Night Dive Unwanted Ascent

[mikerault]

Usually when I am neutral horizontal, if I go vertical I am slightly negative. Sounds like a definate weighting issue. Was the second dive with a new tank or the remainer of the old one? If it was the old tank it simply may have gotten buoyant enoug to make a real difference. Try doing weighting with a nearly empty tank (500 PSI) before your next dive.

Mike

 

[60feet]

Greg,

Did you do a 15' safety stop at the end of your first (90') dive and were able to stay neutrally buoyant at 15' for a couple of minutes? Afterward, were you able to perform a slow, controlled final ascent from the safety stop?

Did you have about the same amount of tank psi at the end of both dives (or higher tank psi in the 30' dive)?

If the answer to all the above is "yes," then it's probably not a problem of being under-weighted. Rather, I would be suspicious of your BCD still containing some air when you were clearing your mask in the second dive. Completely venting all air out of a BCD takes some practice. Others have mentioned inhaling a full breath (to clear your mask) will move you up. If you were finning at the same time (without realizing you were doing it because you were focused on the mask clearing), then that would do it.

If you had less air left in the tank for the second dive than the first dive, then yes, you were more positive, may have been under-weighted, or may still have had some residual air in your BCD, or both. Couldn't hurt to do a proper weight test if you haven't done one in awhile.

 

[gcbryan]

I like to carry a couple more pounds of lead on the shallow dives than the deeper ones to help control buoynacy. I have a question, tho...

Where was your buddy? A newby on a night dive needs a buddy who sticks very close.

I'm just picking on you a little Don....but if you were to carry more lead wouldn't it make more sense to do it for the deeper dive? You should be correctly weighted at all times but after a deeper dive it's even more critical to be able to hold a shallow stop than would be the case for a shallow dive.

 

[Charlie99]

I'm just picking on you a little Don....but if you were to carry more lead wouldn't it make more sense to do it for the deeper dive? You should be correctly weighted at all times but after a deeper dive it's even more critical to be able to hold a shallow stop than would be the case for a shallow dive.

Playing devil's advocate a bit .......

on a deeper dive I'll be doing a series of deeper stops, and will only be at 10' or less for just a couple of minutes. I can change my breathing pattern a bit and reduce my buoyancy at least 2 or 3 pounds for those couple of minutes. OTOH, if I'm doing a dive while I'll be in the 10' range for the whole dive, then I'll want to be weighted such that I will be neutral at 10' with a near empty tank and FULL lungs. That gives me the extra adjustment range so that I can both ascend and descend using my lungs, even when at 5' or 10' with a near empty tank.

I'll only make the adjustment for a dive where I'll be spending a lot of time shallower than 15'.

I'm not talking a huge difference -- just a pound or two.
If you get much more lead than that, then you will have to have excess air in the BCD on the shallow dive. That excess air will make buoyancy control difficult.

 

[Butch103]

Scottb is bang on correct! You shouldnt have to adjust your weighting for any diving.

 

[gcbryan]

Playing devil's advocate a bit .......

on a deeper dive I'll be doing a series of deeper stops, and will only be at 10' or less for just a couple of minutes. I can change my breathing pattern a bit and reduce my buoyancy at least 2 or 3 pounds for those couple of minutes. OTOH, if I'm doing a dive while I'll be in the 10' range for the whole dive, then I'll want to be weighted such that I will be neutral at 10' with a near empty tank and FULL lungs. That gives me the extra adjustment range so that I can both ascend and descend using my lungs, even when at 5' or 10' with a near empty tank.

I'll only make the adjustment for a dive where I'll be spending a lot of time shallower than 15'.

I'm not talking a huge difference -- just a pound or two.
If you get much more lead than that, then you will have to have excess air in the BCD on the shallow dive. That excess air will make buoyancy control difficult.

Personally, I wouldn't make an adjustment either way. I'd be correctly weighted in each case but on a deeper dive you may be spending a while at 20 fsw and staying there is more crucial than if your entire dive was shallow. You should be able to stay at 10 fsw on a near empty tank either way although strictly speaking you shouldn't have a near empty tank.

 

[KMD]

Honestly this is a skills/experience issue. It should take no more then a 1 to 2 seconds to clear your mask. Even if you were currently rocketing to the surface at 60ft/min you would have only moved up 2 feet.

I suspect as you worked on clearing your mask you became positive (very common as inexperienced divers often take big breaths when clearing) and the darkness contributed you not realizing that you were ascending. Sometimes people can determine they are rising by listening to the subtle sounds of their ears crackling.

Dont know if your profile is correct, but if it is may I humbly suggest to get some more experience before making the 90' night dives.

 

[Hutchel]

I can attest the challenge of trying to maintain decent boyancy control at 30' in heavy neoprene.

My wife and I know we need to do some more work in getting our boyancy under control. So in that spirit we set out for the local quarry in Northern Virginia about a month ago. Now this water is not warm, so we are in full 7 mm wet suits with a 7 mm hooded vest. The quarry is 85' deep - but much past 60' the water is too cold for comform in a wet suit < 50 deg. We decided to try a stair step pattern for the dive profile - go in and down to 30' hold - establish good boyancy control for a few minutes, decend 10 feet and do it again, and so forth, at 50' we decided that was cold enough and came back up to 40' and then to 30'. We had a float with a rope that was suspended from it and had a 5 lb weight on it that went all the way to the bottom. The quarry was lousy vis so it provided a reference point and we could go anywhere and not get in the way of the other student groups using the platforms.

We had a heck of a time trying to maintain any sense of neutrality. Detecting sinking was easy - the water got noticably colder (~70 deg on the surface) but ascending was much tougher without watching the rope or our gage. And YES any ascent rapidly snowballed as the wet suit decompressed and the air in the BC expanded. Now we are not expert divers by any stretch (we have completed Rescue Diving but are just under 30 dives), but we felt like complete failures. We'd dump air from the BC and then start sinking again - trying to be subtle was definately a challenge. We started with sufficient weight to be neutral on the surface - i.e. no air in the BC and floating at eye level with a full set of lungs - decending with no air in the lungs. That was about 18 lbs of lead shot for myself.

We understand the importance of a controlled safety stop and have had trouble in the past doing that so we wanted to try some practice concentrating on our boyancy. We did better in the ocean on a regular dive (little or no exposure suit protection - yes I know that makes a big difference) - we just had issues with not enough weight to overcome an "empty" tank ~500 psi . In this case we had "full" tanks > 1500 PSI and full suits (our own gear - yes we're still getting used to it).

We have more practice to do - will probably have to wait until spring - it's too cold out there now - in the mean time we will do some tropical diving with much less neoprene to have to compete with.

It's an important skill to master - I suspect in the regime we were practicing in, I set the bar pretty high to maintain good neutrality since physics was definately working against us. Fortunately at this depth going up and down 20' and occasionly being on the surface wasn't a huge deal - it was just frustrating. Practice, practice, practice.

Lee

 

[greg454]

Thanks for all the answers. I read them all. I was busy at work so I couldn't asnwer before.

 

[sardiver]

Does sound like a case of expansion of neoprene due to change of attitude in the water. I have one wetsuit, a 6 mil farmer john that is really plush. It takes 36 lbs with an OMS steel 85LP to submerge. I can blow all my breath out and paddle down and I still bob right back up without the heavy weights. Of course, once I hit 25-30' I start dropping like a rock!! I do however, come up nice and warm {
Glad you are okay and are smart enough to be thinking this one through. There are always lessons to be learned, thank goodness!