DAN advocating using drysuit for buoyancy control while diving

[kwinter]

Ken, I use the suit to control trim all the time, being able to pack air in your feet is one of the wonders of drysuits, but I am bloody miserable if I have too much air in my legs and have to move around. I'm not saying you should only put enough air in to offset the squeeze, that is practical for warm ish dives, but once you're below 60f or so you need more air in their for thermal protection. Put enough in until your comfortable and take up the slack with the wing. I disagree with the statement, but for a non-training agency to be giving training advice is my issue. If they had stated, "put as much air in the suit as you need to stay comfortable" that would have been fine, but with the ever progressing amount of high volume steel bottles in the recreational market, 6lbs in al80 or LP72 is a lot different than 11lbs in a 130.

In that context, I will agree with your objection. It is also important to note that rebreather divers do not see the large swing in tank volume that OC divers do unless there is a bailout situation. That can affect the method preferred as well since there is a smaller volume of gas for buoyancy maintenance.

I dive with an HP120 and see almost a 9 lb swing from full to 'on-the-boat' That is a lot of air in a suit, no thank you.

This one doesn't make a whole lot of sense to me. You are going to be venting gas as you breathe your tank down, not inflating. Are you suggesting that you would have to start your dive with too much air in your suit because of an extra few pounds of gas in your tank? You are still going to be adding gas as you descend. You just might need less than others.

 

[Storker]

a number of the above posts refer to/say "if your properly weighted"
what is the method you use to achieve proper weighting for your style of drysuit diving

Start with a decent weight guesstimate, dive. Take off a little weight, dive. Repeat until I'm uncomforably shrink-wrapped at my safety stop and have some problems staying down. Add a couple of kilos or so. Dive. Fine-tune.


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---------- Post added March 26th, 2015 at 05:33 AM ----------

Are you suggesting that you would have to start your dive with too much air in your suit because of an extra few pounds of gas in your tank? You are still going to be adding gas as you descend. You just might need less than others.

Au contraire.

At the start of the dive, you're "overweighted" by the weight of the gas you're carrying. The more gas you're carrying, the more "overweighted" you are, and the more air you need to add to your buoyancy control device (wing, suit or both) at the beginning of the dive.

Which is why I fill air in my wing immediately after descending and then gradually vent it during the dive and have an almost empty wing at the end of my dive. The more gas I carry, the more air is added to the wing and gradually vented during the dive.


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[HungoverDiver]

I'm assuming you did take an open water course.

Boyle's law - Wikipedia, the free encyclopedia

Volume is decreased by 1/2 as density increases X2 @ 2ATA. Volume is 1/3rd and density X3 @ 3ATA, etc. So NO, the volume is not the same @ depth vs. the surface.

I did complete OW, and 4 years of university chemistry and my PhD. Hopefully that's enough to convince you to actually read what I wrote. To rephrase my previous point: the volume of the container, the space around you in your drysuit to alleviate squeeze, is the same at depth as it is at the surface. The amount of gas you have to inject to get this same volume is different at depth than it is at the surface, and is dependent on how deep you are.

And yes, if you have a fixed amount of gas and you double the pressure, the volume that gas takes up is now half. Which is a separate question all together since with drysuit physics, we generally aren't talking about a fixed amount of gas.

 

[Storker]

I'm assuming you did take an open water course.

Boyle's law - Wikipedia, the free encyclopedia

Volume is decreased by 1/2 as density increases X2 @ 2ATA. Volume is 1/3rd and density X3 @ 3ATA, etc. So NO, the volume is not the same @ depth vs. the surface.

Boyle's law is only relevant here in that it's a special case of the idealized gas law, PV=nRT. We keep the volume constant, so we vary the amount (moles, grams, or volume AT STANDARD TEMP AND PRESSURE) with pressure.

If you want to be snarky, you had better be right. You're wrong.


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[ScubaInChicago]

I did complete OW, and 4 years of university chemistry and my PhD. Hopefully that's enough to convince you to actually read what I wrote. To rephrase my previous point: the volume of the container, the space around you in your drysuit to alleviate squeeze, is the same at depth as it is at the surface. The amount of gas you have to inject to get this same volume is different at depth than it is at the surface, and is dependent on how deep you are.

And yes, if you have a fixed amount of gas and you double the pressure, the volume that gas takes up is now half. Which is a separate question all together since with drysuit physics, we generally aren't talking about a fixed amount of gas.

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

 

[Storker]

Like I said upthread: if you want to be snarky, make sure you're right.

And another piece of advice: when you're standing in a hole, it might be a good idea to stop digging.


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[ScubaInChicago]

Boyle's law is only relevant here in that it's a special case of the idealized gas law, PV=nRT. We keep the volume constant, so we vary the amount (moles, grams, or volume AT STANDARD TEMP AND PRESSURE) with pressure.

If you want to be snarky, you had better be right. You're wrong.


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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

 

[Storker]

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.

*sigh*

No.

R is the only constant in the ideal gas law (it's approximately 8.314 J/mol·K), all the others are variables which may or may not change depending on your actions. To keep the squeeze off as P increases, we need to keep V constant. We do that by increasing n (adding air to the suit). To avoid runaway ascents, we need to keep V constant as P decreases. We do that by decreasing n (venting the suit).

Re-read the chapter on gas behavior in your OW book. It's pretty low level, but at least it's fairly correct. Look again at the pictures showing the air molecules as small "balls". n is the number of "balls". They may be close together (P high), which means you need more of them to take up the same space (V). Or they may be further apart (P low), which means you need less of them to take up the same space (V). But in a DS we try to keep V constant regardless of pressure.

 

[ScubaInChicago]

So your saying we don't add a volume of gas to our drysuits to equalize the pressure exerted by atmospheric pressures?

 

[Storker]

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. You're also quite happy you've got a hard bottom beneath you, since you're quite negatively buoyant. 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. PV=nRT. Ideal gas law.


Your undergarments are again comfortably lofted, you can move freely, you are neutrally buoyant, 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.