How to determine proper weight, lift capacity of gear, and buoyancy? Tobin?

[bennygill]

I've recently inquired about buying a BP&W setup from DSS. Tobin was very helpful by suggesting what rig I should buy based on what I'll be diving with. But I was hoping to read up on this subject for my own learning.

What calculations does he use to determine what gear I need? For example if I was using different tanks or wet suit thickness how would that affect the decision of what gear I should buy?

I know the general stuff, salt water more buoyant than fresh. Al80's more buoyant than HP100's. Thicker wetsuits more buoyant than thin. Etc... BUT I don't understand how I can give that information to someone and they come back to me with an answer of what a correct setup would be.

I did all my training a long time ago and I'm only recently getting back into diving in a big way... the only thing I remember about all this is the PADI "take a breath and hold it" method for determining how much weight to use.

Also is it possible to ever have a setup that does not require a weight belt?

Sorry if this is in the wrong section.

 

[DevonDiver]

Have a look at my article series that deals with buoyancy, weighting and equipment calculations, it should answer many of your questions.

Scuba Buoyancy Masterclass 1of9 - Buoyancy Control for Scuba Divers
Scuba Buoyancy Masterclass 2of9 - The Need for Buoyancy Control
Scuba Buoyancy Masterclass 3of9 - Achieving Great Buoyancy Control
Scuba Buoyancy Masterclass 4of9 - Assessing Your Weight Requirements
Scuba Buoyancy Masterclass 5of9 - Trim and Position
Scuba Buoyancy Masterclass 6of9 - Breathing and Buoyancy
Scuba Buoyancy Masterclass 7of9 - Ascent, Descent and at the Bottom
Scuba Buoyancy Masterclass 8of9 - The Balanced Rig

What calculations does he use to determine what gear I need?

There are a number of formulas given to help calculate an approximate weighting need. These typically account for your body weight, salt/fresh water and exposure protection. More advanced methods also include factors such as cylinder size/type.

PADI have the 'Basic Weighting Guidelines' (see my articles). These typically bring you out too heavy, but do offer an approximate stating point, which you can then fine-tune with in-water weighting checks.

if I was using different tanks or wet suit thickness how would that affect the decision of what gear I should buy?

Tanks and wetsuit characteristics will affect your buoyancy. They should be factored into your calculations.

BUT I don't understand how I can give that information to someone and they come back to me with an answer of what a correct setup would be.

If you account for the buoyancy characteristics of each piece of equipment, it can be totalled to create an overall picture of your buoyancy requirements. As always, you'd still want to fine-tune this with an in-water weight check though.

I did all my training a long time ago and I'm only recently getting back into diving in a big way... the only thing I remember about all this is the PADI "take a breath and hold it" method for determining how much weight to use.

That's the basic weight check. At the surface, empty your BCD, take a full breath and you should float at eye level. Exhale and you should sink. Ideally, you should do this with a nearly empty tank - otherwise you'd have to add weight to approximate the weight of gas you'd consume during the dive. You can also calculate the weight of this gas, if you desired.

However, the best weight check is to descend to 5m, empty your BCD and with a minimal gas reserve in your tank (35bar/500psi) perform a hover. Slowly reduce your weight until you reach a stage where you can no longer maintain neutral buoyancy and begin to float (positive buoyancy). That gives you an accurate picture of your maximum weighting requirements; where your exposure suit is not compressed and you have no negative buoyancy from your breathing gas.

Also is it possible to ever have a setup that does not require a weight belt?


It is possible. Especially when diving in warm water with no exposure protection, a negative cylinder and other negatively buoyant equipment. However, it's critical that you can always access the surface if your BCD were to fail (leak). That means either having ditchable weight or selecting your overall equipment so that you are never too negative to swim easily to the surface.

 

[bennygill]

AMAZING ANSWER!! So detailed and exactly what I was looking for. Will start reading those articles. Thanks

 

[cool_hardware52]

I've recently inquired about buying a BP&W setup from DSS. Tobin was very helpful by suggesting what rig I should buy based on what I'll be diving with. But I was hoping to read up on this subject for my own learning.

What calculations does he use to determine what gear I need? For example if I was using different tanks or wet suit thickness how would that affect the decision of what gear I should buy?

I know the general stuff, salt water more buoyant than fresh. Al80's more buoyant than HP100's. Thicker wetsuits more buoyant than thin. Etc... BUT I don't understand how I can give that information to someone and they come back to me with an answer of what a correct setup would be.

I did all my training a long time ago and I'm only recently getting back into diving in a big way... the only thing I remember about all this is the PADI "take a breath and hold it" method for determining how much weight to use.

Also is it possible to ever have a setup that does not require a weight belt?

Sorry if this is in the wrong section.

If you have specific questions for me it's best if you post them in the DeepSeaSupply forum. I may not see them otherwise.

On the most basic level:

Some of what you take diving will be buoyant, i.e. exposure suits, padded bc's (ugh) possibly empty tanks (al 80's) and maybe you

&

Some of what you take diving will be negative i.e. back plates, regs, negative steel tanks, canister lights, weight belts

If you want to actually *GO DIVING* you need to get these two lists to match pretty closely.

BTW, for temperate to cold water diving I am not in favor of hanging 100% of your ballast on your rig. 4-8 lbs in a weight belt is easily managed by virtually all divers and it allows the use of a smaller wing than would be required if you place 100% of your ballast on your rig.

For warm water diving so little ballast is necessary that often 100% of the required ballast will be furnished by the divers backplate, harness and reg.

Tobin

 

[bennygill]

Thanks Tobin I think I've got my head around all of this now. Just want to confirm a few things...

In the email you sent me you said "with a full hp 100 your rig will be about -19 lbs and will provide about 11 lbs of ballast, about right for a 5mm suit."

So does that mean my rig weighs about 19lbs, and guessing my reg and octo weighs about 3 lbs, so my complete rig is about -22lbs buoyant. If I have the Torus 26 wing you recommended that should be more than enough lift for my gear. Is that what you base your decision on what type of wing to recommend?

Then adding a 5mm wetsuit which is 17lbs buoyant (i read somewhere it is 10% of body weight - I'm 176lbs) theoretically everything would now be -3lbs buoyant (-22lbs + 17lbs = -3lbs). And then add the buoyancy of my body (not sure how to estimate this part) and I should be close to neutral. Does that sound correct??

If using Al80's instead of HP100 I'll add a little more weight because they are more buoyant at the end of dive.

Also I'm not sure what you mean by "will provide about 11 lbs of ballast".

Again thanks for helping me understand all this!

 

[cool_hardware52]

Thanks Tobin I think I've got my head around all of this now. Just want to confirm a few things...

In the email you sent me you said "with a full hp 100 your rig will be about -19 lbs and will provide about 11 lbs of ballast, about right for a 5mm suit."

So does that mean my rig weighs about 19lbs, and guessing my reg and octo weighs about 3 lbs, so my complete rig is about -22lbs buoyant. If I have the Torus 26 wing you recommended that should be more than enough lift for my gear. Is that what you base your decision on what type of wing to recommend?

Then adding a 5mm wetsuit which is 17lbs buoyant (i read somewhere it is 10% of body weight - I'm 176lbs) theoretically everything would now be -3lbs buoyant (-22lbs + 17lbs = -3lbs). And then add the buoyancy of my body (not sure how to estimate this part) and I should be close to neutral. Does that sound correct??

If using Al80's instead of HP100 I'll add a little more weight because they are more buoyant at the end of dive.

Also I'm not sure what you mean by "will provide about 11 lbs of ballast".

Again thanks for helping me understand all this!

Plate and harness is about -6, typical reg is -2 and a full hp 100 is about -11 that totals -19 lbs.

A 100 cuft ctylinder holds ~8 lbs of gas. If your rig is -19 with a full tank it will be about -11 with an empty tank.

Exactly how buoyant is *your* 5mm wetsuit? I don't know, but 10-12 lbs positive is not unreasonable.

You can test your suit by rolling it up and throwing into water an adding lead until it just sinks.

Tobin

 

[bennygill]

Ok got it now. Thanks again.