Dual Bladder BCD

Do I need a dual bladder BCD?

Redundancy

Redundancy is a core requirement for doing technical diving.  “3 is 2, 2 is 1, 1 is none.”  If a single source of buoyancy control fails and the diver is negatively buoyant – as is typical when executing decompression dives with stage or deco cylinders – the diver may be too negative to return to the surface – resulting in drowning.

BCD Failure

How might a BCD fail?

• Corrugated Hose failure
• BCD Dump Valve unscrews or fails open.
• Seam failure
• Abrasion or cut to the wing

(I’ve personally witnessed each of these events at least once.)

PADI Standards

As a consequence, we need a redundant buoyancy control device available – and not a lift bag or dSMB.  A drysuit or dual bladder BCD is generally the recommended solution to this requirement.  However, PADI Tec Deep standards require that students use a dual bladder BCD except in the case where lightweight cylinders are being used – in which case a drysuit may be used for backup buoyancy.

The definition of lightweight cylinders in not defined – and as such subject to interpretation.  On one end – Aluminum 80’s would certainly be considered lightweight and permit the usage of a drysuit as backup buoyancy under standards.  On the other end, my Steel LP 108 Faber cylinders would certainly not fit the definition of “lightweight cylinders.”  (Faber 85’s?   Use your own judgement.)

Real World Drills

I’ve executed failed BCD drills – even with my LP108 Steel Cylinders and two deco bottles – and yes – it is possible to use a drysuit under these circumstances.  However, I will note that during this exercise:

• my drysuit is heavily inflated with the dump completely closed
• I must maintain a horizontal position or the neck seal will burp causing me to loose buoyancy
• there is considerable task loading reducing my ability to respond effectively to any additional problems of my own or my team
• not every drysuit will have sufficient buoyancy capacity

Recommendations

Unless your technical diving always involves a drysuit with lightweight cylinders (Al80’s)  – I recommend using a dual bladder BCD.

And, if you DO decide to use your drysuit as a backup BCD – make sure you drill sufficiently to ensure that you can effectively use the drysuit to safely complete your dive.