Two divers, with an identical body type and physical fitness, who practice all of the gas saving techniques previously listed will have different gas consumption rates while conducting the exact same dive with the same profile. This is because there are many factors that influence air consumption beyond what we have discussed. Therefore, when planning a dive, especially a deep dive, it is necessary for each member of the dive team to know approximately what his gas consumption rate is.

**Surface Air Consumption**, or SAC can be calculated by establishing how much gas a diver consumes on the surface for a specific amount of time. Dividing the amount of gas the diver consumes by the amount of time spent swimming and breathing will provide the diver with the amount of gas he used per minute.

**SAC = Bar/Time** or **SAC = PSI/Time**

It is important to remember that the data obtained by this calculation is an approximation of the surface air consumption rate. In fact, there are so many factors that influence air consumption that it is practically impossible to create an exact calculation of what a diver’s consumption rate will be on a particular dive. However, the result of this calculation is a good approximation of what a diver’s air consumption may be, and allows the dive team to plan how much gas may be required for a predetermined depth and time.

To obtain SAC, a diver must first swim on the surface for a specified amount of time and record the gas that he used during that time. The cylinder should be acclimated to the water temperature before the exercise begins. If it is not, the results will be far less accurate due to cooling (or warming) of the cylinder once it is submerged in the water. For example:

*Metric System*

Using an 12 liters cylinder, a diver records his starting cylinder pressure as 200 bar. After swimming on the surface for 10 minutes, his ending cylinder pressure is 185 bar, a change of 15 bar.

To calculate this diver’s SAC, we divide 15 bar by 10 minutes, resulting in a 1,5 bar/minute breathing rate using an 12 liters cylinder.

*Imperial*

Using an 80 cubic foot cylinder, a diver records his starting cylinder pressure as 3000 psi. After swimming on the surface for 5 minutes, his ending cylinder pressure is 2800 psi, a change of 200 psi. To calculate this diver’s SAC, we divide 200 psi by 5 minutes, resulting in a 40 psi/minute breathing rate using an 80 cubic foot cylinder.

The SAC rate is based on the cylinder size used when the test was conducted. That means that a diver’s SAC will change when the diver changes cylinder sizes.

A diver may calculate his **Respiratory Minute Volume (RMV)** to avoid having to recalculate his SAC each time he changes cylinder sizes. RMV represents the amount of breathing gas, in cubic feet or liters, a diver uses on the surface every minute. It is a cylinder size independent number, and therefore can be used for dive planning when the cylinder size is unknown.

*Metric System*

Once SAC is calculated, multiply the cylinder’s tank volume (in liters) by the SAC to obtain RMV.

**RMV = SAC × Tank Volume (liters)**

The diver’s SAC in our example was 1.5 bar/minute using an 12 liters cylinder, we can calculate the diver’s RMV = 1.5 bar/min × 12 liters = 18 liters/minute.

If a different diver determined his SAC was 1 bar/minute using a 15 liters cylinder, RMV would be 1 bar/min × 15 liters = 15 liters/minute.

*Imperial*

Once SAC is calculated, the **Tank Conversion Factor (TCF)**should be calculated. Do this by dividing the cylinder’s tank volume (in cubic feet) by its working pressure (in psi). These values are stamped on the neck of the tank. Multiply the tank conversion factor by the SAC to obtain RMV.

**TCF = Volume / Working Pressure **

**RMV = SAC × TCF**

We calculated that the diver’s SAC in our example was 40 psi/minute using an 80 cubic foot cylinder that has a 3000 psi working pressure. The TCF for this cylinder is 80 cubic feet/3000 psi = 0.0267 cubic feet/psi.

Using this tank conversion factor, we can calculate the diver’s RMV = 40psi/min × 0.0267 cubic feet/psi = 1.068 cubic feet/minute.

If a different diver determined his SAC was 40 psi/minute using a 130 cubic foot cylinder with a 2400 psi working pressure the TCF and RMV would be:

TCF = 130 cubic feet/2400 psi = 0.054 cubic feet/psi.

RMV = 40psi/min × 0.054 cubic feet/psi = 2.16 cubic feet/minute.

Armed with this knowledge, a diver can calculate his hypothetical air consumption for any given dive.

*Metric System*

A diver with a SAC of 1.5 bar/min, calculated using an 12 liters cylinder, wants to make a dive to 30 meters using the same 12 liters cylinder. To calculate his gas consumption at 30 meters the diver must multiply the diver’s SAC rate by the ambient atmospheric pressure (= 4 atm).

Gas Consumption at 30 meters = 1.5 bar/minute x 4 atm = 6 bar/minute.

Continuing with the example, the diver wants to stay at 30 meters for 10 minutes, which would result in the diver using 60 bar of gas (6 bar/minute × 10 minutes = 60 bar).

This equates to the diver’s gas use for 10 minutes at a maximum depth of 30 meters. Gas used on the descent, ascent, safety stop and buoyancy control must also be considered when calculating total gas requirements.

Alternatively, the diver’s SAC from the previous example can be calculated based on the depth of 30 meters, time of 10 minutes and gas used 60 bar: SAC =60 bar/10 minutes /4 atm = 1.5 bar/minute.

Calculating the consumption for this dive using the diver’s RMV is just as easy: RMV = 1.5 bar/min × 12 liters = 18 liters/minute

Gas Consumption at 30 meters = 18 liters/minute × 4 atm = 72 liters/minute

Gas Consumption at 30 meters for 10 minutes = 72 liters/minute × 10 minutes = 720 liters.

*Imperial*

A diver with a SAC of 40 psi/min, calculated using an 80 cubic foot cylinder, wants to make a dive to 100 feet using the same 80 cubic foot cylinder. To calculate his gas consumption at 100 feet the diver must multiply the diver’s SAC rate by the ambient atmospheric pressure = 4 atm.

Gas Consumption at 100 feet = 40psi/minute × 4 atm = 160 psi/minute

Continuing with the example, the diver wants to stay at 100 feet for 10 minutes, which would result in the diver using 1600 psi of gas (160 psi/minute × 10 minutes = 1600 psi).

This equates to the diver’s gas use for 10 minutes at a maximum depth of 100 feet. Gas used on the descent, ascent, safety stop and buoyancy control must also be considered when calculating total gas requirements.

Alternatively, the diver’s SAC from the previous example can be calculated based on the depth of 100 feet, time of 10 minutes and gas used, 1600 psi: SAC = 160 psi/ 10 minute / 4 atm = 40 psi/minute.

Calculating the consumption for this dive using the diver’s RMV is just as easy:

TCF = 80 cubic feet/3000 psi = 0.0267 cubic feet/psi.

RMV = 40 psi/min × 0.0267 cubic feet/psi = 1.068 cubic feet/minute

Gas Consumption at 100 feet = 1.068 cubic feet/minute × 4 atm = 4.272 cubic feet/minute

Gas Consumption at 100 feet for ten minutes = 4.272 cubic feet/minute × 10 minutes = 42.72 cubic feet.

Knowing your SAC and RMV can be useful when selecting a dive buddy. Buddy with someone that has a RMV close to yours and you will both be able to maximize your dive time under water.

However, your SAC and RMV are only estimations of your gas consumption based on your physiology and mental status during the SAC exercise. They are not precise calculations.

Many factors have an affect on your breathing rate during the exercise including how mentally and physically rested you are, your comfort level, the altitude, the water temperature, the fit of your exposure suit, and your overall cellular chemistry and hydration, among others. Also, as your diving becomes more streamlined and you become more comfortable in the water, your breathing rate will improve. It is a good idea to conduct an SAC exercise multiple times during a diving season so that you are better able to calculate your estimated gas consumption.