Lung volume

[glbirch]

I'm getting confused. Lung volume is measure in Tidal Volume, Total Lung Capacity, Residual Volume and Vital Capacity.

A standard 80cu' tank at 3000psi (203 bar) hold 2265 litres. If I breathe it down in an hour at 33' (2 atmospheres), I'm consuming 38 litres/minute or somewhere between 2.5 and 3 liters/breathe assuming 12 to 15 bpm. Is this correct, or am I missing something? Or is average Tidal Volume of .5 liters only a resting number?

If this is the case, then at 4 atmospheres that tank would last 15 minutes, at 16 atmospheres it would last 3.75 minutes. Doesn't seem right somehow.

 

[beche de mer]

I'm getting confused. Lung volume is measure in Tidal Volume, Total Lung Capacity, Residual Volume and Vital Capacity.

A standard 80cu' tank at 3000psi (203 bar) hold 2265 litres. If I breathe it down in an hour at 33' (2 atmospheres), I'm consuming 38 litres/minute or somewhere between 2.5 and 3 liters/breathe assuming 12 to 15 bpm. Is this correct, or am I missing something? Or is average Tidal Volume of .5 liters only a resting number?

If this is the case, then at 4 atmospheres that tank would last 15 minutes, at 16 atmospheres it would last 3.75 minutes. Doesn't seem right somehow.

The lung parameter which is important in scuba is minute volume, which is the amount of gas consumed in one minute, which is the tidal volume x breaths per minute. The starting point for gas consumption calculations should be minute volume on the surface.

Your calculations are correct to a point.

80 cu ft is 2265 litres.

If you consume this in an 60 minutes at 33' (10 metres or 2 atmospheres) it will last you 120 minutes on the surface.

2265 litres in 120 minutes is ~19 litres per minute - about average, or slightly on the high side. At 4 atmospheres, your minute volume would be 19x4 = 76 l/min.

At 4 atmospheres (30 metres) 2265 litres would last you 30 minutes. (120/4, or 2265/76)

At 16 atmospheres it would last you 7.5 minutes. (120/16)

Assuming 19 breaths per minute, you would have a tidal volume of ~1 litre on the surface.

The oft quoted average tidal volume of 0.5 litres is, as you say, a resting parameter, and doesn't have much real world relevance.