Gas Consumption while SCUBA Diving

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Why we should know our Gas Consumption?

We as trained and certified SCUBA Divers learn already at the Open Water Diver Course, to take a close look at our Gas Supply. But are we really aware of our Gas Consumption?
We learn at our entry-level courses the different Markings of the SCUBA Tank:

• DOT Number and Material
• Manufactory
• Serialnumber
• Hydrostatic Test
• Tank Size

Also, we learn to determine the remaining Tank Pressure by using the Pressure Gauge (SPG) or AIR integrated Dive-computer. Skills like Low on, Out of Air and CESA are helping us to understand the importance of the Breathing Equipment. And in the very unlikely event of an Out of AIR situation to react the correct way and safely accent to the surface.
But do we know about our AIR Consumption in-depth? Most certified divers never really heard about SAC Rate and how to Calculate it.
The Gas-Consumption Calculations coming to effect if you really plan your dive with your dive buddy yourself. And you would like to make sure from the beginning that your gas supply is sufficient for your planned dives.

In technical diving, these calculations are there for a long time and standard in the Decompression Diver course. But also as a Sports Diver, it is a benefit if you know about your SAC rate.

How to determine your Gas Consumption

Know the size of your SCUBA Tank: S-80 Aluminium = 11.1 L

Normal Conditions: Decent to a depth of 10m look at your computer and SPG. start to swim normally for 10min and check how much gas you used.

Hard Conditions: Decent to a depth of 10m look at your computer and SPG. start to swim as fast as possible for 10min and check how much gas you used.
Take notes how much Gas you used in ether way on 10m of depth!

You start on 10m of depth with 180 bar after 10min you check the pressure again and you have 150 bar left in your S-80 tank whats your SAC rate?
30 bar * 11.1 L (Tank Size) = 333 L / 2 bar (ambient Pressure at 10m) = 166.5 L / 10 min = 16.65 L / Min. (SAC)
Now we know how many litres of Gas we are consuming on 10 m of depth in 10 min. and like this, we can figure out what kind of Gas Supply we need to have for our Dive-plan.

Also we can use this to determine an average SAC Rate for example:
I dived 60 min consumed 150 bar on an average depth of 12m.
150 bar * 11.1 L (Tank Size) = 1665 L / 2,2 bar (ambient Pressure at 12 m) = 756,81 L / 60 min = 12.61 L / Min. average (SAC)

Calculate a decompression dive plan (ohne die CNS Calculations)

GAS	Planned vol.	With Reserve
N 32	3160 L	4740 L	Bottom Gas *1,5
N 99	320 L	640 L	Deco Gas * 2,0

Depth	Time	Run Time	SAC	Conv F. (ata)	Volume	Gas	Po2
5 m	4 min	(4 min)	20 L	1,5 bar	120 L	N32	0,48 bar
10 m	2 min	(6 min)	20 L	2,0 bar	80 L	N32	0,64 bar
25 m	9 min	(15 min)	20 L	3,5 bar	630 L	N32	1,12 bar
30 m	15 min	(30 min)	20 L	4,0 bar	1200 L	N32	1,40 bar
25 m	15 min	(45 min)	20 L	3,5 bar	1050 L	N32	1,12 bar
10 m	2 min	(47 min)	20 L	2,0 bar	80 L	N32	0,64 bar
6 m	10 min	(57 min)	20 L	1,6 bar	320 L	N99	1,58 bar

Calculate the surrounding pressure ATA

Calculate the surrounding pressure is part of every entry-level SCUBA diving course.
The surrounding pressure increase every 10 m Water for 1 bar + 1 bar Atmosphär

Calculate the Gas Volume

To Calculate the gas volume needed in depth: SAC*Time*ata=
for example 20 L* 15 min. * 4 bar = 1200 L

Calculate the Po2

To Calculate the Partial pressure of Oxygene: Gas*ATA=
for example N32 = 0,32 bar*4 bar = 1,28 bar Po2

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