Exhaust valve setting vs UG thickness

[Jack Hammer]

Presumably, for one's preference for amount of squeeze, which is what lead me down this road in the first place. I understand that I'm sort of asking, "why is the sky blue?" It's the engineer in me, sorry!

The amount of squeeze is controlled by the inflator, not the dump valve. A lot of people dive with a few feet of depth worth of squeeze on the suit so adding more air can loft the Undergarment a tad for more warmth. Once undergarments are fully lofted the water pressure squeezing the suit only allows one to control how big the excess gas bubble at the highest point in the suit gets. The dump valve setting only determines how much a pressure differential is needed to allow excess gas to vent or to fully trap it inside.

 

[Jim Lapenta]

I have two suits with different style valves. The Otter has a dump that you can't press to vent. The USIA has the vent that you can press and no matter where it's adjusted to it will dump. On my Otter I run the dump all the way open and back two - three clicks. On my USIA suit I open it all the way and it dumps so easy that I turn it back a 1/4 turn. Usually I want the suit to dump when I want it to. Not when it decides to. It's just the way the valve behaves. On my old OS suit I couldn't run it all the way open. Just rolling a few degrees and it would vent. I've gotten used to my suits and it's not a big deal to spin the valve a couple clicks during a dive to get it dump a little faster.

 

[inquis]

Diving thicker layers this weekend and looking forward to NOT faffing about with the exhaust valve setting. Thanks again for everyone's thoughts and explanations!

 

[lexvil]

Here’s the bottom line, it needs to be adjusted when it needs to be adjusted, mine is often all the way open unless whatever I’m doing allows it to vent when I don’t want it to in which case I close it down a bit. There is no one answer.

 

[tmassey]

Diving thicker layers this weekend and looking forward to NOT faffing about with the exhaust valve setting. Thanks again for everyone's thoughts and explanations!

Oh, you very well may be messing with the valve. But like @lexvil stated, your messing with it is because of circumstances that are under your control, not just because of the application of PV=nRT.

Inflator all the way open, inflator all the way closed, or something in between: those are personal preferences based on experience, skill, immediate circumstances and balance of risk. And I also understand that our conversation has had nothing to do with that choice, just whether underwear changes the practicality of that choice.

But I will say this: I, personally, am an inflator all the way open person. And I find that in easily 95% of circumstances, I do not have to adjust the valve in order to have a sufficient volume of air in my suit. And that is regardless of the amount or thickness of undergarments that I am wearing. That is also as a person who hates cold and therefore might run a little more air in his suit than someone else would.

The only time that I lose gas that I have intentionally put into my suit is when I get out of trim. If I need to go more head up, in a couple of minutes I might notice that my suit is less filled than it was, and then I have to add some more gas. That, too, is not overly affected by how much underwear I have. It really is just a function of the effect of water pressure relative to orientation in the water. And the more consistent your trim is, the more consistent the gas bubble in your suit will be.

Given that all of this is on your mind, really pay attention on your next dive to how much volume you can get into your suit if you maintain the best horizontal trim you can. Depending on how horizontal that is, you may find that you can put a shocking amount of gas in your suit, even when the valve is 100% open. Now, if that is not very horizontal, you may find out how fast all of the gas in your suit is pushed out. So, for your diving position, you may need to crank the valve down some in order to keep the gas in, whereas a diver in a more consistently and strongly horizontal trim might not.

And that is all a function of position.

I’d love to hear what you discover in this next dive, and see if it gives you a better insight into the relationship between trim and dry suit volume. No biggie if you don’t want to, but it’s always interesting to see how people experience different concepts for the first time. You can only have one first time.

 

[inquis]

OK, back from the dive with thicker layers, and things even make sense to the engineer in me. First of all, I was trying to keep things focused on the garment thickness alone by assuming identical trim and depth. I appreciate the helpful spirit, but many responses focused on differences introduced by factors other than undergarment thickness. I don't disagree with those (e.g., changing from horizontal trim to vertical trim will vent air when the exhaust is at the horizontal critical pressure point).

In summary: the valve sees the pressure of the air within the suit as everyone had said. My earlier reasoning leading to a constant suit volume was total BS, because the total suit volume absolutely doesn't stay the same. A simpler example is a 200 lb person vs. a 175 lb person in the same suit, adding air until the vent just starts to exhaust in each case. The pressure on the vent is the same (since the setting is the same), but the total suit volume (i.e., the water displaced) is quite different.

This statement clenched it in my mind:

Once undergarments are fully lofted the water pressure squeezing the suit only allows one to control how big the excess gas bubble at the highest point in the suit gets.

Intuitively, the amount of air it takes to fully loft different thicknesses of undergarments is different. I felt like the same exhaust setting I used with a thin layer was insufficient to fully loft the thicker layer, and I was warmer after turning it up a couple clicks. (In hindsight, I may have even been running a little low with the thinner layers, but didn't notice because it was "enough" for the conditions at the time.) No big deal to adjust during the dive -- as many of you also said. After a slight ascent, I wanted a smaller bubble, so I opened it back up slightly.
"Preference" as has been said. Again, thank you all for your comments and thoughts.