Cold water dive; Air expansion question

[AquaticLung]

Hi folks, lately I've been trying to master my BARE Nex-Gen shell drysuit. It's used but not abused and in good working order. I live in Michigan and dive in cold water primarily, so I need to find out about the characteristics of air expansion relative to temperature inside the suit. I've had a few instances where I've suddenly had an uncontrolled ascent.
Before I explain the details, I'd like to share with you that Lake Michigan and definately MI inland lake diving is mostly poor viz (less than 5-8ft lately) and in the few instances where I've ascended rapidly (over 30ft/min, but not more than 45ft/min, in my case) it has been very cold water (30-45 Degrees F), so before you conclude that I lost my bearing and became confused and/or failed to anticipate the event that lead to my ascent, please listen to this and, at very least, rule it out before stating the above. Also keep in mind that these conditions are what I'm used to and what I was trained in.
Hear are the details: I was diving at about 30 ffw and hadn't changed depth more than a foot or two for a period of 8-9 min's and I didn't add any air to my suit or BC for at least 7 min's, so 8-9 min's later, I started ascending. I admit that I was a little caught off guard and I'd agree that is why I made it to the surface, but I can't figure out what caused the ascent in the first place. I'm comfortable in the suit especially in shallower waters, but this has happened to me three times with this last time being the only time I made it to the surface before regaining control. I also admit that 8-9 min's at one depth isn't common for me either, so with my constant change of depth, I've rarely had a 7-8 min time period pass where I did not have to make adjustments to my BC or suit.
So, all I can come up with, besides human error than continues to evade my knowledge, is the possibility that the air added to my suit 7 min's prior had increased in temp, due to my body heat inside the suit, and as a result, increased the volume the air space occuppied. I realize that this expansion would've happened gradually, as would the ascent, so even I am not fully convinced by this theory. I do remember using my hands a couple times to lower my position in the water, but the change was minimal and I suppose I thought it was due to a deeper breath or something. I should've paid it more attention. 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.
Anyway, I'm not looking to find the cause of this problem because I believe that it was mostly my lack of situational awareness or just getting a little too lax during the dive is what the main issue was. The MAIN purpose of this post is to find out other opinions about the likelyhood of the temp of the air inside a suit changing and as a result altering bouyancy. Also, do any of you suppose that the movement of the bubble from my calf to my chest area could've allowed the change in bouyancy. Who knows, I guess I'll pay more attention in physics this semster. I'll be keeping a much closer look next time. Thanks for the replies and good diving everyone.

 

[Crush]

I have about 20 dives on my drysuit. Here is what helped me:

1. On my first three dives I ended up using the drysuit for buoyancy control, just to get a handle on putting air in and dumping it;
2. Ever since I only add air to the drysuit when I notice a squeeze, and as soon as I feel a bubble of air rolling about between my shoulder blades I immediately dump some air from the drysuit. I have, on occasion, found myself beginning an unanticipated ascent in both wetsuit and drysuit. The trick is to catch it early and react quickly. If you keep your suit air to a minimum you can often vent your BC air and swim down to avert an ascent. My BC allows be to vent air while inverted, but my drysuit does not.

 

[AquaticLung]

Good point on inverted dumping, adds a backup method to returning to depth, swimming. I found my self using my hands and arms to slow ascent for that very reason of not being able to dump air inverted. Thx for the post thou.

 

[JeffG]

The MAIN purpose of this post is to find out other opinions about the likelyhood of the temp of the air inside a suit changing and as a result altering bouyancy.

not a chance.

Also, do any of you suppose that the movement of the bubble from my calf to my chest area could've allowed the change in bouyancy.

No



Like most new drysuit divers...you were probably reacting too late.

Be proactive...dump all the time. Dive the suit...don't let the suit dive you.

 

[Crush]

I have, on occasion, found myself beginning an unanticipated ascent in both wetsuit and drysuit.

By way of clarification, while I have found myself beginning an unanticipated ascent in both wetsuit and drysuit, I have never completed such ascents - in my cases, the ascents were small in amplitude and corrected well prior to making any substantial progress towards the surface. Be diligent.

 

[Bogie]

Make sure dry suit inflator is not free flowing.

If you use your dry suit for buoyancy control only, it is more unstable because the air bubble can move from toes to neck. If you get in a more vertical the air bubble goes to the highest point and can start the ascent.

The best for buoyancy control is to just get the squeeze off on the dry suit and use your bc for buoyancy control. It is more stable because the air bubble movement is limited and it centers on the center of your body. The bc releases air with more efficiency and quicker than a dry suit dump valve. Horizontal position is critically important. Breath control can fine tune your buoyancy control.

Taking a class from a cave/tech diver or from UTD or GUE would benefit your diving skills tremendously.

PADI teaches dry suit diving using only your dry suit which is wrong.

 

[Bubbletrubble]

Interesting question sparked by book-learnin', in particular Archimedes' principle and the ideal gas law.

The temperature idea that you put forth isn't as crazy as it sounds. Increasing the temperature of the air inside the drysuit in the way you describe has some parallels, on a theoretical level, with how a hot air balloon works. I'll leave it to you to figure out why, on a practical level, that probably can't happen in a closed system (airspace between diver and drysuit).

Getting vertical and shifting a small bubble from the calf area to your chest shouldn't initiate an uncontrolled ascent either. A properly functioning exhaust valve set to open or near-open should dump gas before launching you to the surface (assuming that the exhaust valve is at the highest point in the water column and isn't blocked).

I would definitely check the functioning of your drysuit and BCD inflater valves (assuming that you had some gas inside your BCD during the incident). A partially open inflater valve can definitely make a diver more buoyant.

However, the most likely explanation, considering your inexperience with a drysuit, is that you just simply weren't paying attention to your depth and your breathing changed a little (inhaled more deeply +/- started retaining more air in the lungs). The runaway ascent may have been compounded by a closed/inappropriately set/partially blocked/poorly positioned exhaust valve. FYI, some undergarments are known to clog up certain exhaust valves.

As others have recommended, dump early and often. During the bottom phase of the dive, be mindful of your depth by watching your depth gauge or using a convenient non-moving visual reference (sandy bottom, rock, etc.). Make drysuit and/or BCD adjustments to remain neutrally buoyant. During ascents, stay ahead of the expansion of the air inside your suit and BCD. Feel free to get vertical momentarily in order to move the drysuit bubble into position to vent through the left bicep exhaust. Practice recovering from a feet-first ascent. In open water, the preferred method is to kick downward then tuck-and-roll while placing the exhaust valve at the highest point in the water column. If there's something anchored to the bottom that you can grab, go for it. If you can't do any of these things, then open your neck or wrist seal at the highest point in the water column to let gas escape. For beginners learning how to manage drysuit ascents, it can be helpful to do something close to a left arm salute while holding the BCD power inflater up and to the left of your head. This position should make dumping air easier. Another tip to help with drysuit bubble management is to try to stay trimmed out a little feet-low/head-high. This places the bubble nearer the exhaust valve. In time, you'll be able to dive the suit in proper horizontal trim. You'll dump air from the suit by simply rotating your shoulders (right shoulder down and left shoulder up) slightly.

Find a mentor or take a class on how to use your drysuit (if you haven't already). I'd recommend practicing with the drysuit under controlled conditions before doing any real OW dives to any significant depth. Always keep an open airway. The last thing you want to do is deal with an arterial gas embolism due to an uncontrolled ascent.

I've been diving a Bare Nex-Gen successfully for several years now. It is possible. Hang in there.

Good luck...

 

[bleeb]

Hear are the details: I was diving at about 30 ffw and hadn't changed depth more than a foot or two for a period of 8-9 min's and I didn't add any air to my suit or BC for at least 7 min's, so 8-9 min's later, I started ascending. I admit that I was a little caught off guard and I'd agree that is why I made it to the surface, but I can't figure out what caused the ascent in the first place. I'm comfortable in the suit especially in shallower waters, but this has happened to me three times with this last time being the only time I made it to the surface before regaining control. I also admit that 8-9 min's at one depth isn't common for me either, so with my constant change of depth, I've rarely had a 7-8 min time period pass where I did not have to make adjustments to my BC or suit.

However, the most likely explanation, considering your inexperience with a drysuit, is that you just simply weren't paying attention to your depth and your breathing changed a little (inhaled more deeply +/- started retaining more air in the lungs). The runaway ascent may have been compounded by a closed/inappropriately set/partially blocked/poorly positioned exhaust valve. FYI, some undergarments are known to clog up certain exhaust valves.

Expanding on the issue of breathing (sorry for the bad pun), the OP said no air was added to suit or BC for 8+ minutes and implied no venting either. He/she is in cold water at 30'. Roughly speaking, that means breathing off a minimum of somewhere around 10 cu ft or 0.5 lb of gas in that time. For some people, this is approaching the limit of what they can compensate by adjusting their breathing (200 mL out of a typical lung tidal volume of 1 L), especially if it means breathing less as you get more buoyant.

If this is the case, in the end, the recommendation is the same as what others have already said: adjust your buoyancy more frequently.

 

[sled600]

If you use your dry suit for buoyancy control only, it is more unstable because the air bubble can move from toes to neck. If you get in a more vertical the air bubble goes to the highest point and can start the ascent.


Explain this to me.... Just because the "Bubble" is in your feet - Placing you head down- does not increase the size of the bubble....you should remain neutral (assuming you were) just hanging head down, correct?

I am relativley new to the sport and dive a dry suit in puget sound. I agree that it was tough learning to "stay ahead" of the drysuit.

I am comfortable in my drysuit now and love it, but getting used to it was a challenge.

 

[Bubbletrubble]

Expanding on the issue of breathing (sorry for the bad pun), the OP said no air was added to suit or BC for 8+ minutes and implied no venting either. He/she is in cold water at 30'. Roughly speaking, that means breathing off a minimum of somewhere around 10 cu ft or 0.5 lb of gas in that time. For some people, this is approaching the limit of what they can compensate by adjusting their breathing (200 mL out of a typical lung tidal volume of 1 L), especially if it means breathing less as you get more buoyant.

Excellent point, bleeb. I'm embarrassed to say that I assumed he was dumping a little air (from either BCD or drysuit) to compensate for breathed gas.
Perhaps the OP can confirm this?

No addition/subtraction of gas from either drysuit or BCD, combined with the exiting gas from his tank over the course of 8+ minutes, would definitely make the OP more buoyant. I would find it surprising, though, that after 100+ dives this kind of buoyancy adjustment isn't "automatic." Perhaps the added task-loading of operating the drysuit distracted him from this very basic skill.