COMPUTE YOUR SURFACE AIR
CONSUMPTION (SAC)
Many divers ask about their breathing rate. They
hear others talk about it, but have no idea how to obtain theirs
because it wasn't discussed in their OW course. Many try
erroneously to figure it out by reading their pressure gauges
before they jump and after their return to the boat. We will
try to simplify the procedure and explain some common errors
made by both newbies and veterans alike. Once you know what
your Surface Air Consumtion (SAC) is, you can estimate
how long will your air will last you at any particular depth.
First, the NO-NOs
:
DO NOT include
the descent nor the ascent in your computations. There already
is a large margin of error involved in the procedure to exacerbate
it.
DO NOT think
a few readings will provide you an answer. Your SAC will
vary according to your experience level, age, emotional and
physical condition, gear load, configuration, water
conditions and, most important, your level of activity
(exertion).
DO NOT trust
your readings to memory!!
You'll be surprised how many errors show when this simple task is
done deeper than 70 ft (21m)!
Now the DOs
:
Do try
to average over longer periods of time (20-30 min better than
5-10 min).
Do try
to determine your level of activity (effortless, normal swim,
heavy load, strong current, etc).
Do read
your gauges accurately and write them down.
Do try
to remain at a depth with minimal deviations.
Do ignore
decimals (except atm). These numbers are a broad picture of a
breathing pattern which vary widely with multiple factors. Make
your arithmetic easier, use rounded numbers.
THE PROCESS :
Gas consumption is as simple as it is important. We'll run an
example of a dive to 66 ft (18m) using an aluminum 80 cft tank @
3000 psig.
STEP 1 :
Once you reach your planned depth, take the initial pressure
and time readings
(i.e. 2800 psig; time=3 min; depth=66 ft).
If you change your depth significantly, close the
previous measures by making a final pressure and time
readings. Again, the longer the time between the initial and
final readings, the more accurate your results will be.
STEP 2 :
Include in your ascent preparations the final pressure and
time readings
(i.e. 1000 psig; time = 23 min).
You consumed 1800 psig in 20 min. Now do your regular ascent.
The rest is just plain arithmetic. If it is so simple, how
come there are so many errors when we do it at depth?
(narcosis?)
STEP 3 :
Substract the final pressure from the initial pressure and
divide over the minutes
(i.e. 1800 psig / 20 mins = 90 psig/min).
Now you know how many psig you were breathing per minute at 66 ft.
Remember our SAC is increased (multiplied) by the depth; so
now that we know the consumption at depth, we must divide to get
it at the surface
(i.e. 90 psig / 3 atm @ 66 ft = 30 psig).
This is your SAC!!
It is so simple even a PADI instructor could figure it out!!
Now let's get it complicated.
To divers always using the same tanks, this
information will estimate their consumption at any depth by simply
multiplying it by the depth in atms
(i.e. 30
psig/min X 5 atm @ 130 ft=150 psig/min).
To other divers, who would use tanks of different capacities,
an extra step is needed.
They need to compute their Respiratory Minute Volume
(RMV),
which is a true measure of the volume of gas you breathe per
minute. For this, you need to know the tank capacity and
working pressure. This information is usually obtained from the
tank manufacturer, dive center or even from the Internet!
Let's remember that most
steel tanks
capacities INCLUDE the
10% overfill pressure! You divide the working pressure (i.e.
3000 psig for many aluminum or 2640 psig for many steel) over
the tank rated capacity (i.e. 80 cft aluminum or 95 cft
steel) and get a convertion factor
(i.e. 3000 psig / 80 cft = 38 psig/cft or
2640 psig / 95 cft = 28 psig/cft)
of how many psig per cft in this tank. So, if you breathe 1 cft
/min, you would lower the pressure of an 80 cft AL by 38
psig/min, while only 28 psig/min in the steel 95 cft.
The main advantage of the RMV is
that you can figure your breathing rate in psig for ANY tank
(provided you know the capacity and working pressure) at any
depth. In our previous example, the diver had a 30 psig / min SAC,
that would be
(30 psig/min over 38 psig/cft = .8 cft/min
for the RMV.
This is a true reflection of the volume of gas the diver gets from
the tank every minute. If the diver now dives with a 95 steel @
2640 psig, he/she would use
.8 cft/min X 28 psig/cft = 22 psig/min
at the surface. Multiply this by the depth in atm and you
get consumption at depth.
Let's do another example : A diver uses a 95 cft
@ 2640 psig (with the 10% overfill) at 66 ft and has a 65
cft Alum @ 3000 for a 50 ft second dive. What would he/she
consume? At 66 ft, initial pressure = 2600 psig; time = 2
min.; then, right before ascent, pressure = 800 psig; time
= 32 min. That's
2600 psig - 800 psig = 1800 psig / 30 min = 60 psig/min @ 66 ft.
Which means
60 psig / 3 atm = 20 psig/min (SAC)
Since 2640
psig / 95 cft = 28 psig/cft, this
diver has a RMV of
20 psig/min over 28 psig/cft = .7 cft/min.
Using an Alum 65 cft @ 3000 psig (which means
3000 psig / 65 cft = 46 psig/cft),
the diver would consume
.7 cft/min X 46 psig/cft
= 32 psig/min
(SAC).
At 50 ft (2.5 atm), this diver would consume
32 psig/min X 2.5 atm = 80 psig/min.
So the SAC of
20 psig/min in his first tank is equivalent to 32 psig/min on the
second tank.
Diving takes more planning than what most of us
really do. In more serious type of diving, the SAC
or RMV play
a very important role to ensure enough gas all the way to the
planned end of the dive ..... rather than an abrupt exit due to a
gas shortage. In cases where DECO is
included, short of gas is a situation you MUST avoid. Every
diver should know what his/her SAC
/ RMV is for an
effortless, a moderate and a heavy exercise dive. In all
our TDI / SDI training,
we start hammering these concepts into the habitual dive
planning procedure. By the time the diver reaches higher levels,
SAC / RMV are
an everyday factor in their dives. It
should be yours too!!!
