Question Buoyancy issues with Cressi Commander EVO BC

[fedevivio]

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)

Super clear!
Just to give as much detail as possible
- height: 180cm
- weight: 70kg
- tank: steel 15L
- suit: cressi semi-dry (7mm)
- belt: 8kg of weight

Answering your questions:
- Descent was at normal pace.
- Safety stop at 70 bar left
- No specific feeling of rolling to a specific side

Let me know if you want to know anything else, thanks!

 

[Mobulai]

tank: steel 15L
- suit: cressi semi-dry (7mm)
- belt: 8kg of weight

Answering your questions:
- Descent was at normal pace.
- Safety stop at 70 bar left
- No specific feeling of rolling to a specific side

15L and 200-70bar is ~2kg of air (1950l of air at ~1.03 kg/1000L)

A 7mm semi dry is a lot of “air bubbles”, so if you descend “normal” and not dropping like a stone, (and I am gonna make an assumption that you had a full empty BC at that point); the wetsuit alone becomes super floaty at the safety stop
Given these conditions — you might need no air in the BC at that point

Steel tanks get less floaty btw in comparison to Alu — but I’m more certain that you were under weighted

On your next dive give it a try — lots of videos show that technique (better than my text explanation), probably you’ll need 1-2 KGs to compensate for that Air (in that same config)

For reference, I’m 178cm/70kg, in a 2.5mm wetsuit and Alu 12L tank I need 5.5KG; a 12L steel and drysuit I meed 11KG (ok fresh water so conversion to saltwater ~12kg)
So it’s a bit of an inexact science, and best way is to try it out (weight check) on a dive whenever you have a new “config” and write it in your “logbook” what works/not

 

[Mobulai]

Re Trim (which comes after figuring out buoyancy), I suggest you look at this thread

C

Thread 'Fixing diver trim and weight placement'

Oct 29, 2024

I added this to a post in the cave diving forum a while ago, but it seems common enough that I wanted to post it here in basic scuba as well, to hopefully prevent people from chasing their own tails as I did for a while.

One common issue I encounter with divers working on their trim, especially in a double-tank configuration, is an unintentional "head-up" posture. In almost every case, if a diver’s trim is head-up, it’s often due to being "head-heavy." The solution is to move some weight down toward their feet.
Here are some definitions to clarify:

• Center of Gravity (CoG):...

• crofrog
• Replies: 20
• Forum: Basic Scuba

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