers (if in the form of a weight belt) that you can take the rig off on the surface without fear of losing it.
Having the droppable weight in the form of a weightbelt is not strictly necessary when diving a balanced rig in a drysuit, and I usually never wear one. In fact, the only time I wear a belt anymore is on tendered SAR dives. When it may become appropriate is when you are ocean diving from a small boat such as a RIB, which requires you to take the kit off on the surface before boarding. More of an issue when there is a significantly choppy sea - this prevents an excessively negative kit from sinking on you while dekitting and transfering the gear to the boat. Of course, depending on how negative it actually is you could just as easily lose the light, but when your fingers are not quite 100% dextrous in the cold water - I would rather lose a $30 belt than a $1000 light.
One commonly asked question when wearing a weight belt as droppable weight, is should it go over or under the harness? The answer depends on what the greatest risk is for the diver, ie. accidentally losing the belt and becoming positively buoyant at the risk of blowing past mandatory deco stops, or attempting to ditch the belt when you need to get up, only to have it get hung up on your crotch strap. The former seems to be the greatest risk on any dive that requires decompression stops, especially when you consider that should the latter problem occur, you can still remedy it in a few seconds, you have the drysuit as backup, and even if that had failed, provided your rig was balanced in the first place a gentle kick is all that is required to halt your descent. I would only wear the belt on the outside if the ability to ditch the weight was the primary concern, such as diving in swift or aerated water, or any single tank, non deco diving. If you do use a weightbelt with a DIR harness and backplate, you may find that bullets or curved plates positioned at either side of the belt will prevent interference with the bottom of the plate.
As an aside, you may wonder about the extent of buoyancy loss in the event of a flooded drysuit: the answer to this depends on how much gas escapes from your suit. If you are using the suit correctly - that is to say adding only enough gas to take the squeeze off, then buoyancy loss is negligible. Keep in mind that a suit with a tear in most cases will still hold gas. The real issue here is not really one of buoyancy, but rather momentum. When your suit fills with water, you now need to kick hard enough to move all of this extra mass, and you are getting colder, which reduces the effectiveness of your deco. You are now not getting the full benefit of spending extra time at your stops. You best bet in this case is to get out of the water as soon as possible (reader may refer to techdiver archives on omitted decompression).
The idea of a balanced rig does not wholly apply to wetsuit diving, due to the compressibility and variable buoyancy of the wetsuit material. When wetsuit diving, the droppable component of your weight must compensate not only for the weight of consumed gas, but for the increased buoyancy of the suit at the surface. You must necessarily inflate the wing at depth to compensate for this. This is a good reason to only use aluminum tanks when wetsuit diving, to avoid the excess negative buoyancy of the steel tanks. Restricting the use of wetsuits to relatively shallow depths will minimize this problem.
Keep it simple - dive a balanced rig.
Dive safe.
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The ditchable weight component only needs to be equivalent to the weight of the gas in your tank(s), plus compensation for exposure suit compression if any. An extremely negative rig is is more of an issue for cold water divers, where the exposure suit and insulation act to make the diver extremely positive - the dive rig itself is extremely negative to counter this. My rig only just floats itself with the BC fully inflated. (There have been instances where is hasn't floated at all - I try to avoid that by moving some weight onto a belt, but this is mostly out of concern for accidentally losing the gear. My light canister usually serves as my ditchable weight.) The only time I need to take the gear off in the water is prior to boarding small vessels (RIB, etc.), and I can't have it sinking on me and disappearing.
There is no need for the diver and rig to be independently neutral, because there really is no justification for dismounting underwater (save for an absolute last ditch effort to free yourself from an entanglement in a position where your buddy cannot act to free you). If you can't fit through a particular passage, plan another dive using appropriate equipment (sidemount, etc.) Removing weight from your backplate, tanks, etc. and placing it on a belt is not going to accomplish anything except screw up your trim, and if you have any weight on a belt over and above that which is strictly necessary to be ditchable, when you lose it you are then not able to control or halt your ascent - extremely dangerous.
Your best bet is to weight yourself correctly, making only as much weight ditchable as you absolutely have to to ensure that you can still get up when you have to in a worst case scenario. The rest of the required weight should be distributed in accordance with correct trim and attitude control, and you will find that when you do so, it is much easier to avoid entanglement in the first place.
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Penetrating wrecks or cave diving requires the full doubles rig with isolator. I would have the canister light on a dive like that (dive with it almost always, anyway). If I did not have the light, I would wear my ditchable weight in the form of a belt, but in an overhead environment, or on any dive requiring decompression stops, would wear the belt beneath the crotch strap rather than over it. This does require a couple of extra seconds to ditch, but in the event of the buckle slipping, prevents getting pinned to the ceiling or blowing past mandatory decompression stops - the greater risk.
I wasn't thinking of failure there, but rather an intentional weight ditch. Imagine the worst case scenario: beginning of a dive (tank is full), deepest part of dive (exposure suit has compressed and lost buoyancy), and now something happens which requires bailing to the surface. Since you are correctly weighted to remain below the surface with only a couple hundred psi, the amount of ditchable weight you need must be equivalent to the weight of gas consumed (about 6 pounds for an AL80 filled with air), plus the buoyancy lost due to exposure suit compression (variable). If you ditch this in an emergency, you are now neutral or just slightly positive, and can ascend to the surface in a controlled manner. If, however, your belt consisted not only of the required ditchable weight, but also included some portion of the integral weight total, ditching it would result in excessive positive buoyancy, resulting in an uncontrolled ascent.
Make sense?
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There is no advantage to placing any weight on a belt, other than making it ditchable, which you obviously don't need for the majority of your required weight. Any belt is subject to failure, and you don't want the ability to lose anything other than that which is necessary.
Preferably, the weight should be integral to your rig in the backplate, tank(s), and lead added in that region, keeping it completely rigid with respect to the rig, and as close as possible to your center of mass, but distributed fore and aft, as opposed to being a point load at your hips. This gives you the best control over your in-water attitude.
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In large passages, you tend to keep to the ceiling anyway to reduce the average depth and hence, the decompression obligation. The problem here is that if you do lose your weight, you stand to get pinned to the ceiling by your buoyancy, which is not only tremendously inconvenient, but damages the cave.
Experienced wreck diver commenting: What are we talking about here? There is no reason to ever have to do a dismount underwater. Entanglements are best avoided preventatively (through clean gear rigging, proper wreck penetration technique, and exercising good judgement), and when they do occur, can usually be cleared by the diver (again, good technique, good gear rigging, having the correct tools, etc.), and when they can not, should be able to be cleared by the diver's buddy.
Do not do yourself the disservice of changing your gear and weighting scheme to accomodate the possibility of dismounting underwater. The chance that this will be your only survival option is miniscule in comparison to the risk you take by placing weight where it does more hurt than good, and where it can be lost accidentally. Dismounting is a dangerous practice which should only be performed as an absolute last resort, if at all. It introduces some risks (discussed in prior posts in this thread) that are not worth incurring, not to mention the fact that dismounting and remounting underwater provide an even greater opportunity to become entangled - only this time you may become entangled without your gear. Does that make any sense to you?
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The fixed component of your weight is best positioned close to your center of mass (i.e. the centerline of your torso), and distributed along the length of your back. In my case, I use steel cylinders on my back, a stainless backplate, and a lead weight which fits in the backplate channel, right against my back. I am not a fan of integrated weight pouches, both due to the possibility of ditching more weight than you need to be ditchable, and because most of these weight systems place these pouches low on the diver (forward), where they cause trim problems, and add bulk to an area that needs to be clear - especially true when diving a neoprene drysuit that already hinders your mobility. Ideally, this fixed weight component would be right around your spinal column - practically, of course, it needs to sit a bit higher, but in any case, should be as close as possible so that it doesn't create unfavourable leverage as you pitch and roll in the water. Alternative solutions include V weights (between doubles), filling a plastic backpack with lead, or lead rods on either side of a single cylinder, close to the diver's back. There are "integrated" weight BCs which provide pouches for weight which can sit high, near the diver's kidneys, which is better, but still not optimal since it is not distributed over a long enough length longitudinally, and also is not rigidly fixed. You also need to ensure that the fixed component is indeed fixed in place, and eliminate any possibility whatsoever of inadvertently losing this weight. Just in case you missed that, I'll type it again: ELIMINATE ANY POSSIBILITY WHATSOEVER OF LOSING THE NON-DITCHABLE COMPONENT OF YOUR WEIGHT. This would preclude using a weight belt for this, since weight belts can slide down your body, and the buckles can and do fail.
As for the ditchable component, this is usually small compared to the fixed component - certainly it will be in your case. As I mentioned earlier, my light canister is my ditchable weight, but otherwise I would again avoid the integrated/release systems and stick with a regular weight belt. When weighted correctly to start with, an intentional weight ditch is an all-or-nothing event, meaning that you need to drop all of that (pre-determined) weight to get yourself out of trouble. This should only require one action on the part of the diver (not a release from each side) for simplicity's sake. A weight belt is simple, universally identifiable (so a buddy or any other diver can immediately recognize and operate it if necessary), functions consistently, and has the advantage of being easily replaceable underwater, enabling you to actually repetitively practice weight ditching without having to screw around re-loading an integrated system. Again, you want to avoid having anything obstructing the front of the diver, or interfering with mobility or access to other equipment. When I use a weight belt, all of the weight on the belt is toward the back insofar as possible, keeping with the goal of maintaining that center of mass.
Clear as mud?
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An educated guess will come close, but the only way to nail this down accurately is to get in the water and test it. Here's what you do:
You need to do this in two steps. First, follow the weighting procedure in the balanced rig tip - with full tank(s), fully vent your BC and drysuit, and make sure you have only just enough weight to sink from the surface on an exhale. Now, drain your tank(s) down to 200 psi, and add weight until you are able to hover just beneath the surface with the almost empty tank(s). The 200 psi is a safety margin, low enough to ensure that you can maintain your stops under almost all circumstances, but when you are really completely out of gas, you have no choice but to surface. You have now determined the correct weight total.
Now, the unknown is how much weight needs to be integral, and how much needs to be ditchable. To determine this, keep a good portion of your weight total (determined above) in the form of a bunch of bullets (the smaller the increment, the more accurate this will be) threaded on a belt without stops, so you can remove them one at a time. At your deepest depth, with tank(s) full (note that you need to do this on the bottom, since you're going to be negative), empty your BC and drysuit completely. You're now as negative as you're ever going to get. Carefully, one by one, drop a bullet at a time until you are close to neutral (i.e. can rise or sink via breath control). Look at the pile of bullets on the bottom - that is how much ditchable weight you need in a worst case scenario. That is what needs to be on your weight belt, or in your integrated BC (if you must use one). The rest can be permanently attached in a position consistent with good trim and dynamics, as per my previous responses.
If you follow the procedure above, I think you will find that the required ditchable component is significantly less than you may think. Of course, diving a thick neoprene drysuit puts you in a disadvantage in this regard - as you indicated, you need additional weight to sink the suit, but now when you do ditch in a worst case scenario your buoyancy is going to increase as you ascend, affecting your ascent rate. This is probably the most important argument for diving a shell suit - over and above the extra weight/extra BC inflation/drag issues, since it could be dangerous. For this reason alone I could see splitting the ditchable component into smaller pieces, so that if you need to ditch shallow you don't drop it all; however, I would consider this to be an interim measure ONLY. As I indicated before, it is preferable to be able to weight ditch in a single operation, single handed, in a simple manner that can be trained to be a reflex action, and using a single weight belt as the ditchable component makes the most sense in that regard. At some point, I suggest you give serious consideration to replacing the neoprene suit with something constructed from a non-compressible material.
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The intention is simply to get yourself close enough to neutral that you can easily swim up the rig if you have to. A few pounds negative is no big deal to swim up. Remember - contrary to what your instructor may have told you, a weight belt ditch is not intended to propel you to the surface, but rather to ALLOW you to get to the surface when you have a buoyancy failure - under control. For recreational open water diving within NDLs, aim for becoming just perceptibly negative at the 130 limit (or whatever depth is your deepest planned dive). If you happen to ditch a bit deeper, a gentle kick is all that is required to initiate the ascent. As you get shallower, ditching will result in net positive buoyancy, but within the scope of recreational diving, this is not really a concern. Depending on how much suit compensation is required, you may wish to move the neutral depth up (so you are negative deeper than that) so that you don't ascent too fast as you approach the surface - just make sure that if you are still negative following a weight ditch, the amount is negligible, and easy to swim against.
Diving a shell suit eliminates this problem and allows you to be balanced at all times, regardless of depth.
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