Overweighted at beginning of dive but underweighted at end in shallows

[Zach The Diver]

The risk of uncontrolled ascent is exactly the same for any tank for a given amount of gas consumed during the dive. This is not an opinion, it’s physics. Different tanks have different trim full vs empty but the difference of upward force between beginning and end of a dive is only driven by the amount of gas used.

The composition of the cylinders holding that volume of gas also plays a role.

 

[BRT]

Seems a person would just need to know their appropriate weight for every tank configuration. Yes identical tank capacity will drop the same amount on a dive but different tank material will change the starting weight needed. And tanks of different capacity may drop different amounts on a dive depending on whether you use the extra capacity and have longer dive times at similar depths and task loading.

True. You weight for the end of the dive not the beginning. That is up to your BCD.

 

[BRT]

The composition of the cylinders holding that volume of gas also plays a role.

Some tank materials lose weight at the end of a dive? No, it is only the air in the tank that changes.

 

[Zach The Diver]

Seems a person would just need to know their appropriate weight for every tank configuration. Yes identical tank capacity will drop the same amount on a dive but different tank material will change the starting weight needed. And tanks of different capacity may drop different amounts on a dive depending on whether you use the extra capacity and have longer dive times at similar depths and task loading.

Spot on and I sort of felt that this was an obvious jumping off point that everyone here in this thread understood that we all knew. The fact that this had to be stated bodes poorly for the quality of this thread.

 

[Zach The Diver]

Some tank materials lose weight at the end of a dive? No, it is only the air in the tank that changes.

That’s not what I said.

It’s what RobPNW stated- they have inherently different buoyancy characteristics. Not taking that into account at the beginning of a dive with weighting and whatnot- especially when changing from one configuration for one type of tank to another- may have a detrimental effect on buoyancy…

 

[BRT]

That’s not what I said.

It’s what RobPNW stated- they have inherently different buoyancy characteristics. Not taking that into account at the beginning of a dive with weighting and whatnot- especially when changing from one configuration for one type of tank to another- may have a detrimental effect on buoyancy…

This is what you said.

"The composition of the cylinders holding that volume of gas also plays a role."

It is false, in relation to buoyancy change.

 

[lowwall]

The composition of the cylinders holding that volume of gas also plays a role.

No it doesn't. The only thing that matters is the overall bouyancy (tank+other gear+diver+lead) and the gas used.

There's no bouyancy difference at any point in a dive between a diver with HP80 and the same diver with an AL80 plus an extra 6 pounds of lead on the belt (or wherever).

 

[Zach The Diver]

It absolutely plays a role in the weight required to hold neutral buoyancy with all other variables equal.

I did not say that the weight of the cylinders (ie weight of the material) change during the dive. Not once. Actually read what I said.

The only thing that changes during the dive is the volume (and weight) of the gas in the cylinders.

If you were not properly weighted to begin with, then that’s a potential problem. Particularly if you were properly weighted for steel but failed to adjust when switching to aluminum.

 

[Zach The Diver]

No it doesn't. The only thing that matters is the overall bouyancy (tank+other gear+diver+lead) and the gas used. There's no bouyancy difference at any point in the dive between a diver with HP80 and the same diver with an AL80 plus an extra 6 pounds of lead on the belt (or wherever).

How to Select a SCUBA Tank | Dive Gear Express®

How to choose the best SCUBA tank for specific diving needs and styles of diving.

www.divegearexpress.com

One of many sources readily available that dispels that.

The whole idea of that weighting is to compensate for the difference induced by the different composition of the cylinder.

Steel is much denser than aluminum. A less dense object is more likely to float.

 

[lowwall]

I wrote

There's no bouyancy difference at any point in a dive between a diver with HP80 and the same diver with an AL80 plus an extra 6 pounds of lead on the belt (or wherever).

You responded

The whole idea of that weighting is to compensate for the difference induced by the different composition of the cylinder.

Steel is much denser than aluminum. A less dense object is more likely to float.

You are just flat out wrong. Trim will change. Bouyancy will not.