The compressibility of gasses is also an important consideration for divers due to its affect on
how long a diver can stay underwater. Scuba regulators are designed to deliver
air to a diver at the same pressure as the surrounding water pressure, in other words at ambient pressure. That
means that when a diver fills his lungs at a depth of 33 feet, he is taking in the equivalent amount
of air as two breaths at the surface.
Obviously then, a cylinder will only last half as long at 33 feet as
it would at the surface.
And cylinder that would last 1 hour at the surface would only last 1/3 as long,
or 20 minutes, at a depth of 66 feet, etc.
It is noteworthy that a cylinder of that partiular size, for that particular person, would not be enough for a bounce dive to the recreational limit of 132 feet.
It is important to be able to estimate how long a scuba cylinder might last at a given depth when
dive planning. To determine this, it is necessary to determine a divers Surface Air
Consumption (SAC) rate. SAC rates can be expressed as psi/minute or CF/minute.
As cylinder sizes and pressures vary, the best way is to use CF/minute and have the CF/psi painted on the cylinder.
For Example: A diver starts with 2,900 psi at 10:43 and then had 2,090 psi at 11:04 while at 66 FSW wearing double 80’s
What is the diver’s SAC rate?
| 1. |
Determine cylinder constant. |
| 2. |
Determine psi used @ specific depth. |
| 3. |
Determine cubic feet used @ specific depth. |
| 4. |
Determine time spent @ specific depth. |
| 5. |
Calculate CF / minute @ specific depth. |
| 6. |
Convert specific depth to absolute pressure. |
| 7. |
Convert CF / min. @ depth to CF / min. @ surface. |
| 8. |
Check if reasonable & make some notes in your Dive Journal
(Resting should be about 1/2 of swimming and about 1/4 of hard work) |
First we must do is determine the Cylinder constant.
A. Find the manufacturer specified volume of gas at the rated pressure
If aluminum cylinder (3ALxxxx) , then most are stamped “logically”.
Most notable exceptions Catalina/Luxfer S-80, C-80 and “straight” 80.
If steel (3AAxxxx), then all volumes are specified at the “+” pressure
B. If in doubt, have your dive store measure the internal volume
Example: Double Aluminum 80. Stamped DOT 3AL3000
holds 77.4 CF if filled to its 3,000 psi rating
C. Cylinder constant for this unit is ( 2 x 77.4 ) / 3,000 =.0516 CF/psi
| 1. |
Determine cylinder constant. | Ex. .0516 CF / psi |
| 2. |
Determine psi used @ specific depth. | Ex. 2,900 - 2,090 = 810 psi |
| 3. |
Determine cubic feet used @ specific depth. | Ex. 810 x .0516 = 41.8 CF |
| 4. |
Determine time spent @ specific depth. | Ex. 10:43 => 11:04 = 21 min. |
| 5. |
Calculate CF / minute @ specific depth. | Ex. 41.8 / 21 = 1.99 CF / min. |
| 6. |
Convert specific depth to absolute pressure. | Ex. 66 FSW = 3 ata |
| 7. |
Convert CF / min. @ depth to CF / min. @ surface. | Ex. 1.99 / 3 = .66 CF/min. |
| 8. |
Check if reasonable & make some notes in your Dive Journal
(Resting should be about 1/2 of swimming and about 1/4 of hard work) |
Looking at SAC using psi/min
The process becomes slightly more complex if depth consumption rate (DCR) is determined at a
depth that is not in even atmospheres. (Not at 33, 66, 99 feet etc.) For this situation we use a
formula that is simply an adaptation of Boyle's Law to determine our SAC rate:
SAC Rate = (DCR x 33) / (Depth + 33)
Let's look at an example. Suppose you did a 66 foot dive for 21 minutes and used 1700 pounds
of air. This would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formula
we get:
SAC Rate = (68 x 33) / (50+33)
or: SAC Rate = 2244/88 or 25.5 pounds per minute.
We can then turn the equation around to determine our DCR for any depth.
DCR = SAC Rate x (Depth + 33)/33
Let's assume our SAC Rate is 25 and we want to know how fast will we use 2000 pounds of air
at a depth of 75 feet.
Dropping our numbers into the equation we get:
DCR = 25 x (75 + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81
This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this into
the 2000 pounds, we see this amount of air would last 24.4 minutes.
It is important to note that SAC Rate takes into account the assumption that you are exerting the
same amount of energy at any given depth, and you are using the same size cylinder as you used
when calculating your DCR.
For example, under strenuous diving conditions, you can consume air 4 times faster than when
sitting still taking pictures. Also it is obvious that a 50 cubic foot cylinder would not last as long as
an 80 cubic foot cylinder, even if they were both filled to the same pressure.
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