Back-up Buoyancy by Thomas Knedlik

Thomas is the Director of Training for PADI Asia Pacific as well as a PADI Exmainer, Course Director and TecRec IT. In this article he answers a FAQ by explaining PADI’s philosophy on backing up buoyancy control devices during open circuit TecRec training.

Many tec diving risks either don’t exist in recreational diving, or are more severe than in recreational diving. These risks include drowning due to failed BCD and back up buoyancy control while diving heavily weighted with equipment. It is also possible to drown in heavy gear due to entering the water with all your cylinder valves closed (due to an improper predive check) and sink, unable to inflate your BCD or breathe.

Tc40_0709_432

The philosophy and requirement in regards to backup buoyancy control is that the student must have reliable means for controlling buoyancy and maintaining decompression stops in mid-water with a failed primary BCD. During the PADI TecRec courses these skills are also put into practice. Whether you need a double bladder BCD (sometimes called “double wings”) depends on the dive requirements. In all cases, you should have back up buoyancy. If, for example, dropping your weights in the event of BCD failure would still leave you substantially negative, then a double bladder BCD might be the best way to go. But, you still want the simplest rig that does the job. Typically, if you’re in a dry suit with lighter cylinders, that may provide adequate back up buoyancy control and a single bladder can be appropriate. The heaviest cylinders can weigh too much to use a dry suit reliably for back up buoyancy control, or you may be diving in a wet suit. In those cases, the double bladder BCD provides the back-up you need. If you dive in several environments, you may find you need both single and double bladder BCDs, using whichever one fits the circumstances.

In cases where the student is carrying a relatively small quantity of overall weight (e.g., a single cylinder only) one source of buoyancy control may be acceptable at the instructor’s discretion, provided that there is a reliable alternative method for maintaining decompression stops, such as ascending along a mooring line or decompressing on the bottom if topography allows. However, a lift bag or DSMB is not considered a reliable method of backup buoyancy control.

There are several manufactures who are offering dual bladder BCD’s including the Hollis SMS100D, DiveRite Horseshoe dual bladder (BC2074-DUAL), Dive Rite Dual Nomad Wing,  OMS Tesseract, ScubaPro X-Tek Horseshoe Twin Wing, Apeks WTX6R Double Comfort, EZDIVE Tech Scuba Diving Wing BCD or IST JT65H 65Lbf Double Bladder Twin-Tank Bcd.

15 replies »

  1. Hello.
    I can’t understand how it is possible that a tech diver, do technical diving being overwheighted or with all valves closed. A technical diver, in my understanding, should be well trained and prepared, so such things should never happen. It can happen to untrained but certified OW, but not tech diver. Otherwise that tech diver should take the courses from the scratch ( OW) because he has only a plastic card, but nothing more, in order to be called technical diver.
    But I have one question ( I have more than one, but I resume to one ): why lift bag or DSMB is not reliable source of buoyancy ?
    I mean, a suitable lift bag it is reliable to lift up to the surface 50 kg of something from the bottom of the sea, but is not reliable to provide few kilograms of positive buoyancy for the diver ( if the diver is perfectly wheighted as is supposed to be an advanced divers, like tech divers ) ?

    Thank you.

    • Sadly Dragos, fatal incidents have happened when divers did not follow their training.
      A lift bag or DSMB may have sufficient lift, however, redundant buoyancy needs to be immediately available (imagine a shoulder dump failure during a descent over a drop off – I’ve seen it happen) and controllable. Even if your SMB/lift bag is deployed the effort of hanging onto it, while substantially negatively buoyant, throughout a long deco gives an unacceptable stress and risk.

  2. What about this philosophy ? :
    “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.

    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. This would allow them to verify that they are able to manage their SCUBA configuration in the event of a buoyancy failure.” – quote from “Doing It Right:
    The Fundamentals of Better Diving” by Jarrod Jablonski.

    • I agree that is the ideal. PADI is less prescriptive than GUE regarding equipment and try to allow divers as much freedom of choice as possible. The reality is that some divers, in some cylinder configurations, in some suits, will be significantly negatively buoyant. In this case a double bladder wing is needed if there is a primary wing failure.
      This is not the only solution – as Thomas’s article explains.
      Cheers,

      Vikki

      • Hi Vikki,

        Thanks for your clarification. I’m not sure I’m reading it completely right perhaps. Is it ever permissible, to dive in a single bladder wing with aluminum doubles and wetsuit (shorty, 3mm), if the rig is neutrally weighted? Personally I don’t even have to “swim” my aluminum doubles up, if I completely empty my wing – I can just breathe it up. This is how I’d like to weight students too. Is your own lungs acceptable alternate bouyancy? If a ditchable weight of a kilo or two?

      • Hi Soren- It depends on the course and equipment configuration. Here are the standards, so you can see where there is leeway:

        Backup buoyancy control – the student must have a reliable means for controlling buoyancy and maintaining decompression stops in midwater with a failed primary BCD. This is usually accomplished with a backup BCD (double wings) or, when using light weight cylinders, the use of a dry suit is permitted.
        Note that in cases where the student is carrying a relatively small quantity of overall weight (e.g., a single cylinder only) one source of buoyancy control may be acceptable at the instructor’s discretion, provided that there is a reliable alternative method for maintaining decompression stops, such as ascending along a mooring line or decompressing on the bottom if topography allows.
        Note: A lift bag/DSMB is not considered a reliable method of backup buoyancy
        Hope that helps, Vikki
        control.

      • Hi Vikki,

        Thanks for your kind and prompt reply. Locally here in the Philippines, (where aluminum twins or aluminum sidemount, + rashguards or short wetsuits are the norm) there is quite a lot of debate amongst tec instructors on what the standards you quote ACTUALLY means. Some take it to mean, that without a drysuit you MUST dive with a double bladder wing, even if maintaining buoyancy control on deco stops with a complete rupture of BCD is totally possible with correct weighting. No reason offered, other than that is how PADI standard are.

        Others argue that the standards only require a “reliable alternative method,” and that diving a neutral rig is one such method.

        What’s your take?

    • “capable of swimming against the weight of their configuration with full tanks and all weight in place”
      Not something I want to do if I have to do long decompression. Just use double bladders.

  3. Hello Thomas and Vikki,

    Nice article, in fact there was a little controversy in my country about this.

    What I don’t have clear (maybe because english is not my primarily language) is this part: If you dive in several environments, you may find you need both single and double bladder BCDs, using whichever one fits the circumstances.

    If I understand clearly, the entire article suggests that a diver should have a backup buoyancy system, mostly a dual bladder BCD or a drysuit if you are diving lightweight. My doubt is: In which environments a diver can consider using both single bladder BCD and dual bladder BCS? or what this line suggests is that you can use two single bladder wings or one dual bladder wing?

    Thanks

    Luis Humberto Felice
    PADI 259336

    • Hi Luis,

      I think Thomas was saying that some divers use a single bladder wing when they dive in a drysuit and a double bladder wing when they dive in a wetsuit.

      Cheers, Vikki

    • two single bladders or one double bladder wing is a personal choice. Some people say a single double bladder wing keeps everything nicely together and as you only use one bladder at a time the two bladders will not work against each other. I personally like using two single bladders, as it allows me options of using the correct two depending on weight of my gear.

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