You mean I need metric to know there is more gas in a 104cf then an 80cf?

Tony, are those metric minutes?

I agree that it's easier, but we're still stuck in the mid 1800s and I
doubt we'll ever change.

There was a guy in rec.aviation.homebuilt who was complaining because
his plans were in metric and he didn't want to be converting back and
forth all the time and introducing errors.  Several people told him to
buy metric measureing equipment and just work with that.  His
solutions was to sell the plans and buy something he "could work
with".

Ummm, guess it's not so simple after all.  You got it wrong.  A standard 100
cubic foot tank, pumpted to its standard pressure, contains 100 cubic feet
of gas.  Kind of hard to mess up the calculation.

A normal diver (assumes that there really is such a thing as a normal diver)
consumes about .5 cubic feet per minute.  A 100 cubic foot tank will last
200 minutes at the surface.

doubled too.

Lee

3-4.5.2 Respiratory Rate. The number of complete respiratory cycles that
take place in 1
minute is the respiratory rate. An adult at rest normally has a respiratory
rate of
approximately 12 to 16 breaths per minute.

3-4.5.3 Total Lung Capacity. The total lung capacity (TLC) is the total
volume of air that
the lungs can hold when filled to capacity. TLC is normally between five and
six
liters.

3-4.5.4 Vital Capacity. Vital capacity is the volume of air that can be
expelled from the
lungs after a full inspiration. The average vital capacity is between four
and five
liters.

3-4.5.5 Tidal Volume. Tidal volume is the volume of air moved in or out of
the lungs
during a single normal respiratory cycle. The tidal volume generally
averages
about one-half liter for an adult at rest. Tidal volume increases
considerably during
physical exertion, and cannot exceed the vital capacity.

3-4.5.6 Respiratory Minute Volume. The respiratory minute volume (RMV) is
the total
amount of air moved in or out of the lungs in a minute. The respiratory
minute
volume is calculated by multiplying the tidal volume by the rate. RMV varies
greatly with the body's activity. It is about 6 to 10 liters per minute at
complete
rest and may be over 100 liters per minute during severe work.

3-4.5.7 Maximal Breathing Capacity and Maximum Ventilatory Volume. The
maximal
breathing capacity (MBC) and maximum ventilatory volume (MVV) are the
greatest respiratory minute volumes that a person can produce during a short
period of extremely forceful breathing. In a healthy young man, they may
average
as much as 180 liters per minute (the range is 140 to 240 liters per
minute).

3-4.5.8 Maximum Inspiratory Flow Rate and Maximum Expiratory Flow Rate. The
maximum
inspiratory flow rate (MIFR) and maximum expiratory flow rate (MEFR)
are the fastest rates at which the body can move gases in and out of the
lungs.
These rates are important in designing breathing equipment and computing gas
use under various workloads. Flow rates are usually expressed in liters per
second.

3-4.5.9 Respiratory Quotient. Respiratory quotient (RQ) is the ratio of the
amount of
carbon dioxide produced to the amount of oxygen consumed during cellular
processes per unit time. This value ranges from 0.7 to 1.0 depending on diet
and
physical exertion and is usually assumed to be 0.9 for calculations. This
ratio is
significant when calculating the amount of carbon dioxide produced as oxygen
is
used at various workloads while using a closed-circuit breathing apparatus.
The
duration of the carbon dioxide absorbent canister can then be compared to
the
duration of the oxygen supply.

3-4.5.10 Respiratory Dead Space. Respiratory dead space refers to the part
of the respiratory
system that has no alveoli, and in which little or no exchange of gas
between
air and blood takes place. It normally amounts to less than 0.2 liter. Air
occupying
the dead space at the end of expiration is rebreathed in the following
inspiration.
Parts of a diver's breathing apparatus can add to the volume of the dead
space and
thus reduce the proportion of the tidal volume that serves the purpose of
respiration.
To compensate, the diver must increase his tidal volume. The problem can
best be visualized by using a breathing tube as an example. If the tube
contains one
liter of air, a normal exhalation of about one liter will leave the tube
filled with
used air from the lungs. At inhalation, the used air will be drawn right
back into
the lungs. The tidal volume must be increased by more than a liter to draw
in the
needed fresh supply, because any fresh air is diluted by the air in the dead
space.
Thus, the air that is taken into the lungs (inspired air) is a mixture of
fresh and
dead space gases.

3-4.6 Alveolar/Capillary Gas Exchange. Within the alveolar air spaces, the
composition
of the air (alveolar air) is changed by the elimination of carbon dioxide
from
the blood, the absorption of oxygen by the blood, and the addition of water
vapor.
The air that is exhaled is a mixture of alveolar air and the inspired air
that
remained in the dead space.

The blood in the capillary bed of the lungs is exposed to the gas pressures
of alveolar
air through the thin membranes of the air sacs and the capillary walls. With
this exposure taking place over a vast surface area, the gas pressure of the
blood
leaving the lungs is approximately equal to that present in alveolar air.
When arterial blood passes through the capillary network surrounding the
cells in
the body tissues it is exposed to and equalizes with the gas pressure of the
tissues.

In U.S. measure I am 6ft male, 145lbs, and in excellent health do to
physical exertion of my job. I find on a slow swim in the local quarry,
following one of the sunken roads at 25ft of depth, and doing nothing
more than observing fish swim by, I get about 1.75 hours from 80cuft,
starting at 3000, and ending with 500psi. Total distance covered
horizontally underwater is about .4 mile.