Drysuit vs BCD bouyancy control

[Lightning Fish]

I'm pretty sure the Nerd-O-Meter is fully pegged for this thread now

Yeah... As soon as I get my glasses duct-taped up, I think I'll read a few chapters of White's "Viscous Fluid Flow".

 

[JeffG]

Yeah... As soon as I get my glasses duct-taped up, I think I'll read a few chapters of White's "Viscous Fluid Flow".

I was going to ask and see if everyone knew by memory the G Constant in Newtons gravitational equations. I would guess that everyone would say yes. (my brain is foggy about stuff like this...I was trying to forget, but the number 1.667 x 10^-8 is bouncing around my memory...now to search google and find out how close I was)

 

[JeffG]

I was going to ask and see if everyone knew by memory the G Constant in Newtons gravitational equations. I would guess that everyone would say yes. (my brain is foggy about stuff like this...I was trying to forget, but the number 1.667 x 10^-8 is bouncing around my memory...now to search google and find out how close I was)

My nerd factor is safe. Its G = 6.7 × 10^-11 m3·kg-1·s-2.


Whewwww...that was close

 

[Daryl Morse]

The gas in both the drysuit and the BCD are under the same ambient pressure. The only way there could be a difference is if you create a situation where your BC or Drysuit becomes a pressurized vessel (ie filling it beyond ambient pressure.)

I'm with Jeff on this. Irrespective of bungeed wings and stretchy neoprene drysuits, this has nothing to do with pressure vessels or the ideal gas law.

A BCD and a drysuit are both "bags" that can contain gas. It doesn't matter whether you have one bag or two bags or ten bags, if you have a fixed amount of gas, under a constant pressure, it will displace a fixed amount of water, and thus generate a fixed amount of lift.

As for a bungeed wing, since you can inflate it orally, the pressure required to expand the bungee cords is so small that it's irrelevant.

As for a neoprene drysuit, if you're inflating it so much that it has to stretch, you likely have bigger things to worry about, because you're probably going to be taking an uncontrolled trip to the surface.

(In spite of the above, I still prefer using the BCD for buoyancy, not the drysuit.)

 

[Diver0001]

I'm pretty sure the Nerd-O-Meter is fully pegged for this thread now

ROTFLMAO....

Indeed. I know what you mean. Lightning Fish is a close friend of mine and I can say with some confidence that he invented the nerd-o-meter. He's also the smartest person I know and he's just rounding off his PhD in physics engineering (the very basics of which most people can't even begin to fathom). When he said he had read something about this stuff you have no idea...... I'm sure he's forgotten more about physics than I ever learned.

So....given that I only understand these kinds of things on square-boxes-on-a-white-board level let met get to the crux of the point he's trying to make (sorry, Bill. someone dumber than you has to explain this.... )

Most of us believe that containers are either flexible or rigid. It's either a scuba tank or a BCD. What Lightning Fish is saying is that there is a continuum between totally rigid and toally flexible and that, while a drysuit is a flexible container like a BCD, that a drysuit is stiffer and therefore a "relatively more rigid" container than a BCD.

So what's the big deal?

On a typical dive you're using 20-40 litres of air for buoyancy with either the drysuit or the BCD or a combination of both. With an AL-80 you're talking about +/-150 psi in total pressure. If your drysuit is 15% less efficient because it's stiffer than a BCD then you're talking about a difference in psi of 20-25 psi.

So in other words, if Bill is right, then using your drysuit for buoyancy will increase your air consumption. The difference is, of course, academic (in a manner of speaking) but still interesting as a side line to this thread.

Did I get it mostly right, Bill?

Rob

 

[wcl]

ROTFLMAO....

Indeed. I know what you mean. Lightning Fish is a close friend of mine and I can say with some confidence that he invented the nerd-o-meter. He's also the smartest person I know and he's just rounding off his PhD in physics engineering (the very basics of which most people can't even begin to fathom). When he said he had read something about this stuff you have no idea...... I'm sure he's forgotten more about physics than I ever learned.

Rob

Oh yeah, the nerdmeter is pegged ... it's a bit oxidized from long-term storage (changed careers), but I have my PhD in experimental high energy physics in the closet somewhere ...

I'm sure we can have jolly old time deconstructing the simplest act in diving! :laugh:

-- Walter

 

[Canadian_Diver]

Now, I'm wondering how we can apply all this theory to "off-gassing" in your drysuit on ascent.... it could save the many innocent lives of those unzipping our suits after the dive....

 

[lairdb]

Now, I'm wondering how we can apply all this theory to "off-gassing" in your drysuit on ascent.... it could save the many innocent lives of those unzipping our suits after the dive....

I believe this would likely lead to a discussion of whether the body is a flexible or rigid container. Let's not go there.

 

[Diver0001]

Oh yeah, the nerdmeter is pegged ... it's a bit oxidized from long-term storage (changed careers), but I have my PhD in experimental high energy physics in the closet somewhere ...

I'm sure we can have jolly old time deconstructing the simplest act in diving! :laugh:

-- Walter

Ding Ding Ding Ding Ding !!! We have a winner! Walter has a PhD in something even more mysterious than physics engineering!

So, while we're on the subject, Jeff, what's your gig?

R..

 

[Lightning Fish]

ROTFLMAO....

Indeed. I know what you mean. Lightning Fish is a close friend of mine and I can say with some confidence that he invented the nerd-o-meter.

Thanks! :confused2

... his PhD in physics engineering

Mechanical engineering now, eng. phys. is from years and years ago.

So in other words, if Bill is right, then using your drysuit for buoyancy will increase your air consumption. The difference is, of course, academic (in a manner of speaking) but still interesting as a side line to this thread.

Did I get it mostly right, Bill?

Rob

Not bad, I like what you've said about rigid and semi-rigid. I think there is a difference between the two bouyancy devices, even though it may be insignificant, but I also think my argument is going in a circle.

Bill.