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

[The Chairman]

And how does it come out with adiabatic expansion?

Someone looked up a new word in the dictionary and wants to wow us. So please: show us your prowess and give us those calculations. You have a peculiar way of stating this which leads me to call shenanigans.

 

[kelemvor]

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.

Most fill stations aren't so close to the water that diving 15 minutes after a fill would even be possible if you tried. I'd bet that is even a stretch for a fill station on a livaboard or other dive boat. If you're that concerned about the weight of your tank, put it on a scale instead of trying to calculate what might or might not be inside it.

 

[REVAN]

If you're that concerned about the weight of your tank, put it on a scale instead of trying to calculate what might or might not be inside it.

That's where the 4.4 pounds comes from.

 

[kgilb]

And this thread is now officially...

I really don't understand what Revan's beef is with the BCD. I mean really, who can argue this much against its usefulness unless you have some other agenda. And if 2 lbs can mess up your buoyancy then I don't really know what to say to something like that. Others have already established that you can compensate with your lung volume.

 

[dmaziuk]

I really don't understand what Revan's beef is with the BCD.

He is an underwater swimming champion and wants to be able to leisurely cruise at 4 knots on scuba under his own power. I.e. he has some other agenda and 2 lbs could be significant in that setup.

 

[REVAN]

He is an underwater swimming champion and wants to be able to leisurely cruise at 4 knots on scuba under his own power. I.e. he has some other agenda and 2 lbs could be significant in that setup.

2 pounds would not be significant in that setup. It would not even be noticeable.

I really don't understand what Revan's beef is with the BCD.

I don't personally like BCs for several reasons, none of which particularly apply to this thread.

My message simply inferred that the problem DAN identified with poor weight management skills among the diving population could be improved if divers were initially introduced to diving with a simple kit that did not include a BC. The BC adds a new variable that complicates the process of learning how to avoid overweighting, and as a result, I think, many divers never really learn good skills of weight management. The problem with this idea is that most divers (or at least some very vocal divers) seem to think that diving without a BC is basically impossible to the point that if I bring up the subject it inevitably kicks off an argument with these nay-sayers. I don't know what to do about that.

 

[dmaziuk]

2 pounds would not be significant in that setup. It would not even be noticeable.

I once grabbed a 4lb weight instead of the 3lb one and put it in my shoulder pocket. I spent the entire dive listing like titanic and finning with one leg to try and right myself. I firmly believe that even one pound can be significant, if put in the right place.

 

[KatieMac]

It seems to me that you could achieve the same thing by teaching students to always check their equipment before dives (including weights) and not just trust that the shop will do it for them. Your life after all.

 

[bmbeaty]

The issue isn't that you can't dive without a BC, and I don't think anyone would dispute that diving without a BC simplifies the process in certain aspects, however, it greatly complicates thing in others to the point that very few people would prefer to do without. Eliminating a BC cuts down on one piece of equipment to carry, clean etc and forces people to become masters at controlling their buoyancy through perfect weighting and breath control. Having a BC allows me wear a thick wetsuit and relax at depth by compensating for the compression, eliminates the need to have to breathe shallowly in general, provides a safety flotation device when waiting at the surface, etc, etc... having to carry a 7 lb BC and rinse it off is a small price to pay for all the advantages it offers.

 

[Joneill]

2 pounds would not be significant in that setup. It would not even be noticeable.


I don't personally like BCs for several reasons, none of which particularly apply to this thread.

My message simply inferred that the problem DAN identified with poor weight management skills among the diving population could be improved if divers were initially introduced to diving with a simple kit that did not include a BC. The BC adds a new variable that complicates the process of learning how to avoid overweighting, and as a result, I think, many divers never really learn good skills of weight management. The problem with this idea is that most divers (or at least some very vocal divers) seem to think that diving without a BC is basically impossible to the point that if I bring up the subject it inevitably kicks off an argument with these nay-sayers. I don't know what to do about that.

I don't think that is the problem at all - I believe it is more that a lot of people just don't care enough to improve their technique. In addition, many people don't own their own gear and end up diving a different rig every time and dive a little "heavy" to ensure they can descend - versus taking a little extra time at the beginning and end of their first dives with new gear/gear changes to dial in determine correct weighting.

It's a shame as dialing in proper weight via weight checks at the end of a dive when your air is low is really very easy.

Weight checks are also done via dumping all air out your BC - so I fail to see where training with a BC creates issues with determining proper weighting? It is likely more that many people don't bother or care enough to understand and educate themselves about the benefits of and techniques to achieve ideal weighting. Given that attitude, I'd imagine that throwing those same folks in the deep end without a BC could lead to even more issues!