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Surface Consumption Rate, SCR

The examples below are NOT to be used to calculate decompression dives.
Do not attempt to calculate the amount of gas needed to perform
decompression dives solely on the Information within this text.
Technical / decompression diving requires proper training.

 Surface Consumption Rate, SCR. SCR is the average utilization of volume of
 gas, air, over time. When you know your SCR, planned depth and time of a
dive, you can get a good idea on the amount of gas you will need to support
the dive.

 Get a calculator, an 80cf aluminum cylinder filled with approx. 3000 psi of
air, a water proof  timer, something waterproof to record your observations
on. Find an area where 33 feet of seawater may be reached. A 35 foot deep
area would work best to help guide you towards staying at 33 feet for
ten minutes.

Once at the 33 foot depth, make note of your cylinders pressure. Begin
swimming at a comfortable pace for 10 minutes while maintaining a depth
of 33 feet. At the end of 10 minutes, record your cylinders pressure.

 An example: You start the exercise at 33 feet with 2600 lbs of pressure.
After the 10 minute swim, your pressure is 2100 psi, meaning you spent
500 psi of gas after 10 minutes swimming at a comfortable pace at 33 feet.

 We learned that 33 feet is 2 Atmospheres Absolute, ATA. We know this
by the following formula, ( ( DEPTH / 33 ) + 1 ). For the example the
formula would read (( 33 / 33 ) + 1 ) = ( ( 1 ) + 1 ) = 2 ATA.

 Up to this point, we know we spent 500 psi of air after a 10 minute
comfortable swim at 33 feet. We get 500 psi of consumption by
subtracting our ending pressure from the beginning pressure,
(2600 psi – 2100 psi = 500 psi).

To get the SCR from the information we take the amount of consumed
gas, 500 lbs and divide that by the ATA, 2 ( 500 psi / 2 ATA = 250 psi ).
The  quotient, 250 psi divided by the number of minutes you ran the
exercise, 10 minutes results with the amount of gas usage every
minute, ( 250 / 10 = 25 psi/minute).
Continue to perform the steps above to build an average. Also change
the tempo of the dive from a comfortable pace that range from a hover
to a fast pace, recording data each time. You can then mix and match
data to formulate usage based on the type of diving you expect to perform.

Let’s run through a complete example.
Let presume your average SCR is 30 psi/minute using the formulas above.
Let’s plan a dive to 80 feet.

You plan to dive to 80 feet. How much gas will you need to support the
dive? Looking at the AIR RPD, you are allowed 29 minutes without
entering into decompression. We also see that we need to spend at least
3 minutes performing a safety stop at 15 feet. That is a minimum of 32
minutes of dive time. Let’s begin the calculations.

80 feet is (( 80 / 33 ) + 1 ) = 3.42 ATA. With a SCR of 30 psi/min, at 80
feet the consumption becomes ( 30 psi * 3.42 ) = 103 psi/min.
103 * 29 = 2987 psi. Do you have enough gas to support the dive?
Don’t forget you have to get up to the safety stop and spend at least
3 minutes there.

I say the answer is no. There is not enough gas to support the dive.
That leads us to the next subject of volume.  Presuming you have been
using an Aluminum 80, you can’t keep adding air to the cylinder.
It’s rated pressure is 3000 lbs which provides you with 80 cf of
gas. To make the above dive to 80 feet for 29 minutes, you need more
gas.  Let’s look at using a different cylinder. One that holds more
volume. This leads us into the subject of Respiratory Minute Volume,
RMV. See the section under training on the subject of RMV to
continue.

 You now have the information necessary to calculate the amount of gas
needed to support a dive to specific depth for a given amount of time.
You also see why it is important to include in your log the time, depth,
starting and ending pressure. The information may be used to calculate
a need quantity of gas to support a dive. Of course, your data on SCR and
RMV  is as good as the data you enter into your log.

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