Respiratory Minute Volume, RMV

Respiratory Mimute Volume, RMV, what is it for? In the post on
Surface Consumption Rate, SCR you learned how much gas you
would use and/or need to support a dive to a specific depth for a
specific time. With RMV, you can now identify the volume of gas
you would need to support a dive and perhaps the cylinder you
could use to support the planned dive.

Take the rate pressure of your scuba cylinder, again for this
demonstration we will use an aluminum 80. An aluminum 80,
or 80AL when filled to it’s working pressure, 3000 psi will hold
80 cf of gas. The working pressure of the scuba cylinder is stamped
around the neck of the cylinder. Take the cubic feet of the cylinder
and divide it by the working pressure of the tank. For an 80AL filled
to 3000 psi, the formula would be ( 80 / 3000 ) = .0267 cf  in every
psi of gas. That’s .0267 cubic feet of gas for ever pound per square inch.

Take the value just calculated, .0267 and multiply it your SCR,
(See the article on SCR for how to calculate your SCR) which we will
use the value of 30 from the SCR article (30 * .0267) to determine your
RVM which is, for this example .8 cf of gas per minute on the surface.

Using the RMV, let’s calculate any given cylinders ability, when filled
to working capacity to support a dive to a given depth for a given time.
Again, using the figures from the SCR article, the dive is planned to 80
feet for 29 minutes.

We know 80 feet is 3.4242 ATA. We know our RMV is .8cf a minute.
Multiplying the ATA and the RMV will  tell you how much gas, in cf/min
at 80 feet you will consume. That’s ( 3.4242 * .8 ) or 2.7393 cf/min at 80
feet. An 80AL filled to 3000 psi has 80cf of gas. Divide 2.7393 into 80,
( 80 / 2.7393 ) to get the number of minutes the 80cf tank will last at
80 feet. ( 80 / 2.7393 ) = 29.21 minutes. That is not enough gas to
support the dive.

Let’s try this with another cylinder,  one that hold more volume.
Try the calculations for a 95cf cylinder. ( 95 / 2.7393 ) = 34.68 minutes.
Is that enough? That equates to only 5 minutes more than an 80AL.
I don’t think it is enough gas.

You can also use the fomula other ways, for example if you know your
RMV and the amount of time you would like to spend at depth, you could
calculate the size cylinder to use. For example ( 2.7393 * 30 ) = 82.179cf.
We see here that an 80AL will fall short. Selecting a cylinder that will
support 30+ minutes will be required. A good practice is to dive on the
rules of 3rds, planning to returning to your origin (ie. boat/shore/surface)
with 1/3 as reserve.

Using the article on SCR and this article on RMV you can now use the
fomulas to assist you in planning dives, to get an idea of how much gas
you will spend and will need, as well as the cyclinder you should carry
to support the dive.

Larry Davis

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