Cold water dive; Air expansion question

[Gombessa]

Also, I usually keep some air in my calf area for trim and the ascent came immediately after I went vertical to move the bubble to my chest again.

In addition to what's already been said here, when you are horizontal, the gas in your suit is fairly evenly distributed over a wide area, and you're presenting a larger profile against the water column that helps to buffer against quick buoyancy changes.

When you go vertical, it's not just the gas by your calves that goes to your head, it's pretty much all free the gas in the suit (you immediately feel the squeeze increase in your legs and lower torso as the bubble migrates to your chest and shoulders). That, in addition to the fact that you're suddenly presenting a smaller vertical profile to the water (and probably subconsciously breathing a bit more and finning a bit as well to maintain position and orientation) makes it very common for newer drysuit divers to start ascending, and they often don't notice until they're well on their way up. The changes we're talking about here are small and can be compensated for with breathing, but if you're neutral, even small changes are enough to get you started on your way up.

 

[AquaticLung]

Thanks for the replies, I admit that I'm still inexperienced, but every dive is a learning experience, correct? I didn't properly take into account the amount of breathed gas and I'm right around.6=.8 cfm, I'm sure that was the culprit of the ascent, along with a deeper breath, possibly, a slight increase in depth that I was unaware of, and allowing my drysuit to take control, instead of vice versa. I had control of my trim the entire dive, but I had to go slightly diagonal/vertical to dump because I was allowing a small amount of air to enter my foot area, purposely, at the beginning of the dive, which isn't a smart practice, but it's great for maintaining horizontal trim and, as a bonus, it keeps my feet warmer. I've started to refrain from doing that since the OP incident and now I end up with my legs hanging low without propulsion. I thought about a tank weight or something? I'm open to advice on alternative ways to correct my horizontal trim....
Bottomline, I'm still a rookie in a drysuit and need to keep that in mind during future dives.
Later!!!

 

[Bubbletrubble]

I see absolutely nothing wrong with allowing a small amount of air to enter the foot area. As you stated, it can help with trim and keep your feet warmer. I think if you commit yourself to being very aware of your depth and when you might be getting slightly positively buoyant, you'll do fine.

I see that you have the "solo diver" tag on your profile. Learning how to dive a drysuit is one of those experiences during which having a buddy can be extremely helpful. Not only could a more experienced diver give you tips on how to improve, but just having someone else there is nice in case you need help of a more immediate nature. When my girlfriend and I were first learning how to dive dry, we remained arms length from one another at all times -- in particular, during the ascent phase of the dive. If one person got a little floaty, the other person would be right there in case a sharp downward tug were warranted. It happened more than once, believe me.

Good luck. Stick with it. If you find that the calf area of the Nex-Gen is too baggy, don't hesitate to try out gaiters. They really help limit bubble formation in that area.

Have fun!

 

[TSandM]

I echo BubbleTrubble's sentiments -- I run quite a bit of gas in my feet while I'm diving, for the reasons you listed (makes a HUGE difference in warmth!) You just have to learn to anticipate buoyancy changes, and drop your feet to get that gas back out before it gets you into trouble. You'll figure it out -- it takes time.

 

[themagni]

First, drysuits take a lot of practice, and even if you're good with a drysuit it'll take a good half-dozen dives before you can use a new suit properly. Your initial question as to whether or not the temperature of the suit will change the buoyancy can only be answered with a quick calculation:

Let's math!

The gas formula is as follows:

P1 V1 / T1 = P2 V2 / T2

Assuming a constant pressure then we get V2 = V1 T2 / T1

And as the temperature increases, then yes, the volume of the gas will also increase. The temperature has to be calculated in Kelvin, so the temperature change is

277K -> 310K
(densest water vs human body)

Plugging those values into the above, and assuming that V1 = 1, and then V2 is 1.12 units.

Well now, that is interesting. I was expecting it to be a lot less. So it looks like yes, when you warm up the air in your suit it will expand and thus increase your buoyancy.

However, the difference in volume due to pressure changes will be significantly more.

Moving from 66' to 33' assuming a constant temperature:

V2 = P1 V1 / P2

V2 = 3/2 = 1.5

Moving from 66' to 33' with the same temperature change:

V2 = P1 V1 T2 / T1 P2

V2 = 3 x 1 x 310 / 277 x 2 = 1.68

So yes, there is a ~12% extra increase in air volume due to the increase in the suit air temperature. So vent more.

I've made a lot of assumptions here, first of all the loss in gas weight from your tank, assuming AL80. They get positive when they're empty.

Your lungs would also hold more than the increase in air volume unless the suit is very baggy or the lungs are very small.

 

[themagni]

allowing a small amount of air to enter my foot area, purposely, at the beginning of the dive, which isn't a smart practice, but it's great for maintaining horizontal trim and, as a bonus, it keeps my feet warmer. I've started to refrain from doing that since the OP incident and now I end up with my legs hanging low without propulsion. I thought about a tank weight or something? I'm open to advice on alternative ways to correct my horizontal trim....

My legs drop without air in the boots. Thus, I put air in the boots.

Why is it a bad practice?

 

[j yaeger]

30* temp in fresh water is called "ice"
..unless it's reallllllly fast moving!!!!!

have fun with the drysuit!
mind the weight of the air you breathe
yaeg