re: BCD "lift" capacity

[sytech]

Can someone explain the concept of "lift" as it pertains to BCDs?

I think I have an intuitive understanding but need to know more.

Thanks,

Sy

 

[The Kraken]

One of the purposes of a B/C is the ability to support your gear at the surface should you have reason to doff it.

The B/C needs to have sufficient lift to offset the combined weight of the full tank, harness, weights, and anything else that may be attached to the rig.

A 30# lift B/C or wing generally suffices for supporting a single tank at the surface.

A 45#+ lift B/C or wing will be required to support a doubles rig at the surface.

the K

 

[Diving Doc]

What kraken just said!!

 

[cool_hardware52]

Can someone explain the concept of "lift" as it pertains to BCDs?

I think I have an intuitive understanding but need to know more.

Thanks,

Sy

Sy,

Not sure about exactly what you are asking. Is your question is "how does a BC provide lift?" If thats the case the answer is as you fill a flexible bag with Air it displaces the water around it and because the air is less dense than the water around it, it tends to rise.

If your question is "how much lift does a diver need?"

Any BC (jacket, back inflate, wing etc) needs to meet two criteria. It must offer enough lift to float your "rig" at the surface without you and you buoyant exposure suit in it, and it needs to beable to compensate for the compression of a wetsuit or failure of a drysuit.

The first part is easy to estimate, just add up all the buoyancy figures for all the "parts" of your rig. Start with the tank. How negative is the tank you use when it's full? Google "scuba tank specifications" f you don't know. Then add the regulator, light, integrated lead weights (if you bc is weight integrated) etc. Add everything that will remain attached to your rig if you removed it in the water. Whatever this total negative buoyancy figure is it represents the minimum "lift" you need to float your rig at the surface.

I'll assume you are using a wetsuit. Wetsuits compress and as they do they displace less water. That means they become less buoyant. The diver compensates by adding gas to their BC.

How much lift can a wetsuit loose? No more than it started with. If you have a heavy 7 mil wetsuit on, and it's 25 lbs buoyant, if you go deep enough it can lose all 25 lbs of buoyancy it had at the surface. If you start with a 3mm wetsuit that's 8lbs positive, it cannot loose more than 8 lbs as it compresses.

How buoyant is your suit? Roll it up throw it in the pool and add lead until it's just sinks, and weigh the bag o lead you used.

Which ever is greater, the requirement to float the rig, or the compression of the suit determines the absolute minmum lift you need.

For most single tank recreatinal divers this actual required lift is much less than most BC's offer.

Tobin

 

[TeddyDiver]

For most single tank recreatinal divers this actual required lift is much less than most BC's offer.

Except in cold water! Thick wet suit, like 7mmfull+5mm west (or 7mm 2pcs) makes about 25lbs/12kg at the surface, and the the nominal lift for avarege BC is round 22 to 35lbs/10 to 17kg. Having the BC a bit too tight on can decrease the provided lift capacity by 1/3, so in the worst case scenario one can be 10lbs/5kg negative at 100':shakehead

 

[cool_hardware52]

Except in cold water! Thick wet suit, like 7mmfull+5mm west (or 7mm 2pcs) makes about 25lbs/12kg at the surface, and the the nominal lift for avarege BC is round 22 to 35lbs/10 to 17kg. Having the BC a bit too tight on can decrease the provided lift capacity by 1/3, so in the worst case scenario one can be 10lbs/5kg negative at 100':shakehead

Note I said "most" offer more lift than required. I did not say "all." Cold water, thick wetsuits, big heavy tanks can of course increase the required capacity.

That's exactly why I provided a simple logical means for any diver to determine his own requirements.

If jacket BC's loose 1/3 of their rated capacity in normal use they should be rated accordingly.


Tobin

 

[Tonio Anastasi]

Can someone explain the concept of "lift" as it pertains to BCDs?

I think I have an intuitive understanding but need to know more.

Thanks,

Sy

Lift is the optimal buoyancy capacity that a bcd can give.
This is estimated by having enough positive buoyancy capcity (lift) in the BCD to be able to mantain neutral buoyancy, to assit you during ascents from depth.
And once on the surface, to have enough lift to allow you to mantain positive buoyancy with the head above water without having to kick.

A test that is sometimes done to check the lift capacity of a BCD, is to tie the weights you know you need to the BCD and bottle. Take it down to the deep end of the pool and see that: a) it can take the air required to mantain neutral buoyancy,(if it does when inflated to capacity its too small) b) that it can lift the you rig from the deep end of the pool. c) that it can keep it afolat.

During a rescue exercise, one of my trainees had dropped her weightbelt as requested during a tow. At the end of the scenario I went back down to 10M to bring up her 10kg. (20lbs) weight belt.

I took her bottle and BCD down with me to use as a lifting tool to lift the belt.

Her fully inflated BCD would not provide lift for that 10kgs and I could only bring it up with the assitance of air in my BCD.

Whats wrong with that.

Well the problem is that, if she had to bring herself up from depth, her BCD would not have been of any help.

Always make sure that your BCD has enough lift for the weights that you will carry and for the depths you will be diving at.

 

[cerich]

when looking at lift capacity from different vendors be aware that all is not equal. Some just inflate the unit and see how much weight it takes to sink, others will inflate the unit and submerge it and weigh the displaced water and calculate lift, others will inflate it while the unit is assembled and how it will be when dove(with belts, snap done up and bend as it would around a divers body)

Each method gives widely different results with even the same unit, thus don't trust the weight capacity listed.

When you hear, "you need 40lbs of lift" it always tells me that the "expert" doesn't really know but is just passing along what they have been told.

 

[TeddyDiver]

That's why I like your rating method Tobin!:14: