Simplified Gas Management for Deep Recreational Diving

Many excellent articles on this subject are available for Technical divers, but I find that many Recreational Courses do not present the basics of Gas Management to the Recreational Diver. The worst case scenario is that you encounter a major problem at deep recreational depths (in my definition between 60 and 130 feet) and find that you do not have a sufficient volume of gas for you and your Buddy to safely manage the problem and ascend to the surface - a potentially dangerous situation!

Given the physics of Boyle's Law, we know that as we dive deeper, our air consumption increases proportionately with depth. So how much gas should be reserved as contingency? Many times, divers are simply told to turn the dive at 1/2 the tank pressure without respect to depth, or the fact that many buddy teams may have different tank sizes. Turning the dive at 1/2 (or even 1/3) of starting tank pressure may be sufficient to a point, but as depths increase this method will most probably not provide a proper safety margin should an underwater emergency occur. It is your responsibility to determine your minimum gas requirements. This is why you are required to sign that Liability Waiver!

For this example, I am considering a body of water such as a Lake with no current, since current significantly changes the calculations and complexity of the situation.

From a high level, the steps should be as follows:

1. Calculate and monitor your Surface Air Consumption (SAC) Rate and Respiratory Minute Volume (RMV). Why? These factors tell you how your air consumption is doing both in psi per minute, and cubic feet per minute under normal - relaxed diving conditions.
2. Determine the stress level RMV for you and your Buddy. This should be a conservative value above your normal RMV.
3. Allow time for problem solving at depth - allow 1 minute at maximum depth
4. Allow time for ascent - assume max ascent rate of 30 feet per minute.
5. Allow time for deep stop - I allow 1 minute at 1/2 max depth.
6. Allow time for safety stop - 3 minutes at an assumed 20 feet.
7. Add the volumes above for each step. This is the Minimum Gas required for emergencies.

The above will determine the amount of gas (in cubic feet) that should be reserved (also referred to as "Rock Bottom") at which time you and your Buddy should start your ascent to the surface. This quantity of gas can then be converted to a pressure for convenience.

And now for the math:

To calculate your personal Surface Air Consumption (SAC) Rate - which expresses air consumption in psi/minute, or Respiratory Minute Volume (RMV) -which expresses the same factor in cubic feet per minute, I recommend to record over the course of several dives your rated tank volume, starting and ending air pressure, average dive depth and dive time. Many dive computers (especially hoseless with integrated pressure sensors) will log this data, and calculate these factors for you automatically. This data will provide you with valuable information about the efficiency of your air consumption - just like gas mileage on a car. With these values, use the following formulas for the calculation of values:

SAC Rate (psi/min) = ((Change in psi)/(Dive Time (min.)) /Avg. Depth (ATA)

RMV Rate (cu. ft./min) = SAC * (Rated Vol. of Tank (cu. ft.) / (Rated Pressure of Tank (psi.)

Why are these calculations necessary? It provides a basis for your personal air requirements. A real example (From one of my deep dives yesterday):

• Rated Tank Volume: 100 cu. ft.
• Rated Pressure: 3447 psi
• Actual Start Pressure: 3400 psi
• Actual End Pressure: 1800 psi
• Max Depth: 111 feet
• Average Depth: 56 feet
• Dive Time = 24 minutes

(By the way - I also carried a 30 cu.ft Pony Bottle with independent Regulator as Bailout)

Therefore:

• SAC Rate(psi/min) = ((Change in psi)/(Dive Time (min.)) /Avg. Depth (ATA)
• SAC Rate (psi/min) = ((3,400-1,800 psi)/(24 min.) /((56'/33')+1 ATA)
• SAC Rate = 24.72 psi/min.

and,

• RMV Rate (cu. ft./min) = SAC * (Rated Vol. of Tank (cu. ft.) / (Rated Pressure of Tank (psi.)
• RMV Rate (cu. ft./min) = 24.72 psi/min * (100 cu. ft.) / (3447 psi.)
• RMV Rate = 0.72 cu. ft./min

Referring to the original steps above, and for a theoretical dive to 120' deep:

1. My normal RMV is about 0.70 cu. ft./min. I personally assume a "Stressed RMV" of 1.0 cu.ft./min. You should evaluate this factor for yourself, and determine your own value. For Buddy and myself, a total RMV of 2.0 cu.ft./min. will be assumed. Note: This is the single most critical factor in this discussion. All other factors are mathematical calculations!!! Some divers use a Stressed RMV of 2.0 cu.ft./min. or higher.
2. Allow time for problem solving at depth - allow 1 minute at maximum depth. 120' = 4.64 ATA x 2.o cu. ft./min = 9.27 cu.ft.
3. Allow time for ascent - 120' - total ascent time = 120'/30'/min. = 4 minutes, average depth = 120'/2 (average depth) = 60 ft. = 2.82 ATA. Air for Ascent = 4 min. * 2.82 ATA * 2.0 cu.ft/min = 22.55 cu.ft.
4. Allow time for deep stop - 120'/2 = 60 ft = 2.82 ATA. Air for deep stop = 2.82 ATA * 2.0 cu.ft./min = 5.64 cu.ft.
5. Allow time for safety stop - 20' = 1.61 ATA * 3 minutes * 2.0 cu.ft./min = 9.64 cu.ft.
6. Finally, Adding all this up, you should get something like 47.09 cu.ft to satisfy the emergency gas requirements for you and your Buddy breathing off 1 tank.
7. Converting to pressure this would be (47.09 cu.ft./100 cu.ft.) * 3,447 psi = 1,623 psi. Assuming you had a full fill to start with, your turn pressure to meet the Rock Bottom Requirements would be 1,824 psi. (3,447 psi - 1,623 psi.). These pressures are often rounded up for ease of memory, and can be recorded on your slate.

If you and your buddy have different tank sizes, it is only step 7 above that is different. Minimum gas requirements are only a function of depth and your air consumption (RMV). This should be calculated for each diver, and communicated to each other before the dive. The first diver to reach this turn pressure should signal to the other diver to turn the dive. This turn signal must be non-negotiable! It is a fairly simple matter to take this data and prepare a table for various depths and tank sizes that can be laminated and carried to the Dive Site.

Hopefully this guide has been easy to follow. Gas Management should be a critical skill for the deep recreational diver. Additional equipment for deep recreational dives such as redundant regulators (via Pony or Bailout Bottles) should also be considered for an additional measure of safety.

(Note: for further reference, please click here)

Dive Safe!