DAN advocating using drysuit for buoyancy control while diving

[ScubaInChicago]

Will you lay off the gotchas? Of course we add "a volume" of air. Any amount of gas takes up a volume. Now, let me try once more before giving up on you and assume that you're just trolling:

Imagine you're diving a DS. You're on the surface. Your undergarments are comfortably lofted, but you don't have excessive air in your suit. The volume of that air is V.

Now descend to 10m/33ft without adding air to the suit. P has doubled, so V is half of what it was on the surface. Boyle's law.

You're uncomfortably squeezed, cold as Hel and you have problems reaching your chest inflator because you're shrink-wrapped in your trilam suit. All because V is now reduced to V/2. It's not constant any more. Boyle's law.

You manage to reach the inflator, and you add air to your suit. That air of course has a volume. So yes, you add "a volume of gas" to your DS. You add just enough to increase the volume of air inside your suit back to V.

Your undergarments are comfortably lofted, but you don't have excessive air in your suit. V on the surface, V at 10m/33ft. Constant volume. But the amount of air molecules taking up V is now the double. Double pressure, double the amount of air, constant volume. PV=nRT. Ideal gas law.



I'll leave it up to the student to work out what happens on ascent. Hint: To avoid a runaway ascent you want constant buoyancy. Since your weight is practically constant, what do you want your volume to be? The same, i.e. constant. Constant volume.

So if we need a volume of .25 cf of gas to be comfortable at the surface, we would need a volume equal to .5 cf @ 2 ATAs, and .75 cf @ 3 ATAs. That in no way shape or form sounds like a constant volume when we're adding additional volume on decent. PV=nRT sounds great in a closed system which a drysuit is not.

 

[Redshift]

The problem here is that it became standard to use "volume of gas" as a way to sometimes quantify a volume, other times referring to a quantity of gas... and this can only be done if at the same time other variables of the ideal gas law are specified.
Gas cannot be used to define a volume, because it's compressible. When I say I have a 12l cylinder I have no idea how much gas is inside, but the volume will always be 12l. But then some calculations are done like this "a 12l cylinder filled at 200 bar has 2400 l of gas". No, the gas that at 200 bar occupies a volume of 12l, will occupy 2400l at ambient pressure.

The dry suit has a physical volume. To maintain it, it needs a certain quantity of gas inside and that quantity is measured in number of molecules (or moles because the number is enormous). When going deeper, the molecules get closer together and the volume decreases. Hence we need to add molecules that will increase the volume to the original one.
The volume at which a diver is comfortable in a dry suit (or, if we want to remove any subjectivity, we can think of the max volume of the suit) on the surface and at any depth is the same. The amount of gas inside is not. Amount of gas is measured in moles, not in litres or cubic feet or whatever... But in diving we don't say it like this (because there are underlying assumptions) and usually use a volume at the surface as an indication of quantity.

You could have fooled me. The only "volume" of anything that is going to take the "squeeze" off would be measured in gas volume. Maybe reading a simple definition of volume would help: Volume - Wikipedia, the free encyclopedia

What's the volume of a gas?
A container's volume is given by it's physical dimensions. A gas can be squeezed into a variety of containers and will occupy their volume. Volume is not an intrinsic property of the gas, it only means something when pressure and temperature are specified.

So what you're saying is to keep PV=nRT as it equates to keeping the "squeeze" off, we need to add an additional volume to keep a constant. Otherwise, the constant doesn't stay constant. Hmmm

So, to answer to this, no. To keep a constant volume, we add "n".

Imagine a diver as being a cylinder. Volume is area of the base (a circle) x height.
If you measure the height of a diver and its circumference at 1m, 10m, 50m... and the diver is equally comfortable in terms of squeeze, these two measurements will be the same, the volume of diver and suit is the same. The quantity of gas is different. And the volume that that quantity of gas would occupy on the surface is different.

 

[ScubaInChicago]

The problem here is that it became standard to use "volume of gas" as a way to sometimes quantify a volume, other times referring to a quantity of gas... and this can only be done if at the same time other variables of the ideal gas law are specified.
Gas cannot be used to define a volume, because it's compressible. When I say I have a 12l cylinder I have no idea how much gas is inside, but the volume will always be 12l. But then some calculations are done like this "a 12l cylinder filled at 200 bar has 2400 l of gas". No, the gas that at 200 bar occupies a volume of 12l, will occupy 2400l at ambient pressure.

The dry suit has a physical volume. To maintain it, it needs a certain quantity of gas inside and that quantity is measured in number of molecules (or moles because the number is enormous). When going deeper, the molecules get closer together and the volume decreases. Hence we need to add molecules that will increase the volume to the original one.
The volume at which a diver is comfortable in a dry suit (or, if we want to remove any subjectivity, we can think of the max volume of the suit) on the surface and at any depth is the same. The amount of gas inside is not. Amount of gas is measured in moles, not in litres or cubic feet or whatever... But in diving we don't say it like this (because there are underlying assumptions) and usually use a volume at the surface as an indication of quantity.



What's the volume of a gas?
A container's volume is given by it's physical dimensions. A gas can be squeezed into a variety of containers and will occupy their volume. Volume is not an intrinsic property of the gas, it only means something when pressure and temperature are specified.



So, to answer to this, no. To keep a constant volume, we add "n".

Imagine a diver as being a cylinder. Volume is area of the base (a circle) x height.
If you measure the height of a diver and its circumference at 1m, 10m, 50m... and the diver is equally comfortable in terms of squeeze, these two measurements will be the same, the volume of diver and suit is the same. The quantity of gas is different. And the volume that that quantity of gas would occupy on the surface is different.

Thank you, I understand what your saying, but to clarify if PV=nRT and we increase n, then we've changed the equation for (V)volume which in reality equalized the suit pressure(P) to ambient.

If the above is true, then do we not need more volume of (compressible) gas in our suits to offset the increased atmospheric pressures?

 

[Redshift]

So if we need a volume of .25 cf of gas to be comfortable at the surface, we would need a volume equal to .5 cf @ 2 ATAs, and .75 cf @ 3 ATAs. That in no way shape or form sounds like a constant volume when we're adding additional volume on decent. PV=nRT sounds great in a closed system which a drysuit is not.

If you need 0.25 cf of gas at 1 ATA, then at 10 m (2 ATA) you need the same space surrounding you in your suit. That means 0.25 cf @ 2 ATA, which is equivalent to 0.5 cf on the surface. You have more "volume of gas at surface pressure", which at depth translates into the same volume you had initially.

When you say "we would need a volume equal to .5 cf @ 2 ATAs", what you are really saying is "we would need a volume equal to .5 cf (measured on the surface @ 1 ATA), when we are @ 2 ATAs to have the same physical volume"

On the surface you have 0.25 cf. At 2 ATA, that became 0.125 cf. You then bring 0.25 cf from the surface (imagine a balloon), that when it reaches 2 ATA has a volume of 0.125. You move that gas to your suit and you are again at the comfortable 0.25 cf.
This is what's being done when people talk about volumes to specify amounts of gas, but there is an intermediate step of having compressed gas in a cylinder and details left out (such as using volume without saying where this volume is being measured) that distort the picture of what's happening and confuse some people.

---------- Post added March 26th, 2015 at 12:45 PM ----------

Thank you, I understand what your saying, but to clarify if PV=nRT and we increase n, then we've changed the equation for (V)volume which in reality equalized the suit pressure(P) to ambient.

If the above is true, then do we not need more volume of (compressible) gas in our suits to offset the increased atmospheric pressures?

I think what I have just posted addresses this, but I'll reply to it directly.

We need more gas, as in molecules. That means more volume at surface pressure, that at depth will equate to the same physical volume inside our suit.

When you say that we need more volume in our suit, you are actually referring to surface pressure. That element is often left out, but it's crucial to understand what's going on.
So, yes you need more volume of gas measured at 1 ATA, but once you bring it to depth, it will occupy the same volume you had in your suit, the same volume that is necessary for you to keep comfortable.

 

[Storker]

The problem here is that it became standard to use "volume of gas" as a way to sometimes quantify a volume, other times referring to a quantity of gas... [...]

in diving we don't say it like this (because there are underlying assumptions) and usually use a volume at the surface as an indication of quantity.

Well, those who dive in imperial units do that. We who use metric units usually specify tank volume (and pressure, if the situation is ambiguous). Al80, HP80, HP120 (air volume at surface pressure) vs 11L200bar, 10L232bar, 15L232bar (tank volume and pressure).
:beer:

 

[Redshift]

Those tank measurements drive me crazy. Have no idea what they are talking about

But we (metric people) do similar things sometimes, such as in exercises where there is a 5 Kg weight at 20 m (ignoring volume of weight) with a lift bag and we are asked how much air do we need to raise it to the surface. The simple answer would be 5l The answer they usually want is 5l of air @ 20 m, where the pressure is 3 bar and thus that volume is equal to 3x5 = 15l, also omitting the "of air when measured at normal temperature and pressure".

 

[ScubaInChicago]

I would like to thank Redshift for going over this with me. It's been going around in circles basically arguing the same thing. I'll recap it in a way I hope everyone understands (and please correct me where I'm wrong).

PV=nRT

P=pressure
V=volume
n=moles
R=a relatively constant # that we'll keep out of this discussion.
T=temp in K (we'll assume this doesn't change for this discussion).

Were about to start our dry suit dive and stretch out and inflate our drysuit to where it is comfortable.

At the surface with 1ATA

Imperial 14.7(P) .5cf(V)=nRT
Metric 1bar(P) 5L(V)=nRT

We descend to 10m/33' or two ATAs

Imperial 29.4(P) .25cf(V)=nRT
Metric 2bar(P) 2.5L(V)=nRT

At this point PV=nRT is still a constant in a closed system but we are feeling a squeeze. We want to return V to our happy place and add gas to our drysuit (not a closed system anymore) so we double n which will also double V.

Imperial 29.4(P) .5cf(V)=2n RT
Metric 2bar(P) 5L(V)=2n RT


On our return trip to the surface we need to vent to keep or else we would end up like this:

Imperial 14.7(P) 1cf(V)=nRT
Metric 1bar(P) 10L(V)=nRT

I hope this helps with any confusion.

 

[Storker]

I hope this helps with any confusion.

Yes, it does seem as if it helped you. Nice to see that you've understood the concept we've been trying to hammer into you since the last ~15 posts and which I explained in minute detail in post #50.

Maybe it's appropriate to apologize to HungoverDiver for your snark and condescension (posts #39 & #45) now?

 

[ScubaInChicago]

Yes, it does seem as if it helped you. Nice to see that you've understood the concept we've been trying to hammer into you since the last ~15 posts and which I explained in minute detail in post #50.

Maybe it's appropriate to apologize to HungoverDiver for your snark and condescension (posts #39 & #45) now?

Not sure if you noticed, but the volume did change throughout the dive which was my argument all along. Not that you did anything to explain it any better.

 

[Storker]

Not sure if you noticed,

Sure did. It was the same stepwise changes I used in my example, to illustrate the point.

but the volume did change throughout the dive

Because in your example you waited until you've reached 10m to inflate your DS. If you'd dived sensibly, you'd continually inflate and vent the suit to keep your suit's volume (and thus your buoyancy and squeeze) - wait for it...


wait...



wait....



*drum roll*



Constant.