Lift capacity and depth?

[Specmac]

Thanks Tobin. That's what this discussion needed. Actual facts, data and and the relative science and calculations.

 

[halocline]

An easy, practical way to figure your lift needs is to take your thickest wetsuit and any other neoprene you might be wearing, put it in a mesh bag, and toss it in the pool. Start adding weights until it sinks. There you have the maximum buoyancy of the wetsuit. You'll need at least that much lift. It can't lose more than it has.

Then figure what your heaviest gear configuration might be, meaning a full steel tank, steel plate, reg, canister light, bowling ball if you take one diving, any lead that's on the rig, not on your belt, and make sure you have enough lift to float that.

Whichever of those two numbers is higher is the amount of lift you need. If you want to be ultra paranoid, you can take the maximum buoyancy loss of your wetsuit and add the the weight of the gas you're carrying. That way you're covered if you were to lose all the buoyancy of your wetsuit with a completely full tank.

That's basically Tobin's simple answer.

 

[Diver0001]

^^ reasonable suggestions, but I have a couple of small adjustments to make

- if you do it in the pool then you may need to adjust the ballast (and therefore the lift) up by about 3-4% for salt water.

- the minimum lift to keep everything afloat might not feel comfortable. Remember you need to be able to float/swim comfortably on the surface too. The mimimun number might not be optimal.

R..

 

[cool_hardware52]

^^ reasonable suggestions, but I have a couple of small adjustments to make

- if you do it in the pool then you may need to adjust the ballast (and therefore the lift) up by about 3-4% for salt water.

I use 2.5% or 1 lbs of ballast for every 40 lbs of diver and gear. For most divers that's about 4-6 lbs moving from fresh to salt water. Salt content does vary, and can impact density.

I should point out that if one tests a wetsuit in fresh water using the "throw it in the water and add lead until it's neutral" there is no real need to adjust this number, very few wetsuits are more than 10 lbs total...

If one is testing the buoyancy of a drysuit by putting on their undergarment, suit and jumping into neck deep water they DO need to adjust for the weight of the diver.

- the minimum lift to keep everything afloat might not feel comfortable. Remember you need to be able to float/swim comfortably on the surface too. The mimimun number might not be optimal.

Depends on your initial weighting. For single tank diving in buoyant suits (7mm, drysuit) my target weighting is "eye level at the surface with no gas in the wing + a full cylinder"

This approach is easy to achieve as it allows ballast adjustments to be made at the beginning of the dive with a full cylinder.

A thick suit will loose considerable buoyancy from the surface to 15 ft. This loss of buoyancy offsets the weight of the 6-8 lbs of gas in a typical single cylinder.

If the diver is weighted so they are "eyelevel" with No gas in their wing, and full cylinder they have 100% of the capacity of their wing available for "comfort" at the surface.

Properly weighted divers do not need an oversized BC to be comfortable at the surface.

Tobin

 

[halocline]

A couple of things, one a wing will provide 2.5% more lift in salt water, because it's displacing higher density water. This won't cover all the additional weight you need in salt, but I suppose it will help somewhat.

The other is that Tobin referred to: "eyelevel" with No gas in their wing, and full cylinder they have 100% of the capacity of their wing available for "comfort" at the surface.

If you do a buoyancy check with a full cylinder, you'll be light at the end of the dive. Tobin, you're not saying that proper weighting is eye-level with no gas in the wing and a full tank, are you?

 

[cool_hardware52]

A couple of things, one a wing will provide 2.5% more lift in salt water, because it's displacing higher density water. This won't cover all the additional weight you need in salt, but I suppose it will help somewhat.

We test our wings using fresh water........


If you do a buoyancy check with a full cylinder, you'll be light at the end of the dive. Tobin, you're not saying that proper weighting is eye-level with no gas in the wing and a full tank, are you?

Ah. no you won't. Did you read my post #14?

A Thick exposure suit, i.e. 7mm wetsuit or Drysuit with generous undies will easily loose 6-8 lbs from the surface to 15 ft.

Remember from the surface to 33 ft you are moving from 1 ATA to 2 ATA. What effect does that have on a buoyant neoprene wetsuit?

If you don't believe me get neutral at the surface in a 7mm suit and then swim down to 15 ft. Tell me if you need to add gas to your BC to stay at 15 ft. Remember we wear "BC's" because exposure suits compress.

The "rule of thumb" of "neutral at 15 ft with 500 psi and an empty BC" is simply another way of saying the same thing.

My suggested technique has the added advantage of being a whole lot easier to do, i.e. at the beginning of the dive, at the surface with easy access to spare lead.

Where does my method not work? Two cases;

1) In thin suits, A 3mm shorty can't loose 6-8 lbs at 15 ft if it starts with only 3-4 lbs of buoyancy.

2) Large capacity tanks, doubles for example. With ~20+ lbs of gas one cannot rely on the compression of the exposure suit.

I've started many dives in thick wetsuits and modest sized tanks, i.e. single 72's, where I was positive at the surface at the beginning of the dive and had to swim down start my descent. Was I able to hold a shallow stop? Sure, my suit "lost" maybe 10 lbs, and I only breathed down 4-5 lbs of gas.

Tobin

 

[halocline]

Yes I read your post, and tried to respond politely. In my experience, if I start a dive neutral at the surface with a full tank, I'll be light at the end of the dive.

 

[cool_hardware52]

Yes I read your post, and tried to respond politely. In my experience, if I start a dive neutral at the surface with a full tank, I'll be light at the end of the dive.

What are you using for a suit?

Tobin

 

[halocline]

What are you using for a suit?

Tobin

Never more than a 5 mil, usually a 3 mil. As you mentioned there's not enough compression at 15ft to compensate for the 5lbs or so of gas used.

I also understand the idea of being neutral at 15 ft as opposed to the surface, but to me, what I want to avoid more than carrying extra couple of pounds is being positive at any depth at the end of the dive. This makes it very easy for me to control my ascent from the stop to the surface, which I like to do really slowly.

I'm sure for others, including you, that are diving in colder water, neutral at the stop/positive at the surface works fine.

 

[cool_hardware52]

Never more than a 5 mil, usually a 3 mil. As you mentioned there's not enough compression at 15ft to compensate for the 5lbs or so of gas used.

As noted in my prior posts I qualify my weighting suggestions with a very specific set of gear.

A thick suit will loose considerable buoyancy from the surface to 15 ft. This loss of buoyancy offsets the weight of the 6-8 lbs of gas in a typical single cylinder.

Tobin