To avoid breathing gas emergencies constant awareness is the key to a successful dive. If a critical situation is developing, react to it before it becomes a threat and call or terminate the dive. Remember the most important rule of diving is that anybody can call the dive at any time, any reason, no questions asked. There are various reasons why a diver could and should abort the dive or even not start the dive at all.
Never dive with minor or mayor equipment problems. Be more conservative on penetration distance on wrecks and caves on turnaround pressure with new equipment, a new cave system or wreck, even new dive partners. Don’t dive with divers who in your opinion do have attitude problems. Always dive within your and your team member’s limits and comfort zone. Do not be “pushed” or push others beyond their self-imposed limits. Allow team members to evolve within their own time and experience frames. In the last years a variety of breathing gases has been used successfully ranging from air to nitrox, trimix or heliox and all divers face the same problem when the tank is empty. Humans cannot breathe water and we are limited to our time underwater depending on the supply of our breathing gas in our tanks.
Running out of gas or air is in most instances operator error and self inflicted. Essential to all successful diving operations is the knowledge of your and your team members breathing gas supply including reserves for emergencies. In recreational non-decompression diving the traditional rule of turn around pressure or return to the surface pressure is 500 – 750 psi / 35 – 50 bar while some prefer to turn the dive at ½ 200 psi / 15 bar to allow a larger reserve. In decompression, wreck penetration or cave diving the rule of thirds is to be observed as a minimum.
Some divers feel that the rule of thirds is not conservative enough and turn at a fourth or sixth of their starting gas volume. Similar Tanks When all divers in a team are using the same size tanks the calculation of the turn around pressure is simple. If a diver has a tank fill of 3000 psi / 210 bar he will divide the initial tank pressure by three coming to 1000 psi / 70 bar for each third. The diver has 1000 psi / 70 bar for the penetration part of the dive, 1000 psi / 70 bar for the exit part of the dive and 1000 psi / 70 bar for a potential emergency. Taking 1000 psi / 70 bar away from our starting 3000 psi / 210 bar we will have a 2000 psi / 140 bar turn around pressure. If a diver has less then 3000 psi / 210 bar, for example 2900 psi / 190 bar then the number will be rounded down for easy calculation, in this case we go to 2700 psi / 180 bar. Dividing 2700 psi / 180 bar by three will give us thirds of 900 psi / 60 bar. Taking 900 psi / 60 bar away from our initial start pressure will give us our turn around pressure of 2000 psi / 120 bar. When ever we have a pressure not easy dividable by three we will round down to find a number easy dividable by three and subtract that number from our initial pressure to find the turn around pressure.
If diving in a team it is important to match the penetration and emergency gas supply by using a simple method. If we have a team of three with same size tanks and pressures for example of 3100 psi / 210 bar, 3000 psi / 200 bar and 2800 psi / 190 bar we look for the smallest number, in this case 2800 psi / 190 bar. We round 2800 psi / 190 bar down to 2700 psi / 180 bar because it is easy dividable by three and coming to 900 psi / 60 bar. Now every diver in the team can use only 900 psi / 60 bar for the penetration and will subtract that number from the initial pressure, arriving at turn pressures of 2200 psi / 150 bar, 2100 psi / 140 bar and 1900 psi / 130 bar. Since all divers in the team are matched and use the same amount of breathing gas to conduct the dive they will have enough gas for potential air emergencies and a safe exit.
Dissimilar Tanks In many diving areas of the world divers own their own tanks with their own volume size and pressure rating preferences. When dissimilar tanks are used we can not use the system of calculating thirds on pressure but must first convert the tank pressure into volume, then calculate thirds on the volume and convert the turn around volume into turnaround pressure. SAC rate Calculation Matching the gas supply is to insure reserve gas supply for emergencies. Gas matching compensates for diver with greatest RMV (Respiratory Minute Volume) or SAC (Surface Air Consumption) to exit sharing gas on smallest gas supply. Carefully estimate distances and gas needs for varying conditions.
The calculation of the SAC rate does begin with measure of gas consumption at a stable depth over a specific period of time noted on a slate for later calculation. Example – 500 psi / 34 bar are consumed in 10 minutes from an 80 cft / 11 liter tank at 100 feet / 30 meter. What is the SAC rate ?. First step is to calculate the minute consumption at depth 500 psi / 34 bar in 10 minutes is 50 psi / 3.4 bar per minute at depth. Second bring depth minute consumption to the surface. We convert the depth of 100 feet / 30 meter into ATA ( Atmosphere Absolute ) resulting in 4 ATA. Dividing our SAC rate of 50 psi / 3.4 bar by 4 ATA will give us the SAC of 12.5 psi / 0.85 bar per minute at the surface. Gas Reserves The rule of thirds is the absolute minimum for technical, cave, ice, wreck penetration and decompression diving due to that fact that the diver has no immediate access to the surface but has to exit the real or virtual overhead environment with a potential problem.
When a diver’s ability to access the surface is limited or non existent he or she must maintain larger gas reserves for potential emergencies. One third of the initial gas supply is used for the penetration part of the dive, the second third is used for the exit part of the dive and at least one third is used and reserved for emergencies. It is that last third that does not belong to us but our team mates that are diving with us in case they are running out of air. It is important that divers monitor their gas volume and pressure gauges in order to return safely from the farthest point of penetration while maintaining sufficient gas volume for exit and emergencies.
Primary gas supplies are managed by a concept called the rule of thirds, which states “ When diving in an overhead environment the unexpected can happen. Since it may be a long way out of a cave it is better to be conservative when applying the rules of gas management ”. The dive must be turned when 1/3rd of the initial gas supply has been consumed. The remaining 2/3rds are then available for use when returning to the exit for any gas related emergencies. The diver should surface with at least 1/3rd of the original gas supply remaining. Good examples of gas-related emergencies are free flowing regulators and ruptured hoses. Manifolds equipped with isolators and dual regulator shut off valves are designed for maximum safety. They allow a diver to shut down a defective regulator and to recover by using the remaining functioning regulator to return to the exit.
It is important to understand that the primary reason for diving the rule of thirds is self-sufficiency. When applying the rule of thirds to dive teams, the number of variables increases. Each diver in the team has his or her own swimming style and normal breathing rate. Moreover, it’s highly probable that team members are using configurations with different tank capacities, sizes and volumes. When this type of scenario develops, it’s imperative that the team matches its gas supplies to insure everyone makes a safe dive with sufficient gas reserves to deal with out of air scenarios.
The first step in gas matching is to insure that the gas has been planned to enable both the diver with the least amount of gas and the diver who has the most gas to exit the cave from the maximum point on the smallest available gas supply. If the dive team has properly matched gas supplies with individual consumption patterns, the rule of thirds provides a secondary advantage. If a problem causes a team member to run out of gas, actions can be taken so all team members can safely exit the overhead environment. For divers to be able to swim as normal as possible in a gas sharing situation, the donor’s alternate regulator should be equipped with a hose at least 7 foot / 2 Meter long hose. The standard 40 inch / 1 meter octopus hose simply isn’t practical when major horizontal distances must be traveled. In addition, a short hose will make it difficult for each diver to maintain contact using a guide line in cave or wreck diving and almost impossible to share air through a restriction.
RMV rate Calculation is the Conversion of SAC into RMV. Imperial – How many cft are 12.5 psi out of a single 80 cft tank. In order to calculate that we need to know what the rated pressure of that tank is, stamped into the neck of each Scuba tank. In case of the 80 cft aluminum tank it is 3000 psi. If we calculate 80 divided by 3000 we arrive at a base line of cft per psi, in this case 0.0266 cft per single psi. For our example above we just need to multiply 0.0266 by 12.5 and will arrive at a RMV 0.33 cft of gas per minute on the surface. Metric – How many liters are 0.85 bar in a single 11 liter tank. The Metric system is somewhat easier to calculate with our Sac rate of 0.85 bar multiplied by the tank size of 11 liter resulting in a RMV of 9.35 liters per minute at the surface.
Live long and prosper. Have plenty of gas to breathe.
Matt