The Balanced Rig?

[HenrikBP]

Warning: rank rookie question ...

Executive overview: if I dive HP100 doubles with a 3 mill wetsuit and no ditchable weight, would a large SMB or a lift bag be considered acceptable redundant buoyancy?

The long version:
In Fundies we were taught the concept of the "balanced rig", and as I understand it, the gear set-up/weighting needs to be so that in case of complete loss of buoyancy (wing and maybe drysuit) I will never need to swim up more than 10 lbs. A part of the calculations in the Fundies material involved having ditchable weight (if necessary) for just such a situation.

With that in mind I'm trying to decide on what doubles to use on a dive trip to NC in May. My options are HP100s and AL80s. For gas planning purposes I would prefer to dive the HP100s.

I could dive my drysuit for redundant buoyancy, but would rather dive my 3 mill wetsuit with a 3 mill hooded vest.

If I dive wet with the HP100 doubles I will need no additional weight => no ditchable weight and I will be ~15 lbs negatively buoyant at the beginning of the dive. I could buy an aluminum plate and have 4 lbs on the belt - but don't really want to spend the $$.

If I dive the AL80s I will need an additional 12 lbs on my rig, some of which would be on the belt to make the rig "balanced". But then I run into an issue with enough gas for 2 dives.

So ... I've heard as well as read here, that when diving dry the drysuit is acceptable redundant buoyancy. But how about a large SMB or a lift bag?

Thanks,

Henrik

 

[Indian Valley Scuba]

So many questions! Whenever you get the official answer call me, we'll take care of you!

 

[Brian Gilpin]

Henrik

I dont believe hp 100s with a 3 mil can be cosidered a balance rig and a lift bag would be a problematic solution. Aluminum 80s would probably be the preferred solution with a stage if necessary for gas planning.

If the cost of a used aluminum backplate on Ebay is too much then reconsider where your diving is headed. I assume that since you are going to NC that you will be wreck diving. This type of diving is equipment and training intensive and in my opinion requires the right setup.

I dove NC in May a couple of seasons ago. You may be able to use your drysuit depending on depth and duration. I remember being a little chilled in a 3 mil.
Good luck and have some great dives.

Brian Gilpin

 

[elan]

If I dive wet with the HP100 doubles I will need no additional weight => no ditchable weight and I will be ~15 lbs negatively buoyant at the beginning of the dive. I could buy an aluminum plate and have 4 lbs on the belt - but don't really want to spend the $$....

I think you will be ~-15 lb negative even if your rig is perfectly balanced that would be the weight of the gas, in a 3 mill you would be more like - 30lb negative at the beginning of the dive. AL plate itself will not help you IMHO. If your HP100 are Worthingtons then the rig will be quite heavy, my suit is approx 20-22 lb positive and I do not need any weight with my suit when I dive double HP100, that means the rig is at least 20-22 lb negative not counting in the air, with air it's more like - 35-37 lb..

I think in a 3 mill you would not need any weight with the Al80. if you factor in the weight of the regs, plate (even AL) manifold, bands, light and inflation system if any you will still be negative or close to neutral with the AL80 empty.

 

[Indian Valley Scuba]

Ouch!

 

[ScubaInChicago]

I have to agree with elan on that one....I need 4lbs extra with my PST 100's wearing a drysuit w/ 300 gram Action Wear undergarments @ 400-500 Psi.....But if it makes you feel better, I saw a guy who claimed to be "DIR" wearing double HP130's, split fins, and a 3mm wetsuit

Maybe StreetDoctor can post a pic and spin off a thread

 

[DA Aquamaster]

I think you will be ~-15 lb negative even if your rig is perfectly balanced that would be the weight of the gas, in a 3 mill you would be more like - 30lb negative at the beginning of the dive. AL plate itself will not help you IMHO. If your HP100 are Worthingtons then the rig will be quite heavy, my suit is approx 20-22 lb positive and I do not need any weight with my suit when I dive double HP100, that means the rig is at least 20-22 lb negative not counting in the air, with air it's more like - 35-37 lb..

I think in a 3 mill you would not need any weight with the Al80. if you factor in the weight of the regs, plate (even AL) manifold, bands, light and inflation system if any you will still be negative or close to neutral with the AL80 empty.

Let's check the math as it is not quite that "heavy" - but not far off either.

Worthington X7-100s are 10 pounds negative each when full with valves - 20 pounds total. A set of higland mills 7.25" bands are 2.5 pounds and the average isolator manifold will add about 1.5 pounds more over and above the weight of 2 valves. Add that to a 2 pound plate and you have something slightly less than 26 pounds of negative bouyancy. A bit less because the bands, isolator and plate also displace some water - maybe 2 pounds worth. My back gas regs weigh 6.4 pounds but easily displace another quart of water, so figure -4 pounds net bouyancy. Call it 28 pounds negative with full tanks for the whole configuration.

Then take away 15 pounds of gas and you have about 13 pounds of negative bouyancy if you had near empty tanks at the end of the dive.

The only remaining factor is how much buoyany you will have in a 3mm suit at a 10 or 20 ft stop. I am guessing that will be around 5 pounds for the average sized diver so no extra weight will be needed. On the other hand, you could have only afforded to be 7-8 pounds less negative and still hold a 10 ft deco stop if the tanks were close to empty.

At depth with nearly full tanks and a compressed suit, you would have to swim up maybe 25-26 pounds, and that is a lot of weight to swim to the surface. Even if you had the 7-8 pounds less negative buoyancy you needed to be balanced, that would still be be 17-18 pounds you'd have to swim up, suggesting that 200 cu ft of gas is enough swing weight to make a balanced rig problematic at best.

AL 80s change the picture a bit. They are -1.4 pounds negative with valve when full, or in other words, 2.8 pounds negative when full rather than the 20 pounds negative you have with X7-100s. So in the same configuration as above, you would only be about 11 pounds negative when full. When empty AL 80s are 4.4 pounds positive, a total of 8.8 pounds for 2 of them, leaving you about 1 pound positive at your deco stop before you consider the buoyancy of the suit. I see about 4-5 pounds of additional weight being needed in that scenario with a 3mm suit.

In the real world, you will also have along a can light, maybe a deco bottle, a couple reels etc, meaning no added lead is really needed.

Even with the additional weight, you would only be 12-14 pounds negative at depth early in the dive and that is something you could swim up, worst case, after dropping reels, a can light, deco bottle, etc.

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However, in either case whether it is 25-26 pounds negative with HP 100s or half that much weight to swim up with AL 80s, where exactly are your team mates? Add even a single team member to this scenario and getting to the surface is a non event (assuming they are not devotees of the smallest possible wing size philosophy, which in this case means way too small).

That's not to say that there are not advantages to configuring yourself that allows a high degree of self sufficiency (and with large steel tanks redundancy in the form of a dry suit makes a great deal of sense) but you do want to look at the whole picture.

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On many NC boats, 100 cu ft tanks are the norm as wrecks tend to be in the 80-120 ft range even on recreational trips. So AL 80s would leave you undergunned if you planned to use one set for both dives and would leave you looking at shorter dives than most of the other divers. Some boats will frown on the space needed for two sets of doubles on a "recreational" trip. A possible option here would be to bring along an AL 80 stage to burn on the first dive, leaving more than enough backgas for the second dive and you'll get less objection to a single set of doubles and a stage than you will get with two sets of doubles.

In open water off NC, an SMB makes a viable way to "swim" up some additional weight - but there can be some interesting currents and if you come up some where other than the ascent line and are not tied into the wreck, you can get blown off the wreck and may drift for an hour or so before they can come get you.

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Also, while the water in the gulf stream will run 70-73 degrees or so, on a windy day where the boat may make an inshore dive on the Indra, etc, you will often encounter much colder (and murkier) water where a 3mm will leave you freezing in 60-65 degree water. So at a minimum be sure to bring gloves, a hood and a 5mm chicken vest.

 

[Colliam7]

... if I dive HP100 doubles with a 3 mill wetsuit and no ditchable weight, would a large SMB or a lift bag be considered acceptable redundant buoyancy? ... With that in mind I'm trying to decide on what doubles to use on a dive trip to NC in May. My options are HP100s and AL80s. For gas planning purposes I would prefer to dive the HP100s. I could dive my drysuit for redundant buoyancy, but would rather dive my 3 mill wetsuit with a 3 mill hooded vest.

Interesting question for the DIR forum - about the acceptability of a lift bag as redundant buoyancy. I will comment on a couple of aspects of the scenario, though, rather than the potential DIR answer to that particular question.
1. Off the NC coast, for MOST of the wrecks, I prefer something larger than an 80cf for a single tank dive. The good wrecks are in the 80-120 ft depth range, and with an 80 I run out of gas before I run out of NDL. Consequently, unless I have the rare luxury of being on a boat that allows me to take TWO sets of doubles, I don't find my double 80s to be very useful for a 2 dive charter. DA's suggestion of taking an AL80 stage is a good one for the double 80s. I suggest you focus on the 100s for practicality if you would find that cumbersome. Even then, I would take an AL40 stage.
2. Off the NC coast, you can dive a 3mm wetsuit and hooded vest in May - unless you are doing wrecks like the U-85 or the Jackson, both of which are in much colder water (Labrador current). But, you can easily, and comfortably, dive a drysuit as well. Yes, some feel it is a bit more complicated on a boat dive, but I dive dry off the coast most of the time now, simply because I am more comfortable. I am seldom too warm.
Personally, I don't consider a lift bag / SMB to be acceptable redundant buoyancy, although it definitely works, and I would use it without hesitation if my wing and drysuit, or both bladders of my wing, failed. When I dive wet with double steels off the NC coast, I use an AL BP and a dual bladder wing.

 

[dumpsterDiver]

I thought double steel tanks, and a thin wetsuit was not DIR for ocean diving? But hey what do I know, maybe the rules have changed?

If it were not a DIR question, I would suggest the use of two larger, single steel tanks and do each dive with a single full tank (and pony) rather than drag around half empty doubles on a second dive, especailly if the depth is only like 120 feet. At least that is what I would do.

 

[PfcAJ]

Using a lift bag as "redundant buoyancy" is not DIR. The DIR answer is to dive al80 doubles and a wetsuit, or steel doubles and a drysuit.

If you need additional gas, take an al80 stage. You get almost the same amount of total gas as a set of double 104s to 3600psi.