Air Consumption Rates for Scuba Diving - SAC Rates, RMV Rates, Easy Calculations

Follow these five simple steps to use your RMV Rate and SAC Rate to determine how long your air supply will last on a dive.

STEP 1: DETERMINE YOUR SAC RATE FOR THE TANK YOU PLAN TO USE.

Divide your RMV Rate by the volume of the tank that you plan to use (in liters).

RMV Rate ÷ Tank Volume = SAC Rate

Example: If a diver has a RMV Rate of 20 liters/minute, his SAC Rate calculation goes as follows:

For an 12 liter tank:

_{20 ÷ 12 = 1.7 bar/min SAC Rate}

For an 18 liter tank:

_{20 ÷ 18 = 1.1 bar/minute SAC Rate}

STEP 2: DETERMINE THE PRESSURE AT WHICH YOU WILL BE DIVING.

Use the following formulas to determine pressure in atmospheres (ata) at a particular depth:

• In Salt Water:

(Depth in Meters ÷ 10) + 1 = Pressure

• In Fresh Water:

(Depth in Meters ÷ 10.4) + 1 = Pressure

Example: A diver who descends to 66 feet in salt water will experience a pressure of . . .

_{(20 Meters ÷ 10 ) + 1 = 3 ata}

STEP 3: DETERMINE YOUR AIR CONSUMPTION RATE AT YOUR PLANNED DEPTH.

Use the following formula to determine your air consumption rate in psi/minute at your planned depth:

SAC Rate x Pressure = Air Consumption Rate at Depth

Example: A diver with a SAC Rate of 1.7 bar/minute will descend to 20 Meters. At 20 Meters he will use . . .

_{1.7 bar/minute x 3 ata = 5.1 bar/minute}

STEP 4: DETERMINE HOW MUCH AIR YOU HAVE AVAILABLE.

First, check your tank pressure to determine your starting pressure. Next, decide at what tank pressure you would like to begin your ascent (reserve pressure). Finally, subtract your reserve pressure from your starting pressure.

Starting Pressure - Reserve Pressure = Available Pressure

Example: Your starting pressure is 200 bar and you want to begin your ascent with 50 bar, so . . .

_{200 bar - 50 bar = 150 bar available.}

STEP 5: FIND OUT HOW LONG YOUR AIR WILL LAST.

Divide your available gas by your air consumption rate at your planned depth:

Available Gas ÷ Air Consumption Rate at Depth = How Long Your Gas Will Last

Example: If a diver has 150 bar available and an air consumption rate of 5.1 bar/minute at his planned dive depth his air will last:

_{150 bar ÷ 5.1 bar/min = 29 minutes}

Remember, a diver's air supply will not always be the factor that limits his dive time. Other factors that affect how long a diver will be able to stay underwater during a dive include the no-decompression limit for his planned depth and his buddy's air supply.