Silly question about dry suit air usage

[sabbath999]

Is the amount of air that one normally uses to prevent squeeze in a dry suit noticeable or negligible when it comes to gas management?

Do I need to factor that it, at all, when calculating my rock bottom pressure?

 

[MSilvia]

Oh yeah! It's very noticeable. That's a big part of the reason a lot of dry divers use a separate tank just for suit inflation.

 

[fisheater]

How much do you factor in for your BC when you are diving wet?

 

[sabbath999]

How much do you factor in for your BC when you are diving wet?

None. Didn't think about it before.

Now I will.

 

[Bubbletrubble]

Do I need to factor that it, at all, when calculating my rock bottom pressure?

I thought that Rock Bottom calculation generated a minimum PSI number on your SPG which would allow you and your buddy to safely ascend to the surface (including safety/deco stops and allowing for an increased "stress" respiration rate).

If a drysuit diver is properly managing his ascent, he should not have to add any air to his drysuit. He should be exhausting as he ascends, right? That being said, I have on occasion (especially when I first got the drysuit) been a little aggressive with exhausting and had to add a touch of air to the drysuit to offset squeeze...but this is only a negligible amount of air.

If you are being really anal, you could factor in some increased gas usage into the other phases of your dive (particularly the descent) but this should be a minimal amount. I will say this, though. I found that once I became accustomed to diving dry, my overall SAC/RMV dropped significantly. A warm diver is a comfortable diver. A comfortable diver doesn't burn through his air like he would if he were shivering in a wetsuit. YMMV.

 

[sschlesi]

If you are new to a dry suit, your SAC is bound to go up initially. If you monitor your new SAC you will automatically include suit inflation gas (assuming you use back gas) in your new SAC calcs.

My off the cuff analysis says that your suit should require little to no additional gas once you reach your rock bottom pressure. At rock bottom you should start ascending, and therefore won't need to add more gas to your suit. I would expect the elevated SAC level used to compute rock bottom should have enough leeway for the little gas that may be required for suit inflation if you encounter an emergency when you reach your rock bottom pressure.

 

[paddler3d]

I always thought the reason dry suit divers carried argon in a dedicated bottle was because some gas mixes react with sweat and can create an acidic solution in the drysuit possibly leading to skin irritation? I may be making that up.

Recalculating you SAC with your dry suit should give you the answer you seek.

 

[Rainer]

It's TOTALLY meaningless. Totally. I can get 3-4 looong recreational dives off my AL6 inflation bottle. That's <2cf for the dry suit per dive. MEANINGLESS from a gas management perspective.

 

[Bubbletrubble]

I always thought the reason dry suit divers carried argon in a dedicated bottle was because some gas mixes react with sweat and can create an acidic solution in the drysuit possibly leading to skin irritation? I may be making that up.

I thought argon was used for drysuit gas inflation since it was thought to have better insulative properties than regular air. AFAIK, this hasn't been proven by any scientific study, but anecdotal reports support the notion.

The helium gas present in trimix has higher thermal conductivity than either argon or regular air. For tech divers using helium, I think a stronger case can be made to have a dedicated drysuit inflation bottle.

But, then again, I'm not a tech diver using helium mix, I don't use argon, and I don't have a dedicated drysuit inflation bottle. What do I know?

 

[boulderjohn]

IMO, it really doesn't matter.

There is really very little going into your dry suit unless you are really sawtoothing (going up and down a lot so that you have to keep refilling the suit after it vents.)

If you are determining your sac rate based on what happens when you dive, then that will become part of the calculations anyway.

As has been noted, you will be venting your air during the ascent, so you don't need to account for it.

BTW, if you are properly weighted while diving wet, you will use very little air in your BCD as well. People who orally inflate their BCD while diving in order to extend their bottom times are probably getting a few seconds at best.

All of this is a crude estimate at best--your average sac rate on a particular dive will very from your average, sometimes by a lot, depending upon the conditions and your activity.