AIR Most Important When It’s Not There!
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RESCUE TECHNIQUES
Dive/Rescue Series:
We must always be aware of the important role that back-up divers play in a successful inwater rescue. The back-up diver is there to assist, relieve, or save the main diver, if necessary. There could be many situations that would require a back-up diver, but the most crucial one is if the main diver runs out of air.
We’ve already discussed redundant breathing systems (see “Underwater Breathing Systems: What Are Your Options?”, FIRE ENGINEERING, June 1986). Now, in this fourth article in the dive/rescue series, we’re going to talk about safe diving procedures.
The most dangerous predicament a diver can face is if he runs out of air while he is underwater. In order to avoid this potentially hazardous situation, there must be safe guidelines for divers to follow before they can ever enter the water. A safe ascent is an imperative part of every in-water rescue operation.
In this article we will discuss:
- The length of time that a diver can safely stay underwater;
- The amount of air that should be left in the self-contained underwater breathing apparatus (SCUBA) unit when a diver surfaces and the reasons why;
- The ways in which depth, temperature, and workload affect a diver’s overall bottom time and performance.
HOW LONG CAN YOUR AIR SUPPLY LAST?
Bottom time is measured from the moment the diver leaves the surface to the moment he leaves the bottom for a safe, direct ascent back to the surface. Now, how do we set the guidelines for this definition?
First, let’s examine how long the air supply in a tank can last under normal conditions. We’ll start by discussing an 80-cubic-foot SCUBA cylinder. This means that we are working with a tank that can deliver 80 cubic feet of air when it is at its full pressure of 3,000 psi.
Let’s make a rule of thumb and assume that the average rescue diver breathes 1 cubic foot of air per minute at the surface with minimal exertion. (This is a high estimate, but it will make our calculations easier.) This would mean that this cylinder could last as long as 80 minutes at the surface, according to the formula below:
Available air / consumption rate = available time or
80 cubic feet of air / 1 cubic foot per minute = 80 minutes of time.
As the diver descends underwater, some changes occur:
Surface pressure at sea level exerts 14.7 psi on the diver’s body. For every foot of water that he descends, the pressure exerted on his body increases. One foot of salt water is equal to .445 psi. One foot of fresh water equals .442 psi.
According to Boyle’s Gas Law of pressure, volume, and density, if the pressure is increased and the volume remains the same, the density must be increased directly with the pressure. The formula for Boyle’s Law is:
P1/V1 = P2/V2 = Density
If the ambient (surrounding) pressure is increased by two, the density must be increased by two.
Let’s look at the following example: If we descend 33 feet under salt water, we will gain an additional 14.7 psi. This means that every 33 feet of salt water increases the ambient pressure by one complete unit (14.7 psi). This is calculated as follows:
33 feet * .445 psi = 14.7 psi
So, according to Boyle’s Law, the absolute (total) ambient pressure at 33 feet is 14.7 psi surface pressure plus 14.7 psi for 33 feet of depth, which equals 29.4 psi. In order to inhale at twice the surface pressure, the diver must be taking in air at twice the density.
This means that if he were breathing air at 1 cubic foot per minute at the surface, he would now be using 2 cubic feet of air per minute at 33 feet. The equation for the 80-cubic-foot tank would now look like this:
80 * 2 = 40
One cubic foot of air multiplied by 2 times the pressure equals 2 cubic feet per minute consumption rate at depth, divided into 80 cubic feet of available air, equals 40 minutes of available breathing time at 33 feet.
If the diver descended an additional 33 feet, to 66 feet, there would again be an increase of .445 psi per foot, which would result in an additional 14.7 psi. The equation would now read:
.445 psi X 66 feet = 29.4 + 14.7 surface pressure
The diver would now be breathing air at 3 times the density. He would be using 1 cubic foot of air per minute at the surface multiplied by 3 times the pressure, which equals 3 cubic feet per minute divided into 80 cubic feet of available air, which equals 26.6 minutes of available bottom time (see chart above).
Keep in mind that not all SCUBA cylinders have 80 cubic feet availability or 3,000-psi pressure capacity. Some may have 90 cubic feet at 4,000 psi, or 71.2 cubic feet at 2,475 psi. However, if the tank is full, we can always calculate the available time by using the formula mentioned above and inserting the appropriate number of cubic feet.
HOW MUCH AIR SHOULD YOU MAINTAIN IN YOUR TANK?
Next, we must understand what to do when the regulator’s pressure gauge indicates that the unit is not at full capacity. All regulator pressure gauges measure pressure in pounds per square inch (psi).1
When the gauge indicates that there is 2,500 psi available air in an 80-cubic-foot tank, it means the unit is no longer full. In order to figure this out, use the following formula: Divide 80 cubic feet of air by 3,000 psi, which equals 2.5. Then, multiply 2.5 by the psi, which equals the number of cubic feet of air.
Here are some other examples:
Example 1
For an 80-cubic-foot tank at 2,200 psi, use the following calculations:
22 (drop last two digits) * 2.5 =
55 cubic feet of available air.
Then use the previous formula to compute possible underwater time:
Available air / surface consumption * absolute pressure (depth) = time at depth.
Example 2(a)
For a 71.2-cubic-foot cylinder that is full at 2,475 psi, use this formula:
71.2 / 24.75 = 3
Again, simply multiply three by the psi to get the number of cubic feet.
Example 2(b)
For a 71.2-cubic-foot unit at 1,900 psi, do the following:
19 * 3 = 57 cubic feet of available air
and then use this formula:
Available air / surface consumption * depth (absolute pressure) = bottom time.
During dive practice sessions, it is important to record the amount of air that was used for a certain time period (see chart below). Later, we can figure out what our breathing rate was under various conditions. This will allow us to come up with an average per minute volume rate per diver under different conditions. Record these numbers in the team dive log along with depth, operation performed, etc.
Example 3
14 psi used = 2.5 * 14 = 35 cubic feet of air consumed.
35 cubic feet of air / 20 minutes = 1.75 cubic feet of air per minute at depth.
1.75 cubic feet of air used / 2 * the pressure = .875 cubic feet of air per minute at the surface.
Breathing rate at depth = 1.75 cubic feet.
Breathing rate at surface = .875 cubic feet.
To recheck, take .875 * 20 minutes = 17.5 cubic feet of air at the surface
17.5 X 2 (X the pressure) = 35 cubic feet total at depth.
Although these calculations may seem a little involved, it is essential to be aware of how you or your diver is breathing underwater. It should be part of your basic operation plan.
OTHER FACTORS AFFECTING IN-WATER PERFORMANCE
When temperatures are low, the diver’s breathing rate will be 2 or 3 times higher than normal. Thus, he will demand 2 or 3 times more air for the same amount of time he is underwater. We must consider this when calculating available underwater time during winter rescue work. Use the standard formula and divide the final result by either 2 or 3.
Another instance where the diver’s breathing rate will often be 2 or 3 times higher than normal is under hard-working conditions (i.e., currents, floods, dangerous bottoms). You can also take this factor into consideration when preplanning the dive.
Let’s examine a statement I make quite often: The rescue diver should never leave the bottom with less than 750 psi of air and should surface with no less than 500 psi. Why?
Let’s look at an example. The diver is on a hard-working rescue situation in 25 feet of water. We believe he is breathing 272 times his normal consumption rate. His pressure gauge reads 750 psi (on an 80-cubic-foot tank), and he suddenly realizes he is tangled on the bottom and needs time to adjust for the situation.
Here’s how to calculate the amount of available underwater air time the diver has remaining:
750 psi * 2.5 = 18 cubic feet of available air.
1 cubic foot per minute surface consumption rate * 2 pressure (33 feet) = 2 cubic feet per minute.
Normal consumption rate * 2.5 for heavy workload = 5 cubic feet per minute.
18 cubic feet of available air / 5 = 3.5 minutes available underwater time.
This may sound like an extreme case, but it is in fact what happens when the diver is working hard underwater. You should keep a record for every diver on every dive in order to work up a normal consumption rate for each individual. Divers who have a continually high consumption rate should have their dive techniques checked.
SUMMARY
Our number one priority is the diver’s safety. The diver’s number one priority is maintaining a sufficient air supply.
1Keep in mind that in dirty or dark water even the use of an underwater light often will not illuminate the numbers on your submersible pressure gauge. However, the needle on the gauge may be visible.
If your divers are familiar enough with their gear, they can envision the gauge as a clock. They will then be able to detect the amount of pressure left in the cylinder based on the needle’s position.
However, you must remember to check your gauges during practice sessions so that you will know how the psi amounts correspond to the 12, 10, 9, and 6 o’clock positions on your particular equipment.
