Number one cause of diving fatalities?

[tridacna]

Nobody wants to fly to a dive site carrying a lot of lead in a suitcase. In the dive sites where most divers live nearby, that policy makes sense, but it makes no sense in which a sizable portion of the population has to travel to get there. The one time I dived in Southern California, I had to fly in. I had to rent weights. That's the only place I have ever dived where that happened.

My point precisely.

As an interesting aside, there are a few operators in Bermuda that inform their prospective clients that Bermuda waters have "much more salt" than anywhere else in the world and divers are advised to add 10-20% more weight than they usually use. They are simply sick of newbies not taking enough weight so they load them up. Dangerous practice in my opinion.

 

[kelpqwest]

It is with a normal breath, no air in the BCD, and a full tank. If your tank is empty, you will be about 6 pounds lighter with an AL 80.

This has been standard advice throughout the scuba industry for many decades. It's nothing new.

It is still wrong.
The mouth and nose need to be out of the water, full tank or not.
My understanding was also that an aluminum 80 is 4.4 lbs positive when empty.

 

[John C. Ratliff]

Just so you'll know, here is the U.S. Navy Diving Manual, 1975 explanation of breathing. You'll note that it states that the normal tidal volume is 0.5 liters, which has been mentioned elsewhere here. But you'll also see the circle that shows breathing volumes that differ, and the highest in that circle is around 2.5 to 3 liters. That's where I breathe underwater with scuba or a snorkel.



What that means is that I have much more control over my personal buoyancy than has been explained above. I needed to wait until I got into PhotoBucket to get this graph and make this explanation. The reason is that you don't want to simply move air already breathed (from the snorkel, or the mouthpiece) back into your lungs. You want to increase the inhalation and have a greater exhalation to adequately ventilate the lungs underwater or with a snorkel. This is something that really is apparently not taught much anymore. And, by the way, my Vital Capacity is not 4-5 liters, but rather (for me, each individual is different) 7 liters.

SeaRat

 

[boulderjohn]

It is still wrong.
The mouth and nose need to be out of the water, full tank or not.
My understanding was also that an aluminum 80 is 4.4 lbs positive when empty.

You are totally, completely, 100%--however you want to put it--missing the point.

The weight check is just a test to see how much weight you need to perform a dive. It is not a life saving skill.

You are correct about the buoyancy of an aluminum 80 when empty--so what? That has no bearing on the discussion.

Your posts are baffling me. You claim to have a very large number of dives, but your posts also indicate a lack of knowledge about some of the most basic concepts of scuba diving. It is hard to reconcile those two observations.

 

[dmaziuk]

As an interesting aside, there are a few operators in Bermuda that inform their prospective clients that Bermuda waters have "much more salt" than anywhere else in the world and divers are advised to add 10-20% more weight than they usually use. They are simply sick of newbies not taking enough weight so they load them up. Dangerous practice in my opinion.

Of course: they're lying in order to make newbies dangerously overweight. Because they're evil. Everybody knows salinity has nothing to do with buoyancy.

 

[dmaziuk]

The point of the calculations is that your lungs move -- as seen on the graph -- 2.5-3 litres of air when you breathe underwater. Causing buoyancy shift of, say, 6 lbs. (And tank weight indeed has nothing to do with that. It was Friday afternoon is my excuse. If you're neutral at midpoint of that, you'll be +3 lbs when you breathe in, At 3 m that can easily pop you out of the water.

One of the problem with breathing with lungs empty is that it also reduces your tidal volume and, as SeaRat's pointed out, you're not venting enough CO2 out of the dead spaces. So, you want to hold safety stop comfortably, you want to wear 3 pounds of lead on top your "perfect" weight to compensate.

If you're 3 lbs over the "perfect" weight at the end of the dive, there's no way you'd float with "nose and mouth above the surface", as @kelpqwest wants to, during the weight check. That's just the way it is, if you can't swim up carrying 3-4 lbs and stay up long enough to oral-inflate you bc and/or undo your weight pouches, then maybe you should stick to fishing.

Under normal circumstances, that is -- if your dive goes pear-shaped and you're having a heart attack while out of gas at depth, that's a different story that has little to with weight,

 

[tridacna]

Of course: they're lying in order to make newbies dangerously overweight. Because they're evil. Everybody knows salinity has nothing to do with buoyancy.

It does? I had no idea. I thought that it was the specific gravity of the sea water. Do tell more.

BTW: SG (or salinity) in BDA is no different to anywhere in the caribbean.

 

[John C. Ratliff]

dmaziuk said: ↑
[QUOTE="dmaziuk]Of course: they're lying in order to make newbies dangerously overweight. Because they're evil. Everybody knows salinity has nothing to do with buoyancy.
It does? I had no idea. I thought that it was the specific gravity of the sea water. Do tell more.

BTW: SG (or salinity) in BDA is no different to anywhere in the caribbean.

I'm curious, dmaziuk, why I need to add about five pounds to my weight belt when I dive salt water rather than fresh water?

SeaRat

 

[2airishuman]

As for me, while diving I prefer a few extra pounds beyond optimal weighting because I like being able to dump a tiny bit of air at the end of the dive without having to go through a gymnastics routine to get the air bubble at the exit point.

I do this too, usually adding two pounds beyond what would be "optimal" for better control.

 

[dmaziuk]

I'm curious, dmaziuk, why I need to add about five pounds to my weight belt when I dive salt water rather than fresh water?

Well, you don't, unless you arrre divink in Rrrusshia:
Edit: @tridacna quoted 10%, 5 lbs is 10% of 50 lbs, what I am curious about is why you need to wear 50 lbs on your weight belt. If I wanted to add 10% to my weight for extra salinity between, say, Roatan and Bonare, that'd be a .8 lbs and last I looked they didn't make weights like that.