How much BC lift do I require?

[Charlie99]

Doppler once bubbled...
An easy way is to get your hands on a fishing scale... and go to your local pool. assemble your gear -- BPWings, Full Tanks, etc. and put them into the water. Weigh them. Write that figure down. Now get into the water wearing your thermal protection. How much weight do you have to hold to float with your nose/eyes above water? This is Your buoyancy factor... your lift if you like. Take it off the apparant weight of your gear. The remainder is the lift you need

Subtracting the thermal protection buoyancy from the needed lift isn't very conservative, since wetsuits compress and drysuit can flood.

A conservative approach is to assume 0 lift from thermal protection. Then your required lift just needs to be equal to the negative buoyancy of all of your gear and full tanks, including the lead.

 

[Doppler]

Charlie99 once bubbled...
Subtracting the thermal protection buoyancy from the needed lift isn't very conservative, since wetsuits compress and drysuit can flood.

A conservative approach is to assume 0 lift from thermal protection. Then your required lift just needs to be equal to the negative buoyancy of all of your gear and full tanks, including the lead.

More scientific... good point

Doppler

 

[IzzyTahil]

Thanks to everyone for the input - every opinion was valuable!

FYI Im diving with drysuit and heavy undergarmet and using a Halcyon Pioneer #36 for single tank. I will be switching to steel 104 doubles (and with al stages soon) and was going to get Explorer #55, rather than the #45. Any opinions why I should NOT get a bigger bladder?

 

[Albion]

I just got sent some info from FredT including how to work out your weighting, sorry if i got it wrong fred.

Basically you should consider your heaviest condition, ie full tanks and your lightest consditon 500psi at 5m safety or deco stop.

your bouyancy consists of three components.

1) find out how much wieght YOU need to sink with medium breath in your swim trunks and snorkling gear.

2) how bouyant your wetsuit is at the surface by weighting it till it sinks (not sure how a dry suit would work)

3) the changes in bouyancy for your tanks. for instance a full AL80 is about 0.7kg negative full and 3kg bouyant empty.

So work out how much weight you need to keep you neutral at 5m, bouyancy of you + suit + tanks, is the amount of weight you should need then add to this the weight of your tank (s) when full is the amount of lift you need. you can assume that you are at the deepest part of your dive and so dont allow for bouyancy of your suit.

yuo should also consdier any gear your accarying as well.

please feel free to fill in any gaps i missed

 

[ScoobyDave]

Izzy,
Are you asking how much lift you will get out of your BC or how much weight you will need for proper bouyancy?

Virtually all wings, and most BC's have a weight rating on the attached bladder that you would translate into available lift or bouyancy to counter what you are wearing. Mine, for example, has a rating of 44lb/19kg. This translates to the bladder being able to displace that much weight of water, or provide that much bouyancy. Note however, this does not take into account the inherent bouyancy + or - of the BC itself - only the bladder. Other posts here that tell you to fill the bladder with water and weigh the unit duplicates what a rating tag might say. Also note that the rating is +/- 15% to what you might find in practice (some of this can be fw vs. sw bouyancy).

If you are looking for a table for the amount of weight you will need to be properly neutral in the water, I'd recommend a lot of pool time to find what works for your matrix of gear. As my OW instructor said to me: getting bouyancy right is a never ending practice.

Dave

 

[operabuffa]

I'm in the market for a new bc. Could you help me figure out how much lift I need?
Background: Planing on doing some specialty training (now that I have a dive buddy) water temps 50-60F 7mm wet suit
6'1" 205lbs
Thanks for your thoughts

 

[lairdb]

operabuffa once bubbled...
I'm in the market for a new bc. Could you help me figure out how much lift I need?
Background: Planing on doing some specialty training (now that I have a dive buddy) water temps 50-60F 7mm wet suit
6'1" 205lbs
Thanks for your thoughts

You need enough lead to keep you under for your last deco ("safety") stop. To determine that even vaguely (experiment being the only reliable and precise means) you need to specify
- fullsuit or farmer john? old or new?
- what tanks?
- have you been down successfully in this wetsuit and tank before? How much lead did you have then?

Once you know how much lead you need, then you can narrow in on how much lift you need to become neutral at depth, and how much needs to be ditchable in the event of a catastrophic lift failure.

--Laird

 

[DA Aquamaster]

Individual needs may vary slightly, but for a 7mm wet suit and a single tank, 40-45 lbs of bouyancy is about the minimum you should consider. This will allow enough lift to get you comfortably above the surface in rough water.

Many BC's out there are designed for warm water and will not provide enough lift for a cold water diver in a 7mm wetsuit.

 

[rescuediver009]

I assume that you are wearing a two piece 7mm suit (unless you are really brave) and if you are using aluminum 80s you'll likely need 26-30 lbs. of weight. You will only need to counter this for the beginning half of the dive. At the end of the dive the tank will help out. If you had to you could get away with a 35 lb. bladder, but the nice thing is that they don't make them that much smaller for you size jacket BC. as far as wings are concerned, I thinhk that Zeagle starts at 35. If you are using steel tank you can take away at least 4 lbs. (faber steel 80 ) from your belt. But you would still want at least 35lbs. of lift

 

[PnL]

We dive 7mm full suit (w/ 7mm vest when cold enough) with hood/gloves/booties and our 36lb bladders are more than sufficient w/ Al80's. They foat us plenty on surface when fully inflated. Make sure you are weighted properly, i.e., don't expect the bladder to compensate for being overweight as it could be disastrous if the bladder fails. I don't think you would need anything over 45lbs IMO.