Lift vs. Weight

[Cave Diver]

I was thinking recently (kinda scary huh?) that there are alot of discussions related to how much weight is needed for a diver. But a quick search turned up very few discussions about how much lift is needed by a diver.

I feel that this is an important, and perhaps often overlooked consideration. So, the question is, what methods do you use to determine the correct amount of lift for a given dive situation?

Is it trial and error, or do you calculate all of your gear changes and make appropriate changes to your bouyancy control device to compensate?

How much "reserve" lift do you like to have, or how close do you cut it to increase streamlining?

 

[Hallmac]

I spend alot of time on this one.

Usually strive for neutral at ten feet in a pool with gear I'm using no air in my bc and atmosperic in my dry suit. I then log it in my log book. I add 2lbs if I am going to use it in salt water.

For me I found that at that weight I usually can fully exhale and think heavy to descend from the surface. If I need a correction at the site it is usually two or three pounds I make sure to note it in my log book. next time I hit the site I have it.

I have found the weight I use varies alittle site to site so the records make a big difference

One of the ways I do it is to take a bag with a bunch of two pound weights into the pool with me. I go to the bottom and start removing weights until I float just off the bottom without movement (12 ft pool). When I get that I must be too light feeling but visual referance tells me I not rising or falling I count the weight left in the bag.

As for reserve lift I like 45 to 55 lbs bladders. Just gives me peace of mind or was it a piece of mind oh well!

Hallmac

 

[Cave Diver]

Hallmac once bubbled...
As for reserve lift I like 45 to 55 lbs bladders. Just gives me peace of mind or was it a piece of mind oh well!

Hallmac

Ahh, but the question is WHY do you like this much lift, and HOW do you arrive at WHAT size bladder to give you the reserve you want?

 

[Hallmac]

Truthfully I chose my reserve based upon what the equipment needs to float at ten feet plus the lift I need. I'm negative can't float with full lungs if I wanted to.

Once I know what I need to get neutral I add the lift needed to bring a student or buddy up should they have a failure. I also get lazy and bring weight belts and things up in my grasp. Yea I know Bad Bad Bad. I only bring up what I can hold in my right hand.

Since we dive in cold water with dry suits or 7mm alot of weight belts are between 25 and 40 lbs. Since I'm neutral I can swim my stuff up using breath control. I use the wings for the rest. If I lose grasp I can dump and swim down to stop my ascent.

In warm water I would use a 25 to 30 pound lift. For the same reasons.

Hallmac

 

[MikeFerrara]

It's nice to be able to floar your rig when it's not on you. Dependant on wether you are wearing the weight or if it's part of the rig this usually means having more lift than what you need to dive. Of course you want enought lift to float you if your dry suit is flooded. The exact amount of lift obviously depends on the equipment being used.

 

[Diver0001]

MikeFerrara once bubbled...
It's nice to be able to floar your rig when it's not on you. Dependant on wether you are wearing the weight or if it's part of the rig this usually means having more lift than what you need to dive. Of course you want enought lift to float you if your dry suit is flooded. The exact amount of lift obviously depends on the equipment being used.

Ok, let's logic it through. What do you have to take into consideration:

1) you want to be able to float your rig when it's off. Typical steel tanks are 3-4 kg negative when full. Count on 5kg per tank plus the weight of what ever you're taking with you, lights, batteries, weights, regs, "knife" cameras etc etc.

2) You want to have enough lift to overcome the maximum negative buoyancy you'll have at the deepest (most negative) part of your deepest dive. This is the hard one to calculate but you can assume that with 100% suit compression that this more or less equals the buoyancy you need to float your rig and all your stuff.

3) If you wear a drysuit you need to ahve enough buoyancy to stay on the surface with your suit full of water. This can be compared to 100% suit compression so it should be about the same as the lift it takes to float your rig on the surface.

4) for rescues you want enough buoyancy to lift another diver from the deepest part of your deepest dive using only your own BCD.

5) You need enough lift to overcome the natural negative buoyancy of your own body (in some cases this might be positive )

6) ???

To me it's starting to look like what ever you need to float all your gear on the surface is all you really "need". You might want to add a factor of 20% (more?) to makes sure that you don't sink under the waves in heavy seas etc etc.

comments?

 

[donacheson]

Diver0001 once bubbled...


2) You want to have enough lift to overcome the maximum negative buoyancy you'll have at the deepest (most negative) part of your deepest dive. This is the hard one to calculate but you can assume that with 100% suit compression that this more or less equals the buoyancy you need to float your rig and all your stuff.

3) If you wear a drysuit you need to ahve enough buoyancy to stay on the surface with your suit full of water. This can be compared to 100% suit compression so it should be about the same as the lift it takes to float your rig on the surface.

Drysuit, wetsuit, whatever - for this component of required lift, the worst case is the dry weight of the suit in air.
This is equivalent to assuming neoprene compression to a very high density or a shell suit full of water.

 

[Diver0001]

donacheson once bubbled...


Drysuit, wetsuit, whatever - for this component of required lift, the worst case is the dry weight of the suit in air.
This is equivalent to assuming neoprene compression to a very high density or a shell suit full of water.

Agreed. The dry weight of the suit in air. Good suggestion.

R..

 

[MikeS]

donacheson once bubbled...


Drysuit, wetsuit, whatever - for this component of required lift, the worst case is the dry weight of the suit in air.
This is equivalent to assuming neoprene compression to a very high density or a shell suit full of water.

donacheson,

I don’t understand the relevance of the weight of the drysuit in determining required lift. In the case of flooding, the buoyancy of the actual drysuit remains constant, what changes is the loss of positive buoyancy from the air that escapes. It seems that the underwear would have a much greater impact on the loss of lift then the wetsuit itself.

Mike

 

[Cave Diver]

Diver0001 once bubbled...


Ok, let's logic it through. What do you have to take into consideration:

<snip>

To me it's starting to look like what ever you need to float all your gear on the surface is all you really "need". You might want to add a factor of 20% (more?) to makes sure that you don't sink under the waves in heavy seas etc etc.

comments?

GREAT ANSWER! This is exactly the type of discussion I was trying to start, taking logical steps to account for how much lift is enough, and how much lift is too much.

It's pretty obvious that you have to have enough lift to support your own gear. It's the process of determining that amount, plus what kind of "safety factor" for your buddy that I am after.

If you determine that you need 25lbs of lift and your buddy is configured identically, do you automatically assume that 50lbs is a minimum to get both of you back safely?

It's kinda like figuring rock bottom turn pressures...