Explain this

[sbl212]

I got my OW at a resort in Honduras the beginning of this year.
I was there for three weeks diving and for two of the three weeks I the same dive master/guide. I got to know him a bit but I've noticed a couple things about him (the things he did while diving) that seemed odd to me.

First let me say I thought and still do think that he is a great diver as I'm sure he has been diving longer than I've been breathing.

So the first thing I noticed was that he had excellent buoyancy (which I'm sure comes from being a seasoned DM) but what puzzles me is that I later noticed he never connected his BCD to his tank. At first I noticed this on the boat and just assumed he took it off after the dive but I, being the curious, eager diver I am, decided to follow and watch him closely during the dives. He never connected his pressure hose to the BCD. He had air in the BCD but he never needed to adjust the amount of air whether he was at 30ft or 100ft.
So my question is, does his years of experience allow him achieve perfect buoyancy without any adjustment or is he part fish?
I just don't see how he can be (seemingly) neutrally buoyant regardless of depth.

Also, I noticed he seems to have a SAC rate that defies logic. During one dive we had to try to stay stationary identifying sponges in strong current (I was told ~4 knots, this was after a large storm). All of us were in for about 20min at 30ft starting with 3000psi and came out with less than 500 while he came out with around 2000psi while sharing air with another member of our group who had ran out of air during the safety stop.

Oh, did I mention he wears fins that are too small for his feet? One of those full foot fins, he has to wear them with the heel of his feet sticking out.

Keep in mind I'm just a "noob" diver so it might be that case that all divers who have been diving for 20+ years can do this.

 

[ben_wilson3301]

I would guess he orally inflates his bcd, but I hardly have any air in mine. Bouyancy is all about proper weighting and using your lungs. It has very little to do with your bcd. I use mine mostly to float at the surface. As far as sac rate... we had a class last weekend, I was in the pool for three hours in 8 ft of water. Started with 3100 psi, finished with 2965. It comes with practice and becoming comfortable in the water, along with decent physical fitness.

 

[diversteve]

Also, I noticed he seems to have a SAC rate that defies logic.

They all do... We did two scooter dives with a DM/Instructor a few years ago. We went into the water with full 3000 psi 80's. He went in with a 63. Mentioned he'd already used it for two dives that morning so he was down to about 1800. Coming out after the 2nd dive I noticed he had 1200 left...we were breathing fumes by then which called the 2nd dive.

It's just not right....

 

[WhiteSands]

So the first thing I noticed was that he had excellent buoyancy (which I'm sure comes from being a seasoned DM) but what puzzles me is that I later noticed he never connected his BCD to his tank. At first I noticed this on the boat and just assumed he took it off after the dive but I, being the curious, eager diver I am, decided to follow and watch him closely during the dives. He never connected his pressure hose to the BCD. He had air in the BCD but he never needed to adjust the amount of air whether he was at 30ft or 100ft.
So my question is, does his years of experience allow him achieve perfect buoyancy without any adjustment or is he part fish?
I just don't see how he can be (seemingly) neutrally buoyant regardless of depth.

It depends on what kind of suit he's wearing, and how deep you dive. It's possible to do if you're diving in shallow waters in maybe a rash guard and shorts, and you adjust your breathing range accordingly. It's uncomfortable but doable. But if you are wearing a wetsuit on deeper dives, I don't think it's possible. I might be wrong.

Also, I noticed he seems to have a SAC rate that defies logic. During one dive we had to try to stay stationary identifying sponges in strong current (I was told ~4 knots, this was after a large storm). All of us were in for about 20min at 30ft starting with 3000psi and came out with less than 500 while he came out with around 2000psi while sharing air with another member of our group who had ran out of air during the safety stop.

Dive professionals, especially female ones, have very good SAC rates. One I regularly dive with has 50% of the air left when we end the dive. However, it could also be that he started with much more air in the tank than anyone else. Some operators I've used have tanks that can vary between 175 - 230 bar.

 

[scubastingray]

I was in the pool for three hours in 8 ft of water. Started with 3100 psi, finished with 2965.

And what size tank is that?
If it were an AL80, you're claiming that you used 3.6cf for a period of 180 minutes, or .02cf/min. At a depth of 33feet you're claiming you could stay down for 2066 minutes, or 34 hours.

Maybe you mistyped a number or spent most of that time on the surface but I'm not buying it. If it were double HP150s, maybe.

 

[ben_wilson3301]

Three hours of pool time, some at the surface
talking. Then skills, snorkel reg exchange, cesa, etc. When at the lake I usually get in 3 hours or so of bottom time at 20'-30'.



And no, not all on the same dive.

 

[clownfishsydney]

When I dive in the tropics the only air I need to put in my BCD is a tiny bit just to cause it to open up a bit. I think I would still connect the inflator hose as if the shi$ hit the fan and you had an overweighted diver to assist, you may have some trouble.

Low SAC rates for good divers who dive a lot is not uncommon.

A current of 4 knots is impossible to stay still against, even 2 knots is impossible for more than a few minutes for all but the fittest divers. Was more likely to be 0.5 knot or less.

 

[Doc]

Knowing Honduras as I do, knowing DM moves as I do, his inflator likely had mechanical issues. Simpler for him to just un-plug it.

You will not, by the way, see this behavior at all destination resorts.

Icthyesque SAC Rate, yes. I assume you saw the gill slits, they are located just aft of the ear-like nodules.

 

[herman]

What you witnessed is proper weighting and buoyancy control, something most divers don't have a clue about. BCD stands for buoyancy COMPENSATING devise with compensating being the operative word. It is supposed to be used to offset (compensate for) changes in your buoyancy during a dive, not to cover up overweighting. When diving wet there are only 2 things to compensate for. Those are gas used from a tank and wetsuit compression. The swing on an 80 cf tank, regardless of the material contrary to what some believe, is around 4 lbs, a little more if you suck it dry. Wetsuit compression varies with wetsuit thickness, material and age but for a 3mm shortie with a good number of dives on it, it's only a few pounds. After 15ft a lot of the shift that is going to happen in a wet suit has occurred and at 30 ft half of it has occurred, after 60 ft the remaining compression is minimal. If the DM started off dead on neutral at the surface he will be 2 maybe 3 lbs overweighed at depth with a full tank. An average male has about 10 lbs of total shift in buoyancy from completely full to completely empty lungs which leaves plenty of room to deal with a few pounds of off neutral weight. I find about +/- 4 lbs is the upper limit of a comfortable amount of over/under weighting for me with +/-2 being very easy. As he uses gas, his weighting shifts from a few pounds over weighted to a few under weighted, both of which are very easy to control with breath control. If he was not using a wet suit or it was old (very likely) which means it has little remaining compressibility then the swing becomes just the gas used. In which case he would simply over weight by 2 lbs at the beginning of the dive and end up at 2 under weighted at the end of the dive. 2 lbs is a very easy amount to deal with for an experienced diver. Proper weighting and BC use is one of the reasons experienced DM.s use less gas. Since they are not overweighed, they are not dragging an inflated BC bladder (think sail) around nor are they moving excessive weight.
If you are constantly adding or removing air from your BC during a dive, you are overweighed. Depending on wetsuit and tank size, you should have a little air remaining in your BC at depth at the beginning of a dive and it should be completely empty during your safety stop. Any more than that and you are overweighed which requires more work to move and that uses more gas from your tank.

 

[scubastingray]

Three hours of pool time, some at the surface
talking. Then skills, snorkel reg exchange, cesa, etc. When at the lake I usually get in 3 hours or so of bottom time at 20'-30'.



And no, not all on the same dive.

Right, 3 hours of bottom time is very different from 3 hours at the surface while using ~4.4% of the tank.
If the original set of numbers were an order of magnitude larger (the consumption), you might have had me fooled.


Also, OP, the aspiration in some circumstances of not using a bcd is simply unreasonable. A low sac rate helps, but the larger your gas consumption the more your weight swing. I'd like to see anyone stay neutral while using up a set of 120s.
The point is, the air you breathe has weight and will cause buoyancy characteristics that can be beyond what you can compensate. You will have to use your bladder (particularly on larger tanks at the beginning of dives) to remain neutral.