Image courtesy of Gene Page
An Openwater dive class is designed to introduce the beginning diver to a number of new skills and concepts needed to safely venture below the surface. The new student learns about the equipment, specialized skills, and physiology of diving and is given the information necessary to complete a relatively shallow dive within what is called no-decompression limits (NDL). There is little discussion of breathing gas planning or gas management other than being admonished to have 500 pounds of air remaining in the tank at the end of the dive. The concept of gas management is a topic reserved for more advanced classes, and rightly so, because the average new diver is excited and will breathe all the gas from an aluminum 80 before the NDL is reached. For most new divers gas management means watching the gauge as it rapidly falls toward 500 lbs.
As divers become more experienced and their dives become more complex gas management is key to every dive. Dive planning includes calculating the gas needed to complete the dive, which may include greater depths, wreck or cave penetration, and decompression. The diver must know how much breathing gas will be required to execute the planned dive then add a sufficient safety margin and carry the necessary tanks with the correct mixes. It is an intricate process far beyond the scope of Openwater training but, the reality is that even the new diver will benefit from some knowledge of gas planning and management.
The concept of gas planning is really quite simple but the calculations are slightly more complex. To plan a dive the diver must convert the amount of air he/or she breathes each minute at the surface to the compressed volume at the planned depth and multiply the result by the planned dive time. After adding in safety margins the amount of gas needed for the dive is determined and it’s time to fill tanks. For the new Openwater diver a general understanding is all that is needed and a few simple calculations will open the door to gas planning.
The first step in gas planning for SCUBA divers is determining what is called the Respiratory Minute Volume or RMV for short. This is the amount of gas in cubic feet the diver breathes at the surface. For example, the average male breathes about .80 cubic feet per minute while walking and this is about what a diver will breathe after gaining enough experience to be interested in an article like this.
So, what does this mean for dive planning? With an RMV of .80 if you put a regulator attached to an aluminum 80 in your mouth and start walking down the street the tank should be empty in about 100 minutes. If you were at 33 feet or 2 atmospheres the increased pressure means the tank will be empty in just 50 minutes. If you are at 66 feet or 3 atmospheres the gas will be depleted in about 33 minutes. At this point some light bulbs should be illuminating the darkness. This is Boyle’s law that you learned about in Openwater class! At the surface we have 1 atmosphere of pressure. For each 33 feet seawater we descend we add another atmosphere. As we descend the pressure increases and Boyle made it easy for us. At 33 feet or 2 atmospheres volume is decreased to 1/2. At 66 feet or 3 atmospheres it is reduced to 1/3 and at 99 feet or 4 atmospheres gas is compressed to 1/4. This means you will suck down that aluminum 80 twice as fast at 99 feet as you would at 33 feet. This is where we start understanding gas management. It’s important to know that we all have different RMVs and as we gain experience our RMV will decrease. The above is just an example for illustration purposes. The ability to calculate RMV is an important tool for the experienced diver but it’s easy because there are formulas and calculators available on the internet.
The important message to understand is that the deeper you dive the more gas you will use. We all know when we are working or get excited we breathe faster and use more air. If you are at depth and are exerting or become anxious because of an unexpected problem your RMV will increase and you will use even more gas from your tank. Understanding this and doing a little planning for it will make you a safer diver and increase your enjoyment of the sport. Gas planning is not magic. It is simple math and logic. As you gain knowledge and experience the planning will become more complex but it is all rooted in the basic information in this article. Plan your dive and dive your plan. Carry enough gas to complete the dive. Pay attention to your gauges and computer and make your diving safer and more enjoyable.