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  1. #1
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    Weighting: In Search of a Balanced Rig..

    What is a Balanced Rig?

    In the DIR Equipment configuration, the common goal is to have a Balanced Rig. I guess the first thing we have to understand is what is meant by Balanced? There are many definitions in circulation, but going back to first principles the definitions are laid out in "Doing it Right"1

    "The goal of any SCUBA configuration is to create a system that, when empty, is as near to neutral as possible and that, when completely full, is not excessively heavy."

    Now this sounds sensible, but how do we measure too heavy?

    "The bottom line here, however, is that divers should be certain that, without any air in their buoyancy compensators, they are capable of swimming against the weight of their configuration with full tanks and all weight in place."

    And how do we measure too light?

    "The ideal configuration for a diver is one that, while being as light as possible, allows him/her to remain neutral at 10 (3m) with a nearly empty set of tanks (to allow for decompression/safety stops)."


    OK! So I am looking for a setup on my equipment that I can hold a stop at 3M with less then 30 Bar (500PSI) in the cylinder at the end of the dive, but that when I have 240 Bar (3500 PSI) I can swim up to the surface. Simple enough, right?

    Why is a Balanced Rig desirable?


    So we know what the idea of balance is. Why is this a good idea? Can't I just dive with extra weight just in case? I feel more comfortable with an extra 3Kg to help me descend. Let's look at this a bit more.

    DIR Equipment Philosophy


    The DIR equipment configuration philosophy is a minimalist one. Take the minimum you need with adequate back-up. For weighting, with the minimum amount of weight, you can dive with the minimum amount of gas in your BC. This has benefits in streamlining, buoyancy control near the surface (where pressure change is fast and gas expansion and buoyancy change rapidly). The redundant back-up on weighting is the fact that we choose to weight ourselves to be neutral with only 30 Bar (500 PSI) in the cylinders. This gives us the additional weight of the gas in the cylinder between our minimum gas calculation and the 30 Bar. Usually a Kilo or two (2-4 lbs), it depends on the cylinder sizes and density of the gas mixture.

    Diving a balanced rig - the benefits


    Control during and at the end of the dive

    When diving a properly weighted rig the benefits of streamlining and minimalist equipment are clear throughout the dive. You have less drag, and are more comfortable in the water. The benefits are especially visible when you try to do skills in shallow water, or simulate decompression stops. This is because, at the end of the dive your Wing will be empty or very nearly empty. This makes the stops much easier to hold. If you add weight - even 2 Kg (4 lbs) there will be an additional 2 Litres of gas in the wing. These expand rapidly as you ascend, making control harder to achieve. If we step back and do the maths - 2 Litres expand to 4 Litres between 10M (33 ft) and the Surface. This can be enough to trip a runaway ascent if not correctly managed. Less is definitely more.

    Safety at the beginning of the dive


    The second area of benefit is in safety at the beginning of the dive. We've seen the benefits of correct weight at the end of the dive, but there is the scenario of the beginning of the dive. Imagine:

    You jump off the back of the boat at the start of the dive. Your wing isn't connected properly (missed in the pre-dive check) and now you are heavy in the water, with no BC to counteract the weight on your belt, and the weight of the gas in the cylinders. You start to sink! Better get swimming!

    Traditionally the response is to dump weight to achieve positive buoyancy. This can be hard with a twinset (double cylinders). So a thoughtful approach is necessary to allow for safety at this stage. The DIR approach says you should be able to swim up against the weight, and carry enough ditchable weight to make this possible.

    Practical Mechanics of Balancing the Rig


    A practical approach - small adjustments

    Getting your weighting right is an ongoing process. Many divers start with a lot of weight because they are tense and their breathing is not relaxed. To counteract this situation they end up adding a 2-4 Kg (4-9 lbs) of weight. If this is compounded, by having air in the wing/BC or too much in the drysuit due to inexperience, then this additional weight can go up even more. As a dive guide and instructor, I have seen divers in 3MM wetsuits with more than 9Kg (20Lbs) of weight!

    Over time and with experience, they relax, improve their control of the BC and everything becomes easier. It is amazing the number of divers that do not adjust their weighting - because that is the "right amount" - and end up diving over-weighted for the rest of their career.

    So the DIR diver attempts to adjust the weight carried over time to allow for changes in physiology, environment, equipment and comfort. These changes are incremental, small adjustments are important.

    Basic Calculations - Sources of Buoyancy

    All equipment placed in water is to some extent buoyant. It's just that the mass of a lead weight more than offsets the buoyancy effect of the water. So looking at it from the standpoint of sources of positive buoyancy we have a few types of item to think about:

    Positively Buoyant Items

    Firstly (for most of us at least) the human body can be pretty positively buoyant, especially in seawater. Hold full lungs and you will float. For many people this is the case when you breath out too. Obviously as DIR divers we're all models of health and fitness with a BMI of 23.. yeah right! We're all shapes and sizes and most of us are floaty.

    The second major source of buoyancy is miscellaneous equipment. Light fins etc. These do not compress in the water column as you descend and are buoyant.

    A third source is thermal protection. this breaks into two categories - drysuits and wetsuits. Starting with a wetsuit. As you descend the material that the wetsuit is made of compresses as the water presses on the surface. This changes the thermal protection as you descend (it gets thinner) and also changes the buoyancy characteristics (it gets thinner so displaces less water and is less buoyant). Therefore as you descend you get more negatively buoyant. For a drysuit, when managed correctly a drysuit has approximately constant buoyancy. You add a little gas on descent and dump it on ascent so that it has the same volume and buoyancy. This is tricky and requires practice - it's a whole different topic and I'll avoid it for now with the assumption we know what we're doing! Let's just say it is positively buoyant!

    Negatively Buoyant Items


    I'm sure you can think of a bunch of things in your configuration that are negatively buoyant. For a start - regulators, canister light, jetfins, clips, boltsnaps... etc. These are added to by the weights that you carry. Be it ditchable weight (a weight belt) or fixed weight that is mounted on the configuration and cannot be dropped - like a V weight - it all counts.

    Buoyancy that changes during the dive


    Apart from compressing wetsuits, the most obvious changeable buoyancy item is the cylinder. As you breath the gas out of it, it gets lighter and the buoyancy changes. For a set of double 12L cylinders (approx HP95s) that change can be almost 6Kg (13 lbs) across the emptying of the cylinders. This needs to be managed to hold the stops at the end of the dive.

    The Wetsuit vs Drysuit decision


    There have been huge arguments over the years about wetsuits, drysuits and steel or aluminium cylinders, double or single wings etc. If you think through the consequences of the changes in buoyancy caused by a wetsuit, you will get to the decision on what to wear.. thick wetsuit or a drysuit?

    Cold Water


    I am a notorious cold-water-wimp. However, many people are able to dive in the winter in the UK in a wetsuit. I admire their fortitude! Even a 7mm Wetsuit would leave me shivering! However, when you look at the compression of that 7MM wetsuit on the descent to 21M (70 ft) you will see a buoyancy change of as much as 5 or 6 Kg (up to 14 lbs) possibly even more for a brand new suit. This makes the swim up at the beginning of the dive nearly impossible without being able to ditch weight. It also makes holding stops nearly impossible if you have ditched the weight as the buoyancy returns as you get nearer the surface.

    Therefore for cold water dives, a drysuit is definitely preferable.

    Warm Water


    In warm water (I was lucky enough to dive for many years in S.E. Asia with water temperature commonly around 28 C - 82F) a 3MM wetsuit is common. Therefore the compression is far less noticeable. As the buoyancy characteristics of aluminium and steel cylinders are different, it is here that the debate becomes interesting. The received wisdom is that Aluminium cylinders are better, why is this?

    Essentially at the end of a dive, Aluminium cylinders tend to be positively buoyant. Therefore you tend to carry a little weight to offset this tendency. If you had steel cylinders that are negative at the end of the dive you would not have the weights.

    If you turn this around to the start of the dive. You will be more negatively buoyant in steel or aluminium cylinders as they are full of gas. You will probably be just more negatively buoyant in steel cylinders, but crucially without the ditchable weight on your weightbelt. This will make the swim up situation a lot harder, and is the reason aluminium cylinders are preferred.

    That said - if you can still swim them up you can use steel cylinders.. it will still be balanced.

    Steps in Reaching a Balanced Rig

    Now, in practice how can we make this work? It's actually quite easy if you think step-by-step.

    Get your total weight right


    The first task is to get the total weight correct. This is done with a weight check. Get in the water with a set of cylinders with 30 Bar (500 PSI) or less in them and try to hold a stop at 3M (10 ft). Don't worry about trim or position in the water at this point. You should just be able to hang there! Add or remove weight until this is comfortable. As a technique I recommend using a weight belt to make adding or removing weight easy while getting this right. Your buddy can carry the extra weights.

    Figure out the total additional weight needed. For an example, let's say 9 kg (20 lbs) is the amount needed.

    Try to Swim it up


    Get a full set of cylinders, add your weight and try to swim it up from 21M (70 ft). You should try to hold stops on the way up, as if you were ascending from a dive. Let's say 1 minute at 12M (40 ft) 1 minute at 9M (30 ft) 1 minute at 6M (20ft) and 1 Minute at 3M (10ft). Because you have full cylinders you will be swimming up all the time, the gas is weighing you down.

    If you can do this reasonably comfortably you are ready to move on to the next step.
    If this is very difficult, or you cannot do it at all, you need to be able to ditch some weight. Hand off a few kilos from your weightbelt to your buddy and try again. This should be much easier.

    The amount ditched could be as much as the buoyancy change in the cylinders over the dive.. but is probably a couple of Kg (2-4 lbs) less. Make note of the amount of weight you needed to drop to be able to swim up.

    Make the Trim and other adjustments


    The final steps are the fine adjustments. Now that you know how much weight you need to ditch to be able to swim up to the surface without the BC at the beginning of the dive you can make the trim adjustments and decide how to place the weight in your configuration.

    The important thing here is that if you needed to hand off any weight to make the swim up possible, then you must keep this as ditchable weight! Some people are happy to say they will drop the canister light and other pieces of negatively buoyant equipment. If you're like me.. you'll want some cheap ditchable weight like a weightbelt! The rest of the weight is best located in fixed weights, spread around the equipment to help you get your trim right. Common ways of doing this are V weights - between the cylinders or a double tank setup, P weights in the backplate or "tail" weights to help correct trim.

    That's it... that simple! Just remember that it will vary from time to time, so you should recheck periodically, or when you are making an environmental or equipment change.

    1 Doing It Right: The Fundamentals of Better Diving by Jarrod Jablonski. ISBN 0-9713267-0-3 :
    Last edited by keri.lewis; February 15th, 2009 at 09:28 AM.
    Even a "bad" day diving beats a day in the office...

    Keri

    The more I learn.. the more ignorant I realise I am..

    "When it comes to diving... you're the anti-christ!" - James Sanderson

 

 

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