Ascending/Decending/Buoyancy

[steinbil]

Period. Anything else is bad advice, which you should stop offering.

I'm getting tired of this exchange, as it seems you're just trying to pick a fight. You're either misunderstanding (willfully?) what I'm writing, or you are very bombastically saying that what is commonly taught by many very well regarded instructors (not to mention whole agency standards) is patently wrong.

I'm still not 100% sure which.

When properly weighted, no one needs to use their BC for routine depth adjustment.

Define "no one" and "needs". I'm really trying to get ahead of every possible misunderstanding here. Yes, it is possible to dive without a BC with good weighting. It is also possible to use a BC to achieve neutral buoyancy at any depth, at any point in the dive, regardless of gas weight. So, the only thing worth discussing is: How big of a depth change (or more specifically the relative change in pressure) where it's advantageous to make an adjustment. You seem to say that it's "bad advice" to make adjustments to the BC unless you "descend significantly deeper", without specifying what a significant depth change is for you.

You only need to add air to the BC on initial descent to keep from cratering into the sand.

Why would the initial descent be much different than a descent at say 10 minutes into the dive? It seems you are assuming square profiles, or at least a quick descent to max bottom depth, and mostly ascending with an empty BC.

And you only need to release air from the BC to compensate for the swing in tank buoyancy from gas consumption, and to compensate for wetsuit and air expansion on ascent.

As in air expansion in the BC? But only on the final ascent? And not on an ascent from 25m to 18m after 10 minutes of bottom time?

 

[BlueTrin]

Wow. You are seriously misguided. A tank is not compressible, its buoyancy doesn't change with depth. I think it's time for you stop offering bad advice.

He’s not saying this, he is saying that someone may not understand that you need more gas to create the same volume displaced to compensate for the same loss of buoyancy.

(However, I doubt that someone may get confused over this, this is one of the first thing that is taught …)

 

[L13]

When properly weighted, no one needs to use their BC for routine depth adjustment.
You only need to add air to the BC on initial descent to keep from cratering into the sand.
And you only need to release air from the BC to compensate for the swing in tank buoyancy from gas consumption, and to compensate for wetsuit and air expansion on ascent.

Period. Anything else is bad advice, which you should stop offering.

This is flat wrong.

At the beginning of your dive, if you have enough air in your BC to compensate for the ~4-5 lb of air in your tank to be exactly neutral at 30 ft(about 2 quarts volume), it will be compressed by a decent to 90 ft (about 1 quart volume), and you will be ~2.5 lb negative.

Similarly, if you are neutral at 90 ft (with enough air in your BC to compensate for ~4-5 lb of air, or about 2 quarts volume), and ascend to 30 ft, it will expand (to about 4 quarts volume) you will be ~4-5 lb positively buoyant.

UNLESS you add air on the decent and release air on the ascent! To maintain about 2 quarts volume in your BC.

As you consume the gas in your tank, the amount of volume to compensate for it in your BC will drop from ~2 quarts to 0.

Only when your BC is empty does its boyancy not change with a change of depth.

This is pure physics, and has nothing to do with your skill as a diver.

 

[oya]

When properly weighted, no one needs to use their BC for routine depth adjustment.
You only need to add air to the BC on initial descent to keep from cratering into the sand.
And you only need to release air from the BC to compensate for the swing in tank buoyancy from gas consumption, and to compensate for wetsuit and air expansion on ascent.

Period. Anything else is bad advice, which you should stop offering.

This is the wrongest post in the entire thread that I can find.

Someone else also said something about newer divers using their BCs too much. That's a close runner-up.

I've got something in the range of 5-6000 dives. I adjust my BC volume any time I change depth more than a foot or two which translates to my adding or dumping tiny gas volumes in and out of BC almost constantly throughout the entire dive.

This is one of the things that EVERY SINGLE STUDENT I HAVE struggles with enormously. Because they are frequently coming from sometimes many years of controlling their buoyancy with their breath instead of compensating for their buoyancy with the thing that is called a buoyancy compensator. And on day one of a class with me (or any number of instructors who are more demanding than, "Hold generally still inside of this 10 foot bubble") they realize the crippling shortcoming of having spent so much time having done so.

EDITED TO ADD:
I am not actually responding to this poster. They seem to have their mind pretty well made up about things. I am responding for the benefit of newer divers looking to learn good practice who thinks that there might be any sort of good point in that post. There isn't.

 

[Nemrod]

I am usually a little light even with my wing sucked flat for the first dive of the day until my suit compresses and purges. So, I swim down. Once down I may need a puff of air to get neutral, okay, if lazy with weighting or a new suit that is not gone flat, two puffs of air. So then I swim around. By the time the dive is near completion my (typically rental aluminum 80) has become buoyant more so and by such time I have vented out those one or two puffs and then I swim up. Once reaching my (safety) stop, I can hold it but I am slightly buoyant and I do not care. If I need to loiter on the surface for pickup then I add a few puffs or even let the wing fill and sit back and relax 'till somebody comes and gets me. Which thus far they have. I try to stay at or near horizontal in the water at all times.

If with heavy suits, semi-dry suits, dry suits, negative steel tanks, yes, the game changes. But still, I always endeavor to swim down, then swim around and then swim back up.

 

[Reno73]

Hi,

In my pool sessions I have almost exclusively been using my LPI hose to adjust my buoyancy in the pool (i think im slightly overweighted too). Is this typical or is descending/ascending meant to be based on breath control?

My typical situation is to dump all air in BCD til i hit the pool bottom, then do 5-7 ish presses on my inflator hose to bring me to neutral buoyancy. I've been doing some research the past few days and this seems to be entirely incorrect. Any input is greatly appreciated!

about your amount of weight at the beginning of the dive. d a buoyancy check. deflate all your bc, breath normally and float at the eyes level. with full tank add 1 kg to compensate the weight of the air loose. best is to do this check with a 50bar tank do the same process and you will find how much weight you need this is for aluminum tank