Buoyancy and Ascent questions

[demonic_death]

Hi everyone,

I will be taking my open water checkout dive this weekend and I am hoping that I could get some pointers from experienced divers.

When I did the "hovering" at the pool, I have some difficulty in telling that I had inflated my BC enough and that I should just be relying on my breathing to keep me on neutral buoyancy. How does one know that he has already achieved the neutral buoyancy stage?

My next concern is about normal ascent. Do you simply swim up and inflate your BC
at the same time? My instructor has told me that it is dangerous to inflate the BC (because this can cause uncontrolled ascent) but how do you go about it?

Lastly, I have been reading a lot of threads here and some people say that you can get DCS even if you followed your dive plan and played by the rules. Anything on this?
Your answers will be greatly appreciated. Thanks guys.

 

[The Kraken]

1: If you were "hovering" and breathing normally without significantly ascending or descending, then you had your B/C correctly inflated for neutral buoyancy at that specific depth.

2: If you are properly weighted at depth, all you have to do to ascend is to take a little bit deeper breath and you will start to ascend due to an increase in the volume of air in your body. As you ascend, the air in your B/C will expand due to the reduction of pressure due to ascending to lesser depths. The expansion of air in your B/C will neccessitate your having to vent some of the expanding air from the B/C to prevent an uncontrolled ascent. If you purposely inflate your B/C at depth to a greater volume than required to maintain neutral buoyancy you can possibly create an uncontrolled ascent.

3: DCS is still somewhat a mysterious physical complication. Yes, a diver can incur DCS even if s/he followed a valid dive plan. Individual physical and physiological factors come into play. Such things as alchohol consumption before a dive, smoking, lack of proper hydration and other factors can come into play when dealing with DCS.

 

[danialagua]

Hello!
In a normal ascent, you must do a controlled ascent, keeping a speed of 9metres/min maxium (althought PADI allows 18 metres/min using the RDP). When you surface, then is when you must inflate your BCD, in order to keep a positive buoyancy.
Sometimes, often when you start diving, you can find difficult start the normal ascent. I recommend my students to inflate a bit the BCD and when they leave the bottom 0.5 metres (more or less), desinflate the BCD and continue their ascent with their fins. This skill allows an easier start of a normal ascent.

Even computers and diving tables say that the only way of not getting a DCS, is not dive. DCS depends on so many factors... but don´t worry about it! Try to be a prudent diver and you won´t have troubles.

Cheers. Dani.

 

[ScubaFreak]

I recommend my students to inflate a bit the BCD and when they leave the bottom 0.5 metres (more or less), desinflate the BCD and continue their ascent with their fins.

I agree with this, the main thing to remember is that you're supposed to swim to the surface, as opposed to inflate your BCD and enjoy the ride (which will also get you a ride to the chamber).

So remember, swim to the surface venting air from your BCD as you need to, to control the ascent.

Scubafreak

 

[RICHinNC]

Assuming you have a computer...use it.

This to me is one of the major advantages of a computer. Mine is on my left wrist. It is in plain view as I ascend. I have my inflator hose up and able to vent as needed....I can see the depth changes (plus feel it in my ears....use your senses Luke)
and I can watch the computer tell me graphically if I am going up too fast.
I agree with above...I like to go up heavy and use my fins to keep me going up....not the bouyancy of the bcd. You can get a run away too easily. Remember....it takes time for air in...or air out... to actually effect your bouyancy. That is a point a lot of new divers seem to miss.....or arent trained.

 

[Marek K]

Yes, what everybody said. As you're ascending, you'll have to be venting air out of your BC, as the BC air expands and makes you ever-more bouyant.
As soon as you breach the surface, though, that's the time to -- pssssssst -- inflate your BC in order to have good buoyancy at the surface.

--Marek

 

[DBailey]

My next concern is about normal ascent. Do you simply swim up and inflate your BC
at the same time? My instructor has told me that it is dangerous to inflate the BC (because this can cause uncontrolled ascent) but how do you go about it?

For a normal ascent, I will assume that you are neutrally buoyant already. Since you are neutral, an extra deep breath or a small kick should get you started for your ascent. As you are ascending, the air is expanding; therefore, you will need to vent/burp (not dump) the air from your BC. At no time should the BC be the source of your ascent, this is done by you and your fins. Ideally, with proper venting, you should be able to stop you ascent and be close to neutral.

In class, I doubt you are ever close to neutral when you start an ascent. I am willing to bet that you are still negative. So in class, the practice ascents rarely reflect reality.

 

[SDAnderson]

When I did the "hovering" at the pool, I have some difficulty in telling that I had inflated my BC enough and that I should just be relying on my breathing to keep me on neutral buoyancy. How does one know that he has already achieved the neutral buoyancy stage?

An excellent question to ask of your instructor. You'll practice in class, but the basic idea is to play with your weighting until you can hover in the water column while breathing normally. You'll go up and down a little bit with each breath but should remain within the same vertical "range."​

My next concern is about normal ascent. Do you simply swim up and inflate your BC at the same time? My instructor has told me that it is dangerous to inflate the BC (because this can cause uncontrolled ascent) but how do you go about it?

Do NOT inflate your BC to start your ascent. You should be neutral all the time, all you need to do is gently kick towards the sunshine. You're going to want to start dumping air from the BC almost immediately in order to control your ascent. You'll practice this in class - but it takes some time to get good at.​

Lastly, I have been reading a lot of threads here and some people say that you can get DCS even if you followed your dive plan and played by the rules. Anything on this?

Sometimes called an "undeserved hit," I think it's better to call it an "unexpected hit." Decompression models are all approximations and are based upon a number of expectations. People vary and, sometimes, an individual will fall outside the projected range of results. It's scary but the tables and techniques you are learning allow for some pretty wide lattitude. If you stay within the limits of your training and the tables, the chances are VERY small that anything unpleasant will happen. As a new diver, it's a good idea to dive a little more conservatively until you get some experience under your belt and to push the edge of the envelope oh-so gently.​

 

[jpedwards]

When I took OWC (NAUI), we were taught to completely deflate the BC before ascending and fin our way up. If finning was too much work, we should add *just* a puff of gas to the BC and wait at least 5 seconds to see if that helped the ascent before adding more. Then let all the air out again after ascending 5 - 10 feet. Works like a charm for me ...

 

[jbd]

I use the method described by The Kraken in post #2 paragraph # 2. No finning needed--just let the laws of physics take care of it for you.