"Correct Weighting" Identified as #1 Needed Improvement in SCUBA Diving

[bmbeaty]

Do you have the skills to do an emergency ascent even if you are a couple pounds negative? I would certainly hope your answer to this would be yes. Can you swim down from the surface even if you are a little buoyant?

You could weight yourself for optimum buoyancy for a situation that is possible but very unlikely, and then spend all of your time heavy and fighting to stay off the bottom as a result of that decision. Or weight yourself for how the dive is likely to happen and have everything right unless some rare thing happens and then you deal with the situation with a little kicking.

I never have to add or remove air from a BC while diving, or clean it, or pack it in luggage, or carry extra weight around. Diving a backplate really simplifies diving in a lot of ways.



Still not takers on what assumptions are likely leading to an overprediction of weight here?

What is being referenced is an AL80 being 80cf when it's more like 77 for real. That said, it doesn't really matter. If not using technology makes you feel fulfilled, more power to you. For the average person the work involved to dive without a BC isn't worth the hassle. Also, I am slightly negative in freshwater in a 3/2 wetsuit with an empty bcd, reg etc and 500 psi in an AL80. Not typical, sure. However, there would be no way for me to attain neutral buoyancy without just breathing from the top of my lungs the entire dive. For me, diving is about relaxing. If I'm having to constantly adjust my breathing to remain neutral, I'm focusing on breathing not enjoying my surroundings. Again if it works for you- great. The vast majority of people prefer to utilize technology, and it doesn't make sense to train different than you fight.

 

[Chavodel8en]

How about this, rather than training folks without a BC, you let them have their BC and a few pounds above the absolute optimum weight, and then train them as much as possible to not treat the BC like an elevator

Realistically, you will be very disappointed that new divers arent going to be fully dialed in from the get go, but if that's the model you would like to push, that's fine, I would just urge you to re-think the no BC stance.

 

[KenGordon]

Do you have the skills to do an emergency ascent even if you are a couple pounds negative? I would certainly hope your answer to this would be yes. Can you swim down from the surface even if you are a little buoyant?

You could weight yourself for optimum buoyancy for a situation that is possible but very unlikely, and then spend all of your time heavy and fighting to stay off the bottom as a result of that decision. Or weight yourself for how the dive is likely to happen and have everything right unless some rare thing happens and then you deal with the situation with a little kicking.

I never have to add or remove air from a BC while diving, or clean it, or pack it in luggage, or carry extra weight around. Diving a backplate really simplifies diving in a lot of ways.



Still not takers on what assumptions are likely leading to an overprediction of weight here?

Would I rather be a couple of lb more heavy for a nice relaxed bottom time, or a couple of lb light when doing a stressful AS ascent?

 

[REVAN]

What is being referenced is an AL80 being 80cf when it's more like 77 for real.

Okay. A couple of people have recognized that a standard 80 is actually 77.4 cubic feet at 3000 psi, not 80 cubic feet.

The other assumptions that were made are that the air in the tank is an ideal gas with zero humidity, that the expansion is isothermal and that it is all done at 0 C (32 F).

How many of you have your tanks filled in a freezer? Tanks are generally filled in the non-airconditioned back room or external shed from the dive shop and the tanks get hot when they are filled. A tank that is filled to 3000 psi at 0 C is actually overfilled by nearly 10% at room temp and by about 20% at typical tank fill temperatures.

If the compressor has a good water separator and desiccant filter, the air should be fairly dry, but any humidity will make the air lighter in weight by volume.

A scuba tank will not actually expand isothermally. While the tank is in a water bath, it still takes 15 to 20 minutes for the air temperature inside the tank to get close to an equilibrium state. The average drain time on a scuba dive is around 40 to 50 minutes. So, the expansion will be between isothermal and adiabatic predictions, the main variables being the rate of the tank draining and the temperature of the water. Rest assured though, the tank will be colder than the surrounding water at the end of the dive, so there will not be a direct relationship between pressure and moles of gas content (i.e. - weight) remaining in the tank. Diving in the tropics? That scuba tank sitting out in the sun will be hot. Now put it in the ocean and cool it down. You can see a noticeable pressure drop in the tank. How much weight has left the tank? None.

There's not good way to account for the variations of air and the ideal gas prediction except to use direct measurements. A room temperature standard 80 will shift by 4.4 pounds according to measurements, between 3000 and 500 psi (isothermal). Take into account the normal hot state of the tank at the time of the fill and the actual non-isothermal expansion during the dive and the actual weight shift is usually a little less than that. This is why I personally use 4 pounds for my round number estimate.

So, @The Chairman 's formula is going to predict a weight shift that is probably on the order of 20% high of what will likely be the actual weight shift someone will encounter. That's not likely to be an issue for someone diving with a BC unless they are diving really massive quantities of air. But it's good to know the assumptions and likely errors in the formulas you are using to make predictions. Being off by 20% can be an issue for a minimalist or vintage style diver.

 

[The Chairman]

Okay. A couple of people have recognized that a standard 80 is actually 77.4 cubic feet

Stop guessing and do the calculations. All calculations to three significant digits. I'll use your 77.4 CF that is found in SOME tanks.

• 72-32+460=500R (Converts from F to Rankine)
• 500/460=1.09 (Fraction of Ambient to Absolute Zero
• 77.4x.833 (5/6)=64.5cf (Volume of gas consumed down to 500psi)
• 0.0807/1.09=0.0740 (Calculate weight of air at ambient temperature)
• 64.5x0.0740= 4.73 pounds. CFx(Pounds/CF)=Pounds

It's still not 4 pounds.

0.0807lb/cf x 80cf x 5/6= 5.38 pounds.

• 4/4.73=0.845 or 84.5% You're under by 15.5%
• 5.38/4.73=1.13 or 113% I'm over by 13%.
• I'm still a bit closer, but you're splitting hairs.
• Now do the math for my LP 120 with a cave fill (3600psi)

For the average person the work involved to dive without a BC isn't worth the hassle.

This!

Would I rather be a couple of lb more heavy for a nice relaxed bottom time, or a couple of lb light when doing a stressful AS ascent?

And this!

 

[REVAN]

And how does it come out with adiabatic expansion?

 

[stuartv]

And how does it come out with adiabatic expansion?

Who cares?

 

[uncfnp]

And this thread is now officially...

 

[REVAN]

Why adiabatic?

Because I said it is 4.4 pounds for isothermal expansion. This is 7% away from @The Chairman 's corrected figure vs. his 13%. But then, we also know that the tank won't drain in an isothermal state unless you plan to switch to an alternate air supply for about 15 minutes or so. Nor is it an ideal gas. These differences will push the real number further toward my assumption.

At some point it does come down to splitting hairs. In the end, what matters is that you are using something that works. I think my number is clearly at least as accurate as the other guy's, and it works well for what I'm doing. So, why is my figure an "egregious" error, but it seems everyone thinks it's fine for the other guy to be more inaccurate and trash on me for not using his number?

It's those types of errors and/or assumptions that keep people thinking that it is impossible to dive without a BC and not be crawling around on the bottom.

In the end, maybe the main reason so many people dive with more weight than necessary is simply because they want to.

 

[JamesBon92007]

You missed your own misconceptions which are egregious.


Please show the math for this. Better yet, I'll do it for you:

• Air weighs 0.0807 lbs/cf
• If you drain an 80cf tank from 3,000psi to 500psi, then you've used 5/6 of the available air.
• 0.0807lb/cf x 80cf x 5/6= 5.38 pounds.

.

Actually, since a typical AL80 filled to 3000 psi is only about 77.4 cu/ft then that would be closer to 5.15 pounds.

If we are going to bicker let's be accurate.

Edit: I now see that this has been addressed, however the numbers seem to be different.

In any case for me it's "common knowledge" that the weight difference between an "empty" and "full" AL80 is about five pounds and a Steel 72 is about 4 pounds. I've never needed to know precisely what those weights were and empty and full vary with who filled it and who's breathing out of it. I'm surprised Archimedes hasn't gotten into the argument yet