Please help me understand something about BCD's

[Compressor]

What the TV narrator said makes no sense. The BC does not know how deep it is.

 

[boulderjohn]

What the TV narrator said makes no sense. The BC does not know how deep it is.

Just clarifying the language--I know you understand this, so I am not contradicting.

The buoyancy you need from the a BCD depends upon its total volume after air is inserted. If your total body plus gear requires X volume of air in the BCD to be neutral, you will need exactly that total volume at any depth. As you descend, however, compression due to pressure means you need more and more and more molecules of air in that BCD to achieve that required volume. Thus, at great depths you will need to pour much, much more air into the BCD to achieve the same volume you had at shallow depths, but the ability of the BCD to achieve that volume remains the same.

 

[Compressor]

Just clarifying the language--

Well done for those who did not remember the physics of depth and pressure.

 

[Geobound]

Isn't that why you have quick release weight pockets?

 

[rmssetc]

Depending on the thickness of his wetsuit it coukd have been compressed to a level where his bcd could not provide enough lift to become positivle buoyant.

Without a wetsuit I am -3lb at a start of a dive with my 7mm semidry i need 22lb of lead to get me down and if my wetsuit lost all buoyancy i would be 23lb negative which woukd mean a 20lb lift bcd would be pointless

Unless you drop 3lbs of lead.

Or even if you don't...you can probably swim up the 3lbs difference ... or 10lb if needed....hopefully far enough for the wetsuit to regain some buoyancy.

 

[Altamira]

Until Geobound and rmssetc brought it up, I was wondering how long it would take someone to bring up the issue of ditching weights or swimming up whatever weight is not dropped. Before the introduction of the "horse collar" and follow on bcds, divers had to swim up their weights on every routine dive. Bcds are a great tool, but are usually not critical for diving to rec limits.

 

[Bent Benny]

Just clarifying the language--I know you understand this, so I am not contradicting.

The buoyancy you need from the a BCD depends upon its total volume after air is inserted. If your total body plus gear requires X volume of air in the BCD to be neutral, you will need exactly that total volume at any depth. As you descend, however, compression due to pressure means you need more and more and more molecules of air in that BCD to achieve that required volume. Thus, at great depths you will need to pour much, much more air into the BCD to achieve the same volume you had at shallow depths, but the ability of the BCD to achieve that volume remains the same.

There would be a point when a standard BCD would stop working though, right? If someone was at a depth of 500ft and the ATA exceeds the IP, then would it inflate? Or Does an air balanced reg mean that the IP is 150psi above ambient pressure, whatever that may be.

 

[boulderjohn]

Or Does an air balanced reg mean that the IP is 150psi above ambient pressure, whatever that may be.

It does.

If the diver has reached a depth where the regulator no longer works, there are bigger problems than the BCD not inflating.

 

[herman]

The narrator like the diver was narked. What was useless were their brains.

FWIW modern regulators are balanced, that means they are balanced with the ambient water pressure and will deliver gas at the intermediate pressure (<150 psi) until the pressure in the cylinder equals the absolute pressure. Which for the blue hole is 100m or 10ATM or simplicity ~150 psi. So gas will be delivered to the BCD until the cylinder is down to ~300 psi. At which point one is screwed. Actually one would be screwed well before that at that depth.

This point often gets confused by divers. Balancing and depth compensation are completely different things but do work in conjunction with each other. What you are describing in depth compensation which all dive regulators have and like you describe, the purpose is to keep the IP at a constant value above ambient pressure. This keeps the work of breathing of the second stage constant as ambient pressure (depth) changes.
Balancing of the first stage has a different purpose, it keeps IP at a constant pressure as tank pressure varies which in turn keep the work of breathing of the second stage constant as the pressure of the tank changes.

While the end result is the same, keeping the work of breathing of the second stage constant, the 2 systems compensate for 2 different variables which are not related to each other.

 

[Scared Silly]

Yes, you are correct I should have stated that more concisely, I'll edit my post.