Using a BC

[micknewton]

Yet Another Stupid Newbie Question (YASNQ?) This is basic, and I apologize if I'm being dense. I'm just trying to get a better understanding of the correct usage of a BCD.

The question I have is simple. When do you add air to, or remove air from, your BC?

Let's say that you're doing a drift dive in Cozumel to max depth of 80'. You have a single AL80, a BP/W, and a 3mm wetsuit.

Obviously, you let the air out to descend, right? When do you put air in? How much do you put in? When do you let some out again?

At the end of your dive, when it's time to ascend to a shallower depth, let's say for a safety stop, do you swim up, or do you add air to your BC to make yourself more buoyant and automagically rise to your desired depth?

If you swim up, do you add/remove any air in your BC after reaching your desired depth?

If you add air to your BC to move yourself higher in the water column, do you adjust it after reaching your desired depth, as you're ascending, or..?

Any input will be appreciated. A blow-by-blow anatomy of the correct usage of a BC during a complete drift dive would be ideal.

 

[mlloyd]

You put air in to balance yourself at a certain depth. When decending I start to put a small amount in to prevent myself from gaining too much speed, and continue to do this until I arrive at my depth. Once at depth, fine tune yourself with a bit in or out and then enjoy your dive. After a bit of time, even if you have not risen, you will find yourself a bit light because of the air you have consumed. From here on in, you will be letting air out in small amounts as needed and when rising further up.
You shouldnt have to add air to the BC to ascend, taking a breath and holding it briefly will get you started up.

 

[The Kraken]

A short answer.
You add air to your B/C when you're too negatively buoyant to stay where ya want to be.

You release air from your B/C when you're too positively buoyant to stay where ya want to be.

Your B/C is inflated at the surface to provide positive buoyancy.

You release air from your B/C to become negatively buoyant in order to initiate you descent. (You may need to add air at different times during your descent in order to prevent a too rapid descent)

Once you arrive at your desired depth, you add enough air to your B/C to keep you neutrally buoyant.

When you decide to ascend to the surface, you take a deep breath. The increased air in your lungs will initiate your ascent. As you ascend, the air in your B/C will start to expand causing you to become too positively buoyant. You release enough air to keep your buoyancy just ever so slightly negative, this enables you to control your ascent.

Upon reaching your safety stop, you add enough air to your B/C to maintain neutral buoyancy.

Upon completing your safety stop, you repeat the same process as you did from your initial ascent.

Upon reaching the surface, inflate your B/C to maintain positive buoyancy.

 

[Walter]

Let's say that you're doing a drift dive in Cozumel to max depth of 80'. You have a single AL80, a BP/W, and a 3mm wetsuit.

Obviously, you let the air out to descend, right? When do you put air in? How much do you put in? When do you let some out again? [/QUOTE]

Not so obvious to me. I wouldn't have air in my BC. I would be about 6 lbs negative (the weight of the air in my tank) and since I wouldn't be planning to hang around on the surface, there would be no reason to put air in the BC. As I descend and the wet suit compresses (do people wear wet suits in Cozumel?), I'd add air during my descent. Just enough to make me neutrally buoyant. Use small squirts, don't overdo it.

At the end of your dive, when it's time to ascend to a shallower depth, let's say for a safety stop, do you swim up, or do you add air to your BC to make yourself more buoyant and automagically rise to your desired depth?

Adding air to ascend is dangerous. Dump air as you ascend. Dump some before you start your ascent. Dump a little at a time, don't overdo it.

If you swim up, do you add/remove any air in your BC after reaching your desired depth?

Before, not after.

If you add air to your BC to move yourself higher in the water column, do you adjust it after reaching your desired depth, as you're ascending, or..?

Never add air to ascend.

 

[Monkey Knife-fight]

Never add/remove air from your BC to actually move yourself up or down the water column. Use your lungs or, if that's not enough, fin up/down. You want your BC to be inflated enough to be neutral at your depth. The deeper you go the more air your BC will need and vice versa.

Chris

 

[JodiBB]

At the end of your dive, when it's time to ascend to a shallower depth, let's say for a safety stop, do you swim up, or do you add air to your BC to make yourself more buoyant and automagically rise to your desired depth?

Ironically, I was taught not to add air except through oral-inflation into my BCD. The reason? Too many times newbies put too much air in the BCD and will ascend too quickly. I was taught to kick up to the desired height for a safety stop and then either continue to kick and "tread" U/W or add just enough air into the BCD to be neutrally buoyant at that stop.

Upon continuing up to the surface, I was not allowed to use the LPI hose to help with the ascent...only my fins...when I got to the surface, I had to oral inflate the BCD...

Excessive? Probably...but oral inflating my BCD is now as common to me as breathing...I know how to oral inflate well U/W as well as at the surface...now that I'm not a student anymore, I'll use my LPI at the surface to be + buoyant, but U/W, I still tend to kick my way to the safety stop and then adjust my buoyancy at that point.

 

[Charlie99]

After a while, adding or releasing air from the BCD is an automatic action.

The first, most important step is to really know what your current buoyancy is. If you are close to neutral, then there is a point in your breathing cycle where you can pause (while keeping airway open of course), cease all motion, and you will neither rise nor descend.

If that neutral point is with super full lungs, then add a bit of air. If you have to exhale hard to get neutral, then dump some air.

-----------

I find that I add air a few times during the first 4 or 5 minutes of the dive. Some is to account for wetsuit compression. Some is also to account for the change in buoyancy of the wetsuit as it absorbs water (this is for the 1st dive of the day, where you have started with a dry wetsuit). Once this initial adjustment is over, then I use my lungs to stay neutral, and only add/release air to "re-center" my buoyancy to the desired spot in my breathing cycle. For example, if it is a flat bottomed dive, then eventually as the dive progresses and I breathe down the tank, I'll find that I'm spending more time with my lungs towards the exhale side -- because the tank is lighter and I have more buoyancy. I'll dump a bit of air to make it so that I average out at neutral while spending more time with my lungs full.

At any point in the dive you should be able to stop all motion and pretty much stay where you are. Otherwise you aren't really neutral, and you are working way too hard.

Being properly weighted makes everything easier since you won't need extra air in the BCD to compensate for the extra lead.

Being in a thin wetsuit makes it easier since the variation in buoyancy with depth changes will be less.

 

[Charlie99]

Upon continuing up to the surface, I was not allowed to use the LPI hose to help with the ascent...only my fins...when I got to the surface, I had to oral inflate the BCD...
....but U/W, I still tend to kick my way to the safety stop and then adjust my buoyancy at that point.

If you were neutral at the safety stop, then you would have no reason to use the LPI, and instead might have to dump air (or at least empty your lungs).

For your ascent, you would probably find it easier to first get truly neutral, then just inhale a bit more to rise. (Think back to the "fin pivot" exercise of your OW class. Same thing, except you aren't touching bottom with anything) Then dump just a little tiny bit of air now and then as you slowly ascend. Staying horizontal gives you more drag in the vertical direction and makes controlling the ascent easier. Come to a full stop now and then, particularly around 30'. Then when you get to safety stop, then you should already be neutral, since you kept your ascent under control.

 

[Brewone0to]

The 3mm wetsuit is quite excessive for Coz
A diveskin or 1.5mm suit is more than enough for there(and you will not have to fight with the variable of wetsuit compression)
Before you go down to Coz or any other deep wall dive locations,Get some formal training and/or practice with boyancy skills
(Coz has the potential to HURT you BAD)
Also when you make arangements for dives,make sure that they are aware of your comfort/skill level.
There are many places where the currents there will not give your boyancy skills as bad a workout as some of the others.

 

[micknewton]

Great stuff guys! Thanks for the help!