Exhaust valve setting vs UG thickness

[inquis]

For those who use air in the drysuit for warmth (i.e., wing for buoyancy), it's apparent from other threads that most run the shoulder dump open (or close to open) and leave it alone. This works well for me, but I need to do a colder dive soon. I think I will have to close the valve a bit for comparable feeling/squeeze when wearing the substantially thicker undergarments, but can anyone confirm?

 

[Alekseolsen]

For those who use air in the drysuit for warmth (i.e., wing for buoyancy), it's apparent from other threads that most run the shoulder dump open (or close to open) and leave it alone. This works well for me, but I need to do a colder dive soon. I think I will have to close the valve a bit for comparable feeling/squeeze when wearing the substantially thicker undergarments, but can anyone confirm?

I dive thick undergarments year-around. Squeeze-wise, there's no need to adjust the valve, but I do seem to have a bit more air in the suit when it's cold.

 

[Degenerate]

I dive in cold waters year round and use my suit for buoyancy and my valve is always fully open, never any issues.

 

[Jack Hammer]

I dive regularly in cold waters (39f/4c), I leave the dump valve fully open. No issues keeping air in suit. Also with thick undergarments there is quite a bit more gas to dump on ascent. The more closed the valve is the easier it can be for trapped air to quickly become a potential issue.

 

[inquis]

Thanks for the replies, everyone. I'll trust that I don't have to muck with the valve when I change undergarment thickness (so immediate question answered).

If anyone wants to take a shot at explaining why that is the case, I'm clearly looking at this the wrong way to understand. Does that valve position not dictate the total volume of the suit? Wouldn't the volume that is sufficient for me+thin layers be insufficient for me+thick layers? Without additional total volume, how do the thicker layers "fit"?

 

[tmassey]

I’ll try.

The exhaust valve has *zero* idea of the *volume* of gas behind it. It only knows *pressure*: it releases air above a certain pressure level. Even with the valve all the way open, it still requires a certain amount of pressure above ambient for the gas to leave. This is obviously greater than zero, otherwise there’d be no air in the suit no matter what underwear you had!

Obviously, the more air you add, the more the pressure increases. However, there are a couple of items that work to counteract that. One is your position. Go head up vertical and now your feet are under much greater external pressure (they’re now +/- 5 *feet* deeper than the exhaust valve on your shoulder: that’s roughly 2.5 PSI right there). That moves all the gas up to the valve on your shoulders, and so now it’s at a higher pressure — so it vents. Go head down vertical, and the reverse is true: You could add a *ton* more gas, which would simply fill up your feet, and *not* vent. Of course, underwear doesn’t affect your position in the water, but it shows that the volume of gas you add to your suit does not solely determine the pressure. How much your suit is squeezed from external pressure can change much more.

Another item is the thickness and density of your underwear. For example, if your underwear had a hard shell, or if it had little sticks bristling up from your body, it would prevent your suit from being squeezed down. In that case, the extra air wouldn’t be squeezed by the external pressure (it would be stopped by the hard shell or sticks), and it wouldn’t exhaust out.

Of course, underwear doesn’t have a shell or little beams holding your suit out. But it does have some level of resistance to being squished. The more resistance it has, the more air that will be trapped and contained within it at exactly the same exhaust valve setting. Which is why you should be able to add a greater volume of air with thicker underwear without increasing the pressure relative to the thinner underwear.

At least, that’s my understanding!

 

[Jack Hammer]

I’ll try.

The exhaust valve has *zero* idea of the *volume* of gas behind it. It only knows *pressure*: it releases air above a certain pressure level. Even with the valve all the way open, it still requires a certain amount of pressure above ambient for the gas to leave. This is obviously greater than zero, otherwise there’d be no air in the suit no matter what underwear you had!

Obviously, the more air you add, the more the pressure increases. However, there are a couple of items that work to counteract that. One is your position. Go head up vertical and now your feet are under much greater external pressure (they’re now +/- 5 *feet* deeper than the exhaust valve on your shoulder: that’s roughly 2.5 PSI right there). That moves all the gas up to the valve on your shoulders, and so now it’s at a higher pressure — so it vents. Go head down vertical, and the reverse is true: You could add a *ton* more gas, which would simply fill up your feet, and *not* vent. Of course, underwear doesn’t affect your position in the water, but it shows that the volume of gas you add to your suit does not solely determine the pressure. How much your suit is squeezed from external pressure can change much more.

Another item is the thickness and density of your underwear. For example, if your underwear had a hard shell, or if it had little sticks bristling up from your body, it would prevent your suit from being squeezed down. In that case, the extra air wouldn’t be squeezed by the external pressure (it would be stopped by the hard shell or sticks), and it wouldn’t exhaust out.

Of course, underwear doesn’t have a shell or little beams holding your suit out. But it does have some level of resistance to being squished. The more resistance it has, the more air that will be trapped and contained within it at exactly the same exhaust valve setting. Which is why you should be able to add a greater volume of air with thicker underwear without increasing the pressure relative to the thinner underwear.

At least, that’s my understanding!

Pretty good explanation.

OP, In simple terms a fully loosened valve allows gas to vent easily, a fully tightened valve prevents gas from venting at all. Its adjustable for personal preference.

 

[inquis]

The exhaust valve has *zero* idea of the *volume* of gas behind it. It only knows *pressure*

Sure, but with a non-rigid "container" (i.e., the drysuit), it's the force of the surrounding water that determines that pressure. Assuming you don't change depth or trim, that force is constant. Since pressure is force per area, pressure will be constant as long as the surface area is constant. Constant surface area mandates that the volume remains unchanged. That's why when you tap your suit inflator when the dump valve is on the verge of exhaust, the additional gas vents and you don't rise in the water. (Yes, that assumes an ideal situation -- small amount of air added, the delay between adding and exhaust is not an issue, no hysteresis in the valve, etc.)

Now, since pressure is constant, the ratio of gas (n) and volume (v) occupied by that gas in the suit must remain constant (n1/v1 = n2/v2). Further, thicker underlayers have greater volume (assuming the material density is the same). Therefore, if I were to magically (bear with me) change underlayers at depth, the volume inside the suit for air is necessarily reduced. Since n2/v2 must remain constant, n2 must therefore decrease. Magically changing underlayers at depth should cause air to vent (i.e., more squeeze).

Can anyone point out an error in the above line of thinking? If not, then perhaps the amount one would have to adjust the valve to keep the perception of squeeze "the same" is small enough to ignore. Again, everyone's experience is consistent in this regard, so I'm happy I don't have to worry about it while diving!

 

[inquis]

Its adjustable for personal preference.

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!

 

[tmassey]

You are on your own. You’re 50 feet deep in one tiny area of the logic I outlined, and paperthin in all the others. You have one mathematical equation that you’re focused on, forgetting all of the other elements that I outlined. In fact, your reasoning is very similar to a paper that starts with: imagine human beings as frictionless spheres.

I already gave you an example where the volume will change — it would actually increase, yet the pressure can be the same, because the exhaust valve is still the same. Therefore, there will be a greater volume of air inside the suit. Again, the pressure is still the same, because it would be created by exactly the same valve setting.

Warmer underwear is thicker. It therefore takes more volume. It also resists the pressure of the outside water. Therefore, it absorbs the pressure of the water, transmitting less of the pressure to the environment around it. Just like a spring. Therefore, it allows a larger volume of air yet still creates the same internal pressure.

remember: the pressure is in pounds per square inch. I still think you are stuck on volume not pressure. Because the thicker underwear has increased the volume, you can have more gas, therefore staying warmer, yet still have the same internal pressure caused by the exact same valve setting.

The other thing is: I can have dramatically different volumes of gas in my suit for exactly the same valve setting. This is not a precise mathematical equation conducted upon frictionless spheres. All of the fine details matter: density of the underwear, volume of the underwear, amount of air added to the suit, the physical position that air occupies, and especially the position and orientation of the exhaust valve.

you’re not asking why the sky is blue. You’re arguing about why one mathematical equation relevant for Raleigh light scattering doesn’t match the exact shade of blue that you’re expecting in the sky, when you’re completely ignoring the presence of things like, say, clouds.

in other words, real life will not distill down to simplistic PV= nRT. There’s a reason that’s called the ideal gas law. Our world is far from ideal: tiny details make a huge differences.

if that’s not enough for you, someone else will have to wade into this. And in the end, none of this will matter one wit to actual diving. The real world experience of millions of dry suit divers shows that the rotation of our exhaust valve is one of the least interesting requirements of how much gas we can keep in and manipulate within our dry suits.

ETA: One more thing I thought to add. Maybe you’re missing this: it’s not just air pressure that determines the volume inside the suit. Why do you think we’re wearing different sizes and thicknesses of underwear? If we only depended upon air pressure alone to determine the volume of air in our suit, we could dive completely naked in our suits and have as much warmth as we wanted, simply by increasing the air pressure (and therefore the volume, and therefore the thickness of the insulating layer). But that doesn’t work for a zillion different reasons, not least of which the air would be so difficult to control due to pressure differentials that it would just never work.

That’s why we wear the underwear in the first place: to resist the external pressure using something other than air pressure — the density of the suit – and to help evenly spread that extra air volume around our entire bodies, even the parts that are experiencing greater external pressure because they might be at a lower depth.