Minimum Diving Weight

[peterbj7]

One Liter of air does not weigh 1Kg, it will displace 1 Kg of water

Sorry, under what circumstances will this happen, and what precisely do you mean by "one liter of air"? The reason I'm being picky is that i think you're mixing your units.

 

[Akimbo]

Sorry, under what circumstances will this happen, and what precisely do you mean by "one liter of air"? The reason I'm being picky is that i think you're mixing your units.

Lets see if these different ways of looking at it is clearer:

Respiratory volume is most commonly measured in Liters per cycle or Liters/Minute at ambient pressure. You can convert Liters into any other unit of volume if you prefer. The volume of fresh water displaced by a Liter of air weighs 1 Kg. This is true if it is in a beaker or inflating lungs.

For the sake of discussion, let's say you added one Liter of water to your lungs. You would add 1 Kg to your dry weight without increasing the volume your body displaces. Your positive buoyancy would then be decrease by 1 Kg… I'm not volunteering for that particular experiment.

If you fill a 1 Liter beaker with fresh water, it will increase the weight of the beaker by 1 Kg. We can get obsessive and specify the water is at 0° C, but the variation in volume due to temperature change is miniscule compared to the rough estimations of average respiratory volume.

One of the easiest techniques for measuring the volume of a complex irregular solid object is to submerge the object in a liquid and measure the increase in water level. For example, take a metal object and submerge it in a half-filled beaker of water. Lets say the water level increases 10 ml (milliliters, which is also 10 cc or centimeters³). 10 cc is the volume of the object.

Please note that 1 Liter of respiratory volume was an example and not necessarily any particular individual's volume at any given rate or work load.

So, it this clearer or worse?

 

[fisheater]

Here's a shorter version.

One liter equalling a kilogram is just metric for "a pint's a pound, the world around."

 

[randini]

Good point. I never considered it was an advantage until now, but the pool I trained in was unheated. We were used to full wetsuits and had our weight pretty close by our first ocean dive.

A submersible fish scale would be ideal because it would attach to the line near the diver who could read their plus or minus weight themselves. I wonder if such a scale exists?

How about something like THIS

I've used them for other stuff and they are good enough, even when completely submerged. They are cheap enough that you could easily pick up a dozen and consider them disposable, but the ones I've used have lasted months after getting splashed by waves continually.

They're also really easy to modify to record maximum (during some early graduate work I used them to measure the force required to yank mussels off the rocks): drill holes in the top & bottom corners, add a strings going from top left to bottom left and another for top right to bottom right, then place a marker (I used a milled piece of plexiglass) in the strings that moves down with the indicator lever. Once you get your reading, slide the marker back up and move on to the next measurement.

 

[randini]

Here is my view on this often debated subject. It is always dangerous to make absolute statements, especially on Scubaboard, but I am in my flack jacket so fire away. :nailbiter:

A diver's submerged weight should be no less than neutrally buoyant

• At the shallowest decompression or safety stop
• With a fully deflated BC (if you use one)
• With drysuit deflated to minimum without discomfort (if you use one)
• With nearly empty Tank(s), like 200-300 Lbs or 14-20 Bar
• With lungs comfortably inflated to your normal respiratory inhalation peak

<snip the rest out>

Personally, I prefer neutral at the surface (empty BC, never used a drysuit and don't see the need to for the conditions I'm usually in). Like you mention later, 10-15 ft won't make that much of a difference, but the logistics of adding removing weight seems easier (at least if you're diving off a boat or in a pool).

As for the near empty tank, 200-300 psi won't make enough of a difference to affect trim that much. Might as well do it at 500, or even 700. But I completely agree with you on missing the stop just to come up with the 500 psi, If a stop is warranted (mind you a safety stop, not a deco stop) I'd rather suck a tank almost dry to get that stop in. Not completely dry, since that would facilitate the possibility of humid air or water getting into the tank.

Also, I prefer to do the check before about half way through the breathing cycle, with lungs less than 1/2 inflated during a normal breathing cycle.

One of my first scuba jobs was as a researcher searching for marked Norrisia norrisii (a kelp snail in southern CA) in kelp forests. Looking for these guys, we had to hover about arm's length under the kelp canopy (about 3ft deep), sifting though the kelp to see if any were around. Point is, that's probably why I still have these preferences, achieving perfect buoyancy was paramount to not getting wrapped up in the kelp and being able to move along quickly to get the job done.

Mind you these are my own preferences and I won't tell you how you should do it. Each of us needs to figure out what works best for us (you can take you flame retardant suit off now)

 

[peterbj7]

The volume of fresh water displaced by a Liter air weighs 1 Kg

This is only true at standard surface conditions.

 

[Akimbo]

This is only true at standard surface conditions.

Perhaps I don’t understand your statement. Displacement is ambient volume, regardless of depth. If you have 1 Ft³ of air at 10 ATA or 297 FSW, it will displace 1 Ft³ of sea water, or 64.1 Lbs of sea water, thus providing 64.1 Lbs of added buoyancy. At the surface that same air volume may expand to 10 Ft³, but that does not matter at 297'.

If the gas were contained in a 1 Ft³ lift bag (open bottom), it would lift 64.1 Lbs minus the weight of the lift bag, regardless of depth because the excess gas would spill out the bottom. Put that same 1 Ft³ at 297' in a 5 Ft³ lift bag and it will lift 64.1 Lbs at 297', except this time the gas will expand and be contained on ascent. That gas will expand to 5 Ft³ at 132 FSW, or 5 ATA, and will provide 320.5 Lbs (5x64.1) of positive buoyancy, minus the weight of the lift bag. That lift will then be constant until hitting the surface, also because the excess expanding gas will spill out the bottom after passing the 132' mark.

Put that same 1 Ft³ of gas in your drysuit at 297' and you will be a missile by the time you hit the surface unless you vent the gas off or your suit explodes first. However, it still comes down to the displacement of the gas at whatever ambient pressure you choose. You must analyze the effect of expanding gas on displacement as depth/pressure decreases or increases to determine the effect on buoyancy.

Granted, you could get very precise by adjusting for the added weight of the compressed gas, temperature, and salinity; but the effect is so small that it is irrelevant for an individual diver.

Archimedes Principal works equally well if the lighter than water media is gasoline instead of air — as used on the Bathyscaph Trieste. For discussion, let's say a particular fluid weighs 58 Lbs/Ft³. It will provide added buoyancy of 6.1 Lbs (64.1 – 58) of buoyancy per Ft³ in sea water. Since fluids are not compressible for all practical purposes in our application, depth also does not matter. That is how the Trieste made it to 36,000' and back.

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.

You have to be careful if context. "Standard surface conditions" only apply if you are calculating buoyancy loss due to compression of gas, or expansion if the gas is contained in a closed container… like a drysuit. At any given depth, buoyancy still comes down to the weight of the fluid displaced by that submerged object. Examples:

• A 1 Ft³ block of concrete that weighs 150 Lbs will only weigh 85.9 Lbs in the water (150-64.1)
• A Ft³ block of Styrofoam that weighs 1 Lb will require 63.1 Lbs of added submerged weight to make it neutrally buoyant.
• A 1 Ft³ block of steel that weighs 450 Lbs will only weigh 385.9 Lbs in the water (450-64.1)

A lot people will likely read this so I made the explanation more detailed for their benefit rather that for divers of your experience. Context and semantics can quickly confuse a simple conversation over an international text-based media.

 

[Akimbo]

How about something like THIS

I've used them for other stuff and they are good enough, even when completely submerged. ...

Do they hold up OK in salt water? How is the accuracy? If good, that would be the ideal solution. Even if their calibration was not all that great, all you would have to do is use the same scale to weight out the weights you were adding or subtracting.

You could just tie the rope off to a small float or buoy for the tests. Very cool, thanks.

 

[Akimbo]

Personally, I prefer neutral at the surface (empty BC, never used a drysuit and don't see the need to for the conditions I'm usually in). Like you mention later, 10-15 ft won't make that much of a difference, but the logistics of adding removing weight seems easier (at least if you're diving off a boat or in a pool)…

Everything you wrote is true. It may be harder to defend as the "minimum" weight, but it works. Your procedure is certainly better if you are working in shallower water than a stop. Perhaps I should modify the procedure to specify "your shallowest stop or working depth"?

The lower tank pressure makes a little more difference on doubles, but your point is well taken. However, I do think new divers should experience what being out of air feels like under controlled conditions. It varies by regulator, but knowing what it breaths like and how much time you have in shallow water can be useful. I will take another cut at the procedure in a few weeks and hopefully incorporate everyone's input.

Thanks

 

[randini]

Do they hold up OK in salt water? How is the accuracy? If good, that would be the ideal solution. Even if their calibration was not all that great, all you would have to do is use the same scale to weight out the weights you were adding or subtracting.

You could just tie the rope off to a small float or buoy for the tests. Very cool, thanks.

They do hold up OK to salt water. They are a cheap mix of different metals so once they get exposed saltwater they do start to rust fast (read they're good for a few weeks unless you maintain them properly). The ones I used in the experiment I mentioned worked fine for months after almost continuous use on the shore and constant battering by waves. Just give them a good rinse in freshwater, spray the heck out of em with WD-40 and recalibrate before re-use. For about $4 a pop, don't expect them to last forever (hence why I said you could buy a dozen of them and consider them disposable). Recalibrating is easy, just hang weights of known denominations from it, see what it indicates and make sure it's still reading what it should.

If you really want to continue using them for extended periods, you could even apply regression equations to them after recalibration to get the corrections.

Personally I don't think that this degree of accuracy is necessary. It's all about how you feel in the water. But if you wanted to use a system like this to help a friend/buddy realize when they gain neutral buoyancy, what I was thinking is that you could have one of these at a stage (say 10 ft below the surface or whatever your depth of preference is) and given the conditions you want (low tank, empty BC, etc) hook it onto the diver (not directly, but via a long enough rope that the diver could hang from it like if on a trapeze) and see how negative they still are. That should give both of you an indicator of how much lead they theoretically could shave off.

These scales aren't that precise, but at 1/2 kg (about 1lb) increments that's good enough considering the range of dive weight increments (generally about the same).