Question Buoyancy issues with Cressi Commander EVO BC

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fedevivio

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Location
Barcelona
# of dives
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Hello,
I'm a not-so-experienced diver (only 25 dives until now), which I made on a classic rent BCD, I recently bought a Cressi commander EVO BCD, which is instead a hybrid between a classic and wing one.
Now, while there is no stop to learning and improving, with the renting gear I handled buoyancy pretty well, this was also shared by diver masters and instructors.

I did only one dive with the new BCD, but it was a complete disaster. I had to put in all my effort to avoid skyrocketing to the surface in the safe stop and also during the dive I didn't have complete control over it.
I would like to understand how to improve in using it. What I noticed is the following:
- my BCD after the dive was full of a mix of water and air
- I was not able to understand, during the dive, when the BCD was empty of air or not, so I was probably overusing the valves to purge it


More than eager to hear suggestions on how to improve.

Thanks
 
my BCD after the dive was full of a mix of water and air
This is often a sign of being under weighted.

How were you trying to dump air? (Lifting the corrugated hose? Pull dump in the corrugated hose? Right shoulder pull dump? Hip dump?) It's also possible the air bubble was in the wrong place (and your weighting is fine).

I was not able to understand, during the dive, when the BCD was empty of air or not
Generally, I listen for the bubbles. Depending on the dump method, you may be able to feel bubbles on your hand.
 
I had to put in all my effort to avoid skyrocketing to the surface in the safe stop and also during the dive I didn't have complete control over it.
How was the safety stop depth range at the beginning of the dive (might have been short to notice as descent is quick)?
Was that a bit more manageable?
Also how was the tank pressure diff between both points of the dive?

If so, I would consider doing an empty tank weight check

The closer you get to wing style (back inflate) the more effective the wing is to turn volume into buoyancy

So maybe you were underwighted

Edit: a minute late to bringing it up 😅
 
This is often a sign of being under weighted.

How were you trying to dump air? (Lifting the corrugated hose? Pull dump in the corrugated hose? Right shoulder pull dump? Hip dump?) It's also possible the air bubble was in the wrong place (and your weighting is fine).


Generally, I listen for the bubbles. Depending on the dump method, you may be able to feel bubbles on your hand.

I tried using both valves and also the corrugated hose, even tilting myself to try to move air eventually, but it didn't work out very well besides the first attempts where I did hear some bubbles in fact.

That's a good suggestion I'll try to pay more attention to it, thanks!
 
How was the safety stop depth range at the beginning of the dive (might have been short to notice as descent is quick)?
Was that a bit more manageable?
Also how was the tank pressure diff between both points of the dive?

If so, I would consider doing an empty tank weight check

The closer you get to wing style (back inflate) the more effective the wing is to turn volume into buoyancy

So maybe you were underwighted

Edit: a minute late to bringing it up 😅

I'm sorry but I didn't get your questions, do you mind rephrasing them? My lack of knowledge might be the issue in understanding them ^^.

Thanks!
 
Your height/weight, amount of weight carried and how carried?

I carried 8Kg, and I didn't have any issue descending, even though ofc this doesn't mean anything given that gas consumption counts as well. Unfortunately, all of them were on the belt, something I'm already planning to change and probably put 2 + 2 near the tank.

Thanks!
 
I carried 8Kg, and I didn't have any issue descending, even though ofc this doesn't mean anything given that gas consumption counts as well. Unfortunately, all of them were on the belt, something I'm already planning to change and probably put 2 + 2 near the tank.

Thanks!
And you are how tall, you weigh how much, what exposure protection, you are talking about trim, which will be your next problem.
 
I'm sorry but I didn't get your questions, do you mind rephrasing them? My lack of knowledge might be the issue in understanding them ^^.

Thanks!
Sure thing — took me more than 25 dives to get this concept so you’re in the clear 😅
You might already know most of what I’ll mention, but I’m gonna „over“ explain it foe the benfit of future readers

I’m wondering if it’s a matter of overweight or „trim“ (center of mass/buoyancy placement)

Tanks change weight (significant enough) during the dive; an Al80 tank (12L) is heavy at the start of a dive, and is neutrally buoyant at ~100bar; it gets super floaty at reserve
On a single tank dives, esp. in Alu tanks, if you go from 200bar to ~50bar it’s the equivalent of „dropping“ 1.5kg of weight

If you’re weighted just enough to descend at the beginning of the dive, and you’re in a „thicker“ wetsuit (gets floaty near surface/ not at depth die to trapped air), at the safety stop it’s gonna be near impossible to not to shoot up (or how we call it in Egypt: float like a tea bag)
So, proper weighting is done at the end of a dive; you get as close to exit (calm conditions) as possible; drain the tank to 50 bar, and empty your BC completely — on a „half lung“ you should float just barely, a full inhale should get you to sit on the surface relatively comfortable

Hence my questions, was descent super quick or barely at the beginning (trying to gauge weighting with exclusion of wetsuit effects)
And was safety stop at 100 bar or 50 bar (big or small diff in “lost” weight of consumed air); if my thinking about weighting is the majority of the issue or just a part

Re Trim:
Did you feel a tendency to roll into one direction or the other?
(Bad) Eg.: If I have all my (correct value for bouamcy) weights on the tank near the valve, I would do alright in the sense of decend/stop; but I would be very head heavy that I will be stuck in a star fish pose (inverted) aince thats the heaviest part of my body
That’s a longer topic that I understand but can’t explain well
(What lex is asking about relates more to trim btw)

(PS: I might have wrangled some parts of the explanation as I’m no “true” expert —but the essence is that🤷🏽‍♀️)
 
This is a bit out of scope and excessive details since it’s sidemount config, but it touches on the subject of weighting quite well tbh


Also — at the risk of overwhelming you — this is a great resource to check out
 

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