Air Consumption
Learning Objectives:
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•
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Correctly calculate the surface air
consumption rate in either psi/min. or
ft3/min. given: depth, time at depth, and
amount of air used.
Correctly calculate the air consumption
rate for any depth, given the
consumption rate for any other depth.
Correctly calculate air consumption when
the size of the cylinder used is changed.
Main Points
• Calculating gas consumption rates at the surface
and at depth.
• Converting surface consumption rates (SCR) to
depth consumption rates (DCR).
• Converting depth consumption rates (DCR) to
surface consumption rates (SCR).
• Converting gas consumption rates from
psi/minute to cu. ft./minute
• Converting gas consumption rates from cu.
ft./minutes to psi/minute
Calculating Gas Consumption Rate
at Depth (DCR)
Calculate gas consumption rate at depth
using the following formula
– Gas used/ time
Example:
A diver at 66' in the ocean breathes 750 psi in 10 minutes. What is the
divers air consumption rate at depth (DCR)?
Answer:
750 psi / 10 minutes = 75 psi per minute @ 66' S.W.
Converting DCR to Surface Gas
Consumption Rate (SCR)
Convert a DCR to SCR using the following
Formula:
DCR / Pressure atm
Remember the formula for converting depth
to pressure is
Seawater Patm = (depth + 33) / 33
Freshwater Patm = (depth + 34) / 34
Example 1
If a diver has a DCR of 75 psi/minute at a depth of 66 fsw,
what is that diver’s SCR?
Answer:
DCR / Patm
75 psi per minute /[(66 + 33)/33)]
75 psi per minute / 3 atm
25 psi per minute @ The Surface
Converting SCR to DCR
When converting a SCR to a DCR use
the
following formula:
DCR = SCR x Pressure
Example
A diver has a known SAC of 25 psi per minute @
the surface. What is that diver's DCR @ 78 fsw?
Answer:
DCR = 25 psi per min. x [(78+33)/33]
DCR = 25 psi per min. x 111 / 33
DCR = 25 psi per min. x 3.4
DCR = 85 psi per min. @ 78 fsw.
Converting consumption Rates from
psi/minute to cu ft/minute
Example:
A diver has a known SCR of 25 psi/minute using an Al. 80.
What is the diver's SCR in cubic feet per minute?
Answer:
Determine ft3 / psi
80 ft3 / 3000 psi = .027 ft3 / psi
SCR psi/min x ft3 / psi
(25 psi/minute) x (.027 ft3 / psi ) = .675 ft3 / minute
Check your answer
• 3000 psi / 25 psi min = 120 minutes
• 80 ft3 / .675 ft3/minute = 118.5 minutes
Converting from psi/min to cu ft/min
and from SCR to DCR
A diver, using an Al. 80 filled to 3000 psi.
has a known SCR of 20 psi/minute.
In ft3/min, What is the diver’s breathing
rate, @ 57 fsw?
Converting from psi/min to cu ft/min
and from SCR to DCR continued
Step 1
Convert the SCR from psi/min to ft3 /min
80 ft3/3000 psi = .027 ft3/psi
.027 ft3/psi x 20 psi/min = .54 ft3 / min
Step 2
Convert SCR to DCR
.54 ft3/min. x[( 57 + 33)/33]
.54 ft3/min. x 2.7
DCR = 1.46 ft3/min. @ 57 feet SW
Converting from ft3/minute to psi/minute
When converting from ft3/min to psi/min
first determine how many psi/ft3
Formula
Psi when full / ft3 when full
3000 psi/63 = 47.62 psi/ft3
Converting from ft3/minute to psi/minute
A diver using an AL 63 filled to 3000psi has a known SCR of
.55 ft3/min. Using the same cylinder, what is the diver’s SCR in
psi/min.
Answer:
Determine psi/ft3
3000 psi / 63ft3 = 47.62 psi/ft3
Multiply known SCR x psi/cu ft
SCR = .55 ft3/min x 47.6 psi/ft3 = 26.18 psi/min
Converting from ft3 /min to psi / min
Converting DCR to SCR
A diver using an AL 63 filled to 3000psi has
a known DCR of.8 ft3/min at 27ffw. Using the
same cylinder, what is the diver’s SCR in
psi/min.
Step 1
Convert from ft3 / min to psi / min
3000 psi / 63 ft3 = 47.62 psi / ft3
.8 ft3 / min x 47.62 psi / ft3 = 38.1 psi / min @
27ffw
Step 2
Convert DCR to SCR
38.1 psi / min / (27ffw + 34ffw)/34ffw
38.1 psi / min / 1.79 atm = 21.28 psi / min SCR
Converting air consumption rates and tank sizes
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Convert Consumption Rate to to ft3/min
Determine ft3/psi for new cylinder
Determine psi/min for new tank size
Example
A diver has a known SCR of 25 psi/min using AL 80. What is the SCR
using an Al 63?
Answer
80 ft3 / 3000 psi = .027 ft3 /psi x 25 psi/minute = .675 ft3/min
Psi/FT3 for AL 63 = 3000 psi/63 ft3 = 47.6 psi/ft3
47.6 psi / ft3 x .675 ft3 / minute = 32 psi/minute
Have We Covered
• Calculating gas consumption rates at the surface
and at depth.
• Converting surface consumption rates (SCR) to
depth consumption rates (DCR).
• Converting depth consumption rates (DCR) to
surface consumption rates (SCR).
• Converting gas consumption rates from
psi/minute to cu. ft./minute
• Converting gas consumption rates from cu.
ft./minutes to psi/minute
Can You
•
•
•
Correctly calculate the surface air
consumption rate in either psi/min. or
ft3/min. given: depth, time at depth, and
amount of air used?
Correctly calculate the air consumption
rate for any depth, given the
consumption rate for any other depth?
Correctly calculate air consumption when
the size of the cylinder used is changed?
Last Thoughts
• Most divers have a good intuitive
understanding/guesstimate of how much bottom
time they can get based on cylinder size, depth
and exertion level.
• As you get into advanced diving activities it is
important to have a more accurate
understanding of your gas consumption rates
and gas supply requirements.
• Knowing our gas consumption rates in cu.
ft./minute allows you to select the appropriate
size cylinder for the task.