Trapped air

[Walter]

With over 2000 dives that is easy for you to say but a lot harder for us newbies.

First, keep in mind that I was also a newbie once. I remember that time quite well and my instructor (who is still teaching) still uses me as an example of the worst student he ever had. I struggled with all the issues with which you've had problems. Why is it hard to dive without adding air to your BC? The buoyancy compensator (BC) is designed for two uses. First, it allows you to be positively buoyant while waiting for a period of time on the surface. Second, it allows you to compensate for loss of buoyancy from wet suit compression as you descend. If you are properly weighted, you'll be about 6 lbs negative at the beginning of the dive (to make up for air you'll be using during the dive). Your lungs are large enough to easily compensate for 6 lbs. Assuming no wet suit, as you descend, your average buoyancy won't change (yes you will become more positive as you inhale and more negative as you exhale, but that is no different with or withour air in your BC). Since your buoyancy does not change as you descend, there is no need to add air to your BC. If you have to add air to your BC, you are over weighted. Drop some lead.

 

[scubadiver888]

First, keep in mind that I was also a newbie once. I remember that time quite well and my instructor (who is still teaching) still uses me as an example of the worst student he ever had. I struggled with all the issues with which you've had problems. Why is it hard to dive without adding air to your BC? The buoyancy compensator (BC) is designed for two uses. First, it allows you to be positively buoyant while waiting for a period of time on the surface. Second, it allows you to compensate for loss of buoyancy from wet suit compression as you descend. If you are properly weighted, you'll be about 6 lbs negative at the beginning of the dive (to make up for air you'll be using during the dive). Your lungs are large enough to easily compensate for 6 lbs. Assuming no wet suit, as you descend, your average buoyancy won't change (yes you will become more positive as you inhale and more negative as you exhale, but that is no different with or withour air in your BC). Since your buoyancy does not change as you descend, there is no need to add air to your BC. If you have to add air to your BC, you are over weighted. Drop some lead.

There is one other factor besides the wet suit. If you are diving with aluminium tanks you need to start the dive slightly over weight. As you get closer to empty tank you will be properly weighted.

For me though, the wet suit is a huge factor. Last dive I was on I had neoprene boots, gloves, hood and 14mm of wetsuit. At depth I estimate I could drop 8 to 10lbs (25% of my weight) but I need it when I'm at the safety stop or getting down.

I'm working on buoyancy; once I get enough practice in (a) I'll no longer complain about buoyancy control and (b) I'll no longer be a newbie.

 

[Web Monkey]

There is one other factor besides the wet suit. If you are diving with aluminium tanks you need to start the dive slightly over weight. As you get closer to empty tank you will be properly weighted.

The tank material doesn't matter. You could have aluminium, steel, carbon fiber or kryptonite. 80 cu Ft of air weighs about 6.5 pounds, regardless of what you keep it in.

If you're diving an 80 Cu Ft tank (an AL 80 isn't exactly 80, but it's close) and are properly weighted, you'll be about 6.5 pounds overweighted at the beginning of the dive, and just about perfect when the tank is empty.

Some people like to be overweighted by a couple of "extra" pounds to compensate for lift caused by currents and non-horizontal trim, as well as a fudge-factor for not knowing exactly how much a weight really weighs (weights aren't generally precision devices).

Terry

 

[needforspeed751]

yeah generally I keep almost no air in my bc but my dive buddy tended to overweight himself so as a general rule I carried an extra two to four pounds in order to help them at the end of the dive if necessary. We were using aluminum tanks so he tended to get particularly buoyant toearss the end. The first few dives he forgot the extra weight so I started packing it just in case. Thus the extra bit of air

 

[scubadiver888]

The tank material doesn't matter. You could have aluminium, steel, carbon fiber or kryptonite. 80 cu Ft of air weighs about 6.5 pounds, regardless of what you keep it in.

If you're diving an 80 Cu Ft tank (an AL 80 isn't exactly 80, but it's close) and are properly weighted, you'll be about 6.5 pounds overweighted at the beginning of the dive, and just about perfect when the tank is empty.

Some people like to be overweighted by a couple of "extra" pounds to compensate for lift caused by currents and non-horizontal trim, as well as a fudge-factor for not knowing exactly how much a weight really weighs (weights aren't generally precision devices).

Terry

I have been told, if you take an empty aluminium tank it will float. If you take an empty steel tank it will sink. So the material a tank is made out of does matter. Please correct me if I'm wrong.

Darrell

 

[Walter]

There is one other factor besides the wet suit. If you are diving with aluminium tanks you need to start the dive slightly over weight. As you get closer to empty tank you will be properly weighted.

True, but if that applies to steel tanks as well. Also, if you'll read my post, you'll notice I mentioned that.

If you are properly weighted, you'll be about 6 lbs negative at the beginning of the dive (to make up for air you'll be using during the dive). Your lungs are large enough to easily compensate for 6 lbs.

 

[Walter]

I have been told, if you take an empty aluminium tank it will float. If you take an empty steel tank it will sink. So the material a tank is made out of does matter. Please correct me if I'm wrong.

Darrell

OK, I'll correct you. Most empty aluminium tanks will float and most empty steel tanks will sink, but that's only part of the story. To compensate for that, you'll use less lead with a steel tank than with an aluminium tank. Assuming the tanks hold approximately the same amount of air, the weight loss from using up the air in the tank will be approximately the same. If you use 6 lbs of air, it doesn't matter what type of tank it was stored in, that tank is still 6 lbs lighter.

 

[InTheDrink]

Moving off the aliminium tanks again for a moment, it is worth - particularly for newbies-ishes - to understand that when buoyancy doesn't feel right to trust their gut. I had a rental BC (corregated hose cut 50%) that screwed my buoyancy in ripping current when my reg failed too (nice way to get familiar with task loading . The air appears to have been leaking and getting trapped *somewhere" in or around my BC such that I had to dump every minute or so. I felt like an absolute idiot until on dive 3 I check the corregated hose at the surface (having to abandon the dive alone) and it was totally slit. Just because you're new doesn't mean you're wrong. Two lessons:
1. Trust your instincts
2. Don't use rental kit if you can avoid it

 

[BgDadddy]

Actually, this was a very good experience to post, and thank you! It came as a big surprise to me to learn that a diver directly below me would make me positive -- You don't need a rash guard to trap bubbles. They can be trapped elsewhere in your gear as well. New divers need to know that they have to watch their buoyancy if there is something directly beneath them.

Ditto. I dive in a large sinkhole a lot and often there are numerous groups in the water. I have noticed this happening to me on numerous occasions when I swim above other groups.

Rick

 

[String]

Next lesson - if you are not wearing a wetsuit and you are weighted properly, there's no need to put air in your BC.

Depends entirely on tank choice. Its perfectly possible to be overweighted without a wetsuit or any weights and in that case you will require air in the BC.