Minimum Diving Weight

[peterbj7]

Under what conditions will 1 litre of inhaled air weigh 1 kg? And in what way does your being deeper help to make you "much heavier"?

 

[CT Sean]

I believe the idea is that 1 liter of air offsets 1 liter of water which weighs 1 kg therefore inhaling 1 liter of air adds 1 kg of buoyancy

 

[Akimbo]

Under what conditions will 1 litre of inhaled air weigh 1 kg? And in what way does your being deeper help to make you "much heavier"?

One Liter of air does not weigh 1Kg, it will displace 1 Kg of water, thus change your buoyancy by one Kg. For the benefit of others reading this, here is a more detailed explanation to avoid misunderstandings:

Many of the original definitions in the Metric system were based on the Meter and pure water.

• A Meter was one ten-millionth of the distance from the Earth's equator to the North Pole through Paris. It was represented by a metal rod stored in France that was used as the "standard" in the early years
• A Liter was and is 1/1000th of a cubit Meter
• A Kilogram was the weight of one Liter of pure water at 0° C.
• A Metric ton was the weight of a cubic Meter of pure water at 0° C, which also is 1000 Kg.
• 1° C was defined as 1/100th the difference between the freezing and boiling temperature of pure water.

As these units were defined more precisely over time, all the old definitions were not exactly the same, but are still very very close.

Metre - Wikipedia, the free encyclopedia

Litre - Wikipedia, the free encyclopedia

Kilogram - Wikipedia, the free encyclopedia

My example was an approximation based on fresh water because sea water is heavier, but is close enough for estimating respiratory cycle and the change in buoyancy. For those unfamiliar with Archimedes Principal:

Archimedes of Syracuse
Any object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.

This translation is out of the US Navy Diving Manual. There are several iterations that essentially state the same thing. Therefore a close approximation would be that your total body volume will increase by the volume of air inhaled as you inflate your lungs — more like a lift bag than a submarine. Does this make sense to everyone?

Edit: Sorry CT Sean, your post was up before my cache was updated.

 

[jepuskar]

Some good points in this thread...but I think a weight check on the surface with an almost empty tank..if done properly, is more than suffice.

 

[Akimbo]

Some good points in this thread...but I think a weight check on the surface with an almost empty tank..if done properly, is more than suffice.

Agreed. There is a fair amount of confusion about what constitutes a "proper" weight check so I decided to take a crack at it. That thought process lead me to trying to describe a method that would be easy for people with minimal experience to use, who are the ones that need it most. I figured we could debate the best amount of weigh forever, but the minimum would be simpler to justify.

 

[JohnB47]

So simple, even a caveman can do it.

Uh... did you mean, "So simple, even a cavediver can do it?"

 

[Akimbo]

… And in what way does your being deeper help to make you "much heavier"?

I just noticed I didn’t respond to the second part of your question. The "much heavier" comment applies mostly to wetsuit divers where suit compression reduces their displacement. It does not apply to tropical dive skins or drysuits made from pre-compressed or uncompressible fabrics. There is a much smaller effect in cases where you can’t easily purge all the air out of your BC or an accessory like a rolled up inflatable sausage. Increased pressure can also compress closed-cell padding used in some BCs, back plates, and weight belts.

Rather than try to explain all the variables in detail, I figured it was simpler to just get in the water and weigh yourself.

 

[iztok]

Or just dive 2 steel tanks and no pesky weights are neede.

Which is too negative for at least 3 and 5 mm suits.

 

[Lenaxia]

I had a thread on this a month or two ago, but I can't find it right now. I was also seeing around a 2' change between breaths. Someone (perhaps TSandM) suggested I start breathing out as soon as I notice myself rise, and breath in as soon as I notice I'm falling. That lead to a considerably shorter breathing cycle, and got me down to like 1'. I just got back from a night dive where we burned the last ten minutes at 4-7' and I was starting to get the feel for the timing, although my lungs were pretty empty at 4' with an empty tank.

I gave this a try during my OW training with some free time my instructor gave me with a bit of success. I think if I practiced it more I would probably get better at it.

You don't necessarily need to alter your breathing all the time due to porpoiseing. When you need to be spot on in position then you breathe as needed with precision. In general you can usually breathe at a relaxed pace. Timing of your breaths, your kicks and nuances in posture can all level out the flight path. Whatever variation remains is not a crime.

Spending time in the shallows figuring out just what you can achieve is a great way to get in touch with the secrets of buoyancy control.

Pete

Point taken

 

[KY_BOB]

Or just dive 2 steel tanks and no pesky weights are needed.

That's what I've been doing lately. I've pulled them down to 300psi and I'm still negative at 15'.