Overweighted at beginning of dive but underweighted at end in shallows

[LFMarm]

The implication is that the “buoyancy swing” is likely rooted in poor weighting from the outset of the dive, which is easily masked in underweighted divers until they start running low on gas at a relatively shallow depth.

This is the crux of our disagreement. The buoyancy swing has nothing to do with weighting (including type of metal used for the tanks). The swing is only driven by the amount of gas consumed and its density (bigger swing with nitrox than trimix).

 

[Zach The Diver]

This is the crux of our disagreement. The buoyancy swing has nothing to do with weighting (including type of metal used for the tanks). The swing is only driven by the amount of gas consumed and its density (bigger swing with nitrox than trimix).

The swing itself does not. But the resulting effect on the diver’s ability to maintain buoyancy as the cylinder(s) empty has everything to do with their starting weighting which is influenced by the composition of tank (among other things).

Your comment about the different compositions of gas having an effect is correct. The presence of helium would render a gas much less dense.

 

[EdThatchLongfins]

This is why RAID teaches that a diver should check for correct buoyancy with a partially filled cylinder (70bar/1000PSI). It helps compensate for when the cylinder starts becoming light at the end of a dive.

 

[JMarc]

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

The volume of the gas in the cylinders doesn't change, the amount (and thus the weight) does.

You'll perhaps consider this as nitpicking as well, but I've spent more time than I'd have liked to clear up the confusion due to the usage of volume units at an implied pressure and temperature (often 1bar, 15 °C) to give measure of amount of gas to let extend that source of confusion from the unit domain (where it is too common to be avoidable) to the physical quantity names.

 

[Zach The Diver]

The volume of the gas in the cylinders doesn't change, the amount (and thus the weight) does.

You'll perhaps consider this as nitpicking as well, but I've spent more time than I'd have liked to clear up the confusion due to the usage of volume units at an implied pressure and temperature (often 1bar, 15 °C) to give measure of amount of gas to let extend that source of confusion from the unit domain (where it is too common to be avoidable) to the physical quantity names.

You’re correct as the pressure and amount changes but occupies the same volume. That’s a fair correction and I’m glad you brought it up.