Messed up and ascended like a missile

[Storker]

There’s a lot to take into consideration. Suit compression and cooling, working air out of padding/hard to vent areas, etc. which affects the final outcome by a few lbs.

Srsly, "a few lbs"?

Ok, I'm a drysuit diver. I know that drysuit weighting is kinda different from wetsuit weighting. But still, "a few lbs"? Last time I dived wet, I was issued a belt with a pretty good estimate of what I'd need. Trying to submerge I discovered I was a tad light, so I asked for a kg or two extra. Stuffed the weights inside my WS and dived just fine.

Two lbs is (roughly) one kg. For a normal sized diver, that's within the margin of error. I wouldn't stress with being a couple kg too heavy compared to absolute minimum weighting.

 

[Roger Hobden]

Speaking of weight, is it better for a beginner diver to purchase an aluminum cylinder, and then to switch to steel eventually once he becomes more confident of his ability to weight himself correctly, or immediately start with a cylinder that will always be negatively buoyant throughout the dive ?

 

[MichaelMc]

Speaking of weight, is it better for a beginner diver to purchase an aluminum cylinder, and then to switch to steel eventually once he becomes more confident of his ability to weight himself correctly, or immediately start with a cylinder that will always be negatively buoyant throughout the dive ?

Steel tanks are not always the best 'final destination'. If, tank aside, you need 6 lb. of ballast, a HP80 at -1.7 lb. empty gives you less flexibility in your lead placement and ditch ability than a +4.2 lb. AL 80. Having 10 lb. of lead between a belt and trim pockets is easy to manage, needing only 4 lb. does not give you much flexibility, particularly if you'd like some to be ditchable.

For total weighting, including tank, how much weight the tank loses matters, which is a function of volume. That it goes from negative to positive is not the issue, except if you have very few pounds of lead, and the tank ending up positive gives you a few pounds of lead more to play with in terms of how you distribute it.

Also, a negative tank above you gives you more tendency to turtle, while a little bit buoyant tank keeps you more stable. In the great white north, the AL80's cost of requiring an extra 3 lb. of lead may be annoying, and you likely do not lack for lead to move around as needed regardless of tank choice. Something like an LP80 or HP100 might be a happy middle ground on that, with more capacity than the AL80.

A dated thread: Aluminum vs Steel & why?
A more recent thread: Steel 100 vs Aluminum 100

 

[Magnus Lundstedt]

Speaking of weight, is it better for a beginner diver to purchase an aluminum cylinder, and then to switch to steel eventually once he becomes more confident of his ability to weight himself correctly, or immediately start with a cylinder that will always be negatively buoyant throughout the dive ?

For me the choice of tank is more due to what suit I diving in. When diving in a thin wetsuit I am now a tad heavy with AL80 and no weights (SS BP). When diving dry in cold water i dive steel 12l and still use 8-10kg weights. same equipment otherwise.

 

[boulderjohn]

The decision on what kinds of tanks to purchase depends upon a number of actors. For me the biggest factor is the amount of gas it will hold.

The change in weight during a dive has nothing to do with whether or not it is aluminum or steel. You need to carry a certain amount of lead in order to make the total weight of you and all your gear, including the tank, appropriate for the dive. If you are using aluminum tanks, you will need to start the dive with more lead than if you have steel tanks. During the dive, your total weight will drop as you use up the gas in your tank. If you use up 6 pounds of air, you will weigh 6 pounds less at the end of the dive than you did at the beginning, regardless of the composition of your tank.

 

[Roger Hobden]

The change in weight during a dive has nothing to do with whether or not it is aluminum or steel. You need to carry a certain amount of lead in order to make the total weight of you and all your gear, including the tank, appropriate for the dive. If you are using aluminum tanks, you will need to start the dive with more lead than if you have steel tanks. During the dive, your total weight will drop as you use up the gas in your tank. If you use up 6 pounds of air, you will weigh 6 pounds less at the end of the dive than you did at the beginning, regardless of the composition of your tank.

That was my understanding also.

 

[boulderjohn]

That was my understanding also.

It is surprisingly hard for many people to understand. I bet I have read 100 posts on ScubaBoard over the years telling people not to use aluminum tanks because, unlike steel, at the end of the dive the tanks float and will be pulling them to the surface.

 

[Roger Hobden]

It is surprisingly hard for many people to understand. I bet I have read 100 posts on ScubaBoard over the years telling people not to use aluminum tanks because, unlike steel, at the end of the dive the tanks float and will be pulling them to the surface.

But a diver does need some extra detachable weight with AL cylinders, compared to a steel cylinder of the same capacity, right ?

 

[Eric Sedletzky]

Srsly, "a few lbs"?

Ok, I'm a drysuit diver. I know that drysuit weighting is kinda different from wetsuit weighting. But still, "a few lbs"? Last time I dived wet, I was issued a belt with a pretty good estimate of what I'd need. Trying to submerge I discovered I was a tad light, so I asked for a kg or two extra. Stuffed the weights inside my WS and dived just fine.

Two lbs is (roughly) one kg. For a normal sized diver, that's within the margin of error. I wouldn't stress with being a couple kg too heavy compared to absolute minimum weighting.

When wetsuits compress they don’t necessarily reinflate to normal thickness at the same rate that a diver ascends. They also can cool down considerably in cold water. Both of these factors affect the buoyancy of a suit. I wear a 7mm wetsuit where I dive and the water is 48 degrees. Yes, my suit will easily lose at least two lbs or more of buoyancy from the time I first get in to when I complete a full length dive to depth and exit. When I first get in my suit has been in the sun and it’s warm and plump. After I do my dive it has been compressed and cooled and has lost a considerable amount of buoyancy. I use a BP/W so the only part of my rig that will hold any sort of residual air in a wrinkle or fold would be my wing. Getting under water is really the only way to get all the air out, by having water pressure force it out. This is difficult to do on the surface. Jacket BC users will experience even more residual air pocket problems with padding, sewn straps, piping, design features, hollow plastic components that need to fully flood, bungeed wing folds, etc.
So yeah, by doing a weight check as soon as you get in, it’s possible to overweight yourself by as much as 5 lbs from the start by not taking these factors into consideration. That’s 5 lbs I’d rather not drag around and not have on me at the end of my dive on the surface.

 

[boulderjohn]

But a diver does need some extra detachable weight with AL cylinders, compared to a steel cylinder of the same capacity, right ?

Whether they are detachable or not, you will need to carry more weight with an aluminum tank than a standard steel tank, sure. My point is that the change in buoyancy during the dive depends upon the amount of gas you use. An aluminum tank starts lighter than a steel tank and ends lighter, but they both lose the same amount of weight.

It is the total weight of you and all your gear that determines your buoyancy, not just the weight of the tank.