back mount b/c size

Question
I am buying my own SCUBA equipment for the first time.  I'll be diving primarily on salvage sites in the Florida Keys--bottom excavation, not ship infiltration.  OMS offers several B\Cs with different lift ratings.  My question is, how do I choose the correct lift rating for me?  I am 6'3", 230lbs. I'll be doing mostly single tank dives in 20-40 foot depths.  Thanks for you help.


Answer
Hi John

Thanks for the question about lift capacities of BC's.  As you've noticed, they can differ considerably.  First, let's establish a key point...the BC is NOT to be used as a lift bag.  It is designed to provide adequate lift for the diver only and not to lift both the diver and a heavy object.  The reasons for this are fairly obvious.  If the diver loses his grip on the heavy object during the lift, he will undergo a rapid, uncontrolled ascent and may suffer severe lung overexpansion injuries before he can dump the excess air and regain control.  The large lift capacities are designed primarily for the cold water diver wearing a drysuit.  Drysuit divers use the inflator hose on their drysuit to control their buoyancy.  If the drysuit should fail, the BC is a backup buoyancy contol device to permit the diver to end the dive quickly and safely. It is also used to support the drysuit diver at the surface.  These divers are generally wearing quite a lot of weight compared to tropical divers, hence the larger lift capacities.  Most of the OMS line of BC's is back inflating and the primary differences have to do with lift capacity.  You want enough lift to comfortably lift you and your wearable diving equipment to the surface and keep your head above water.  If most of your work is going to be bottom excavation, you'll probably wear about six to eight pounds more than the average diver so you can maintain a footing on the bottom.  I'm guessing that you probably wear around 18 pounds if diving in the Keys during the summer and probably around 24 if you're really bundled up for winter diving.  If you add an extra 8 pounds to stay firmly planted on the bottom, you're looking at about 32 pounds.  If that's the case, you only need a BC with about 42 pounds of lift capacity to ensure that you have sufficient buoyancy to keep your head above water on the surface.  If my weight estimates are a bit off, you can adjust accordingly.  Make sure you aren't overweighted to start with.  You should be able to SLOWLY sink beneath the surface with a fully deflated BC and holding a normal breath.  Of course, that's BEFORE you add your extra weight to allow you to stay planted on the bottom.  I know that's not exactly the way the "book" teaches it but it's a much more practical technique and it will work well for your type of diving.  If you're properly weighted, the lift capacity of the BC will only be used to support you at the surface.  It doesn't take much air to do that and the extra lift capacity is only a "safety factor".  The lift capacity of the BC is only achieved when the BC is fully inflated and you shouldn't need to do that during the normal course of your diving.  Keep in mind that a back inflating BC will tend to push your face down in the water if it's fully inflated at the surface so never plan to use the full capacity of the BC at the surface.

In summary, John, I think a BC with a lift capacity of 40-45 pounds will be more than enough to handle anything you might run into in your salvage work.  If you're handling tools or other heavy equipment, use a separate lift bag to lift and transport them underwater.

If you have any questions, please don't hesitate to write back.

Sincerely,

Mike Giles
Mike's Dive Center
mikescuba.com