Fire_medic_RescueDiver:
I have started diving in my new dry suit. I have never dove in a dry suit before, I am taking a PADI dry suit class next weekend, but still have a few questions about it. Do you always have some squeeze to the suit even after adding air at different times. How much air should you add? I have dove the suit 3 times now and have only had the mild squeeze once and that was on a deeper dive (95feet), and I did air thru out the dive, it is not a really bad squeeze feeling just noticeable. I also leave the exhaust valve open fully.
Sounds like you're adjusting the air in your drysuit about right, at least based on prevailing opinion, which is to have just enough to avoid significant squeeze. Going beyond that by adding more air is often considered "excess" air.
Many drysuit divers also appear to use their bcd for their primary buoyancy adjustments, just occasionally adjusting the drysuit to have only a slight squeeze.
Not me, on both counts.
I prefer having a moderate amount of "excess" air in the suit, primarily because I feel it's warmer.
Plus, I like to use
only the drysuit to control buoyancy during the dive, not the bcd. That means even more excess air.
Obviously, using only the drysuit for buoyancy control appears to be a minority opinion, but, here in the NE, it seems to be shared by quite a few local "year-round" divers.
That opinion is also shared, with good reasons, by the makers of DUI drysuits, on page 33 of their drysuit manual, (1991): "WARNING: Do not inflate your buoyancy compensator under water while using your drysuit. Controlling two independent air bladders (the BC and the dry suit) is a very difficult skill to master. If not carefully performed, you may experience a rapid and uncontrolled ascent. Rapid ascent is dangerous and may cause air embolism or decompression sickness, either of which can result in serious injury or death."
Someone please correct me if they have revised or changed that guideline since publication in 1991.
Other drysuit makers differ from that opinion, I believe.
Drysuit courses may differ also. I'd be curious to find out.
What should you do? Avoid a rapid and uncontrolled ascent, of course.
I think both methods of adjusting buoyancy can work when drysuit diving, but which one is more safe or managable is debatable.
On the one hand, since raising raising one's arm to vent a bcd can also vent the drysuit at the same time, controlling two air bladders seems quite easy.
I can't make a strong case to support DUI's position, except that the diver might
not be aware that the drysuit was
not venting enough in the scenario above.
If the drysuit is your
only buoyancy concern, it makes sense that you're more likely to know if it's venting adequately or not.
For me, this really comes down to simplicity, but I can understand the other view, especially since they may argue that the "excess air" in the drysuit is kept minimal and venting it won't be a problem.
Unfortunately, even the
minimum amount of air contained in the drysuit insulation can often amount to 20 lbs or more of buoyancy, offset by weight, of course. In shallow water, such as 10 to 20 feet, where one might be trying to take a safety stop, counteracting a significant increase in buoyancy can require pretty rapid venting of the suit, and one may be busy trying to vent the bcd at the same time.
Still, if one uses only the drysuit for buoyancy control, there are valid concerns about
excess air in the drysuit shifting rapidly or getting trapped or upending the diver and causing an uncontrolled ascent.
Nevertheless, I think a
moderate amount of excess air can be managed easily with a little bit of practice.
Again, why bother? Warmth.
How much excess air?
For those of us using
only the drysuit to control buoyancy, that means we start with significantly more air in our suits than those who use the BCD for primary buoyancy control.
How much more air? Enough to offset the
change in buoyancy of the tank when it goes from full to empty. That's enough air in the suit to offset about 6.5 lbs for an al80, or about 9.6 lbs for my steel 120.
The guys who use the bcd for primary control, the air needed to counteract the change in tank buoyancy is in the bcd, not the suit. Obvious advantage there, for sure.
What's the volume of that air?
Since water weighs about 8 lbs per gallon, with my steel 120, I'd need to
start the dive with a little over a gallon of
excess air in my suit.
By the end of the dive, when the tank is near empty, most of that excess would be gone, having been vented to adjust to the gradual change in tank buoyancy.
Is a gallon of excess air going to shift drastically in the suit?
Possibly, if the suit is a loose fit. A loose fit would let the air bubble move quickly in the gap between the shell of the suit and the outer layer of the insulation.
That's much less likely with a suit that fits better or is more filled out with insulation. In a normal fitting suit, the shifting air would have to work it's way through the insulation. That can take a number of seconds for a significant shift, which is plenty of time for a diver to adjust attitude in the water.
Now, here's a curve ball for you....
I go one step further to increase my warmth by carrying about 5 lbs more weight than the minimum needed, which lets me put
even more excess air in my suit.
Should you do that? No.
At least, not until you see what kind of air shifting you get with your suit, with certain insulation, and at "normal"
minimal weighting.
And certainly not until you've learned how to counteract an upended, inverted uncontrolled ascent, right?
One more thing, just for the sake of passing some time during this seemingly endless winter....
For the experienced drysuit divers around my area, it's not uncommon to see a diver add up to 4 pounds prior to a second dive to increase the warmth.
That's often because they know their insulation will be a little less effective when slightly damp from the body moisture of the first dive.
I have a little theory that excess air in the drysuit feels warmer, not only because it allows more lofting of the insulation, especially in the areas highest in the water column, but also because the excess, shifting air moves moisture away from the skin and from the first layer of insulation and leaves it condensed on the inner surface of the drysuit. That improves wicking and keeps the skin drier and warmer feeling, I think.
Just some more food for thought, perhaps.
Hope I haven't given you indigestion!
Dave C
) For me the half a militre of air I'd put into my BC is just one more thing I'd have to remember...so I use my suit for primary bouyancy control. 
