"Balanced Rig"

[lamont]

i think the quote from george answered it:

- neutral in a gas-loss emergency
- not overweighted at the beginning of the dive

there's a few other corrollaries, or related items, as well:

- don't use heavy/negative double steels with a wetsuit in open water
- use a drysuit for redundant buoyancy
- use a drysuit to reduce exposure compression at depth vs. wetsuit
- use helium to reduce buoyancy swings
- use tanks that are closer to neutral when empty
- size wing to float your rig at the surface
- size wing for at least neutral buoyancy at depth with full tanks and full wetsuit compression
- keep wing size minimal to reduce runaway buoyancy with oversize wings
- position your weighting appropriately to maintain trim

 

[b1gcountry]

First, the rig must be negative enough that at the end of the dive you can maintain buoyancy with little or no air in the wing...
Second, the rig must not be so negative that you can't swim it up at the beginning of the dive if you were to suffer a wing failure that made it useless.

I would only point out that you can control one of these, but the other will be based on your tank size plus your buoyancy loss at depth...(I would prefer to stay clear of discussing ditchable weight for the time being.)

 

[JeffG]

I would only point out that you can control one of these, but the other will be based on your tank size plus your buoyancy loss at depth...(I would prefer to stay clear of discussing ditchable weight for the time being.)

You control it all.

You pick what wing to use.
You pick what tank to use.
You pick what gas to use.
You pick what exposure suit to use.
You pick what amount of lead to use.

etc etc

and you can't stay clear of discussing ditchable weight if you want to talk about balanced rigs.

Some rigs need them to be balanced.

 

[rainman_02]

I would only point out that you can control one of these, but the other will be based on your tank size plus your buoyancy loss at depth...(I would prefer to stay clear of discussing ditchable weight for the time being.)

What buoyancy loss at depth? Oh, that's right...we dive drysuits with heavy steel tanks.

 

[b1gcountry]

You control it all.

You pick what wing to use.
You pick what tank to use.
You pick what gas to use.
You pick what exposure suit to use.
You pick what amount of lead to use.

I agree, you just don't have direct and independent control over most of the above like you do over the amount of lead you wear. The tank size and gas should mostly be determined by the diving profile, and conditions will definitely limit your exposure suit choice. I would also argue that the wing size is mostly a result of your other choices more than a driving choice in itself.

and you can't stay clear of discussing ditchable weight if you want to talk about balanced rigs.

Some rigs need them to be balanced.

I'm curious what you mean by this. Please explain.

Tom

 

[Warren_L]

I agree, you just don't have direct and independent control over most of the above like you do over the amount of lead you wear. The tank size and gas should mostly be determined by the diving profile, and conditions will definitely limit your exposure suit choice. I would also argue that the wing size is mostly a result of your other choices more than a driving choice in itself.



I'm curious what you mean by this. Please explain.

Tom

Ultimately, you do control where you dive and following the logic you've laid out, everything else falls into place, so in the end, it is ALL under your control.

As for the ditchable weight, you can't not consider it's application if it is required. Sometimes having ditchable weight is a requirement.

 

[DIR-Atlanta]

and you can't stay clear of discussing ditchable weight if you want to talk about balanced rigs. Some rigs need them to be balanced.

I'm curious what you mean by this. Please explain.

When diving with a wetsuit, you must adjust your weighting to deal with the following extremes:

• You want to be NEGATIVE enough that at the end of the dive with (nearly) empty tanks, you can comfortably hold a stop at 10 feet
• You want to be POSITIVE enough that at the beginning of the dive with full tanks and no air in the wing, you can swim up from the planned maximum depth of your dive.

Unfortunately, it is sometimes the case that the amount of weight that is required to address the first requirement, is too much weight to allow for the second requirement, due to loss of buoyancy from wetsuit compression. In those cases, you need some form of removable ballast. Note that "ballast" is different from "weight", and may consist of:

• Actual lead weights on a weightbelt, or
• Other items of gear that can be removed to decrease negative buoyancy (stage or deco bottle, canister lights, etc)

Note also that these items are "removable", but should not necessarily be considered "ditchable". For example, you will probably want your deco bottle back at some point, when you get ready to do your decompression. You will probably also want that weightbelt back when you get to shallow water, to enable you to maintain your stop depths. A can light is important for signaling (and is very expensive to boot ), so you probably don't want to ditch that either.

So now you have a bunch of stuff that is making you excessively negative, but you can't really afford to "ditch" and safely complete the dive. The way to deal with that is to have your buddy(ies) help you. For example, they can temporarily hold onto a weightbelt or a deco bottle, until you get shallow enough (and the wetsuit expands enough) that you can get your buoyancy control back. At that point you can safely take the items back and finish the dive.

So the way to balance a rig is to test it first in shallow water with an empty wing and (nearly) empty tanks (including deco bottles), but all other required equipment in place (can lights, etc). Adjust your weight so that you can comfortable hold a stop at 10 feet.

Then test the same rig at your maximum planned depth, with an empty wing and full tanks, and see if you can comfortably swim it up. For safety purposes, you should do this test with a buddy (or two!) nearby, and over a hard bottom that does not exceed the MOD of your mix. If you can't swim up, then remove negative ballast until you can. The "ditchable weight" is the difference between the amount of weight that is required in the first (shallow) case, and the amount of weight that is required in the second (deep) case. Note that I dislike the term "ditchable weight" as it implies something that may not be true - the items that must be removed to achieve neutral buoyancy may not be truly "ditchable", and they may not necessarily be "weights". I prefer the term "removable ballast", but that is probably just an issue of semantics.

That is the concept of a balanced rig as I understand it. Please feel free to add any additional comments or clarifications as you see fit.

 

[JeffG]

That is the concept of a balanced rig as I understand it. Please feel free to add any additional comments or clarifications as you see fit.

Here some addition comments, a practical application of a balanced rig.

When talking about balanced rigs, some people quote the often misunderstood one liner "Don't dive steel with wetsuits".

Almost always they blame the steel tank. Actually, its usually the wetsuit to blame. Not the small thin 3mm ones, but the thick 7mm (or the 2 piece farmer john/jacket 7mm). It takes a lot to sink the suckers, but at depth they compress and give no buoyancy to the system. This leads to an unbalanced rig, where you can't swim it up at depth, but if you drop weight to swim it up, you might not be able to hold stops near the surface.

So the line shouldn't be "Don't dive steel with wetsuits"...it should be "Thick wetsuits aren't DIR. They lead to unbalanced rigs, and they don't provide any thermal protection at depth.

I agree, you just don't have direct and independent control over most of the above like you do over the amount of lead you wear. The tank size and gas should mostly be determined by the diving profile, and conditions will definitely limit your exposure suit choice. I would also argue that the wing size is mostly a result of your other choices more than a driving choice in itself.

You have direct control over everything. You buy it don't you? You pick an exposure suit, not just for conditions, but for its characteristics. Same reason why certain Al tanks are used for deco/stage tanks. (for their characteristics, not by how much gas they hold) ie take the above problem and replace thick wetsuit with a drysuit and most of the issues disappear. Just change your undies type, to compensate to temperature.

 

[DIR-Atlanta]

So the line shouldn't be "Don't dive steel with wetsuits"...it should be "Thick wetsuits aren't DIR. They lead to unbalanced rigs, and they don't provide any thermal protection at depth.

That's an important point, and one that comes up a lot when you are talking to people whose only exposure to DIR principles is via "Internet soundbites". The recommendation of "don't dive steel with wetsuits" was actually made in reference to steel doubles, not steel tanks in general. Steel doubles in a wetsuit can cause all kinds of problems with excessive weight, and there are at least a couple of fatalities that I am aware of that were attributed directly to that particular configuration of gear.

It is perfectly acceptable to dive steel singles in a wetsuit, as long as the principle of the "balanced rig" is applied. For example, I dive a steel single on ocean dives all the time, in a 3mm jumpsuit and no weightbelt. With an aluminum backplate and unweighted STA, that particular setup is just about perfect for me, and does not cause any weighting problems on recreational profiles down to 100 feet or so.

You pick an exposure suit, not just for conditions, but for its characteristics. Same reason why certain Al tanks are used for deco/stage tanks. (for their characteristics, not by how much gas they hold)

Another important consideration is choice of back gas. For deeper dives, you should be using some helium in the mixture, and that will also help mitigate the negative buoyancy of the tanks at the beginning of the dive. For example, most Nitrox mixtures (including Air) weigh about 0.08 lbs/cf, while a light trimix like 30/30 or 21/35 has a density of only 0.06 lbs/cf. So the gas weight of a light Trimix fill will be about 25% less than the gas weight of a Nitrox fill - for a single 80 cf tank, that translates to a difference of about 1.5 lbs (or 3 lbs for doubles).

The buoyancy of an empty tank is always going to be roughly consistent, independent of whatever kind of gas you had in it. However, the negative buoyancy of a tankful of Trimix will be less than a tankful of Nitrox, by the amount of weight calculated above. That means that if you balance yourself for neutral buoyancy in shallow water with a set of double 80s, then you will be about 3 lbs less negative with the tanks full of Trimix than full of Nitrox. I know that's not a lot, but it might make the difference between being able to swim the rig up, versus not.

I think one of the problems that people have with a lot of this stuff as that they don't really see things in terms of the "big picture". There is definitely a "method to the madness", and the apparent inflexibility of the system is actually one of its greatest strengths. Everything about DIR gear, procedures, and philosophy is tied together, and the whole is definitely the sum of the parts - change one thing, and it can throw everything else out of whack. That's why it's so frustrating to see a statement like "Don't dive steel with wetsuits" taken out of context (as it so frequently is). As JeffG pointed out, it doesn't really convey the true essence of the concept, and often leads to misinterpretation.

 

[TSandM]

A beautiful discussion, Jeff and DIR-Atlanta!