Knowing How Fast You Use Air Will Make Your Dives Safer

Air consumption rate is the speed at which a diver uses the air in the tank. Air consumption rates are usually given in terms of how much air a diver breathes in one minute on the surface, at one atmosphere of pressure.

Knowing your air consumption rate is useful in scuba diving because:

• It allows you to calculate how long you can stay underwater at a planned depth and whether you have enough breathing gas for the dive.
• It’s useful in determining the proper tank reserve pressure for a dive. Divers are often surprised to find that for deeper dives, more than the standard 700 to 1,000 pounds per square inch of reserve pressure might be required to get a buddy team safely to the surface.
• In certain types of technical diving, such as decompression diving, air consumption rates are essential in determining how much gas to carry for decompression stops.
• It’s beneficial to assess a diver’s stress or comfort level during a dive. If you typically use 200 psi in five minutes at 45 feet and notice that you have used 500 psi, that high air consumption rate might indicate that something is wrong.
• A diver who is breathing calmly but using breathing gas more quickly than normal might have a major leak. Breathing resistance and an elevated consumption rate might also indicate that a diver’s regulator requires servicing.

Ideal Air Consumption Rate

Divers have been known to ask each other, “How much air did you surface with?” because they take pride in staying underwater longer than most.

There is no “normal” breathing rate among divers. Different divers require different quantities of air to adequately oxygenate their bodies. A diver need only be concerned with calculating his or her average breathing rate. Trying to lower air consumption to “beat” another diver may accumulate carbon dioxide or under-oxygenate a diver’s body, which can be dangerous. A diver should focus on slow, calm, full breaths that properly ventilate the lungs and not compete to use less air.

Surface Air Consumption Rate

Divers typically express air consumption using surface air consumption (SAC) rate and respiratory minute volume (RMV) rate. The SAC rate is a measurement of the amount of air a diver uses in one minute on the surface. SAC rates are expressed in pressure units, either psi (imperial) or bar (metric). Because SAC rates are given in terms of tank pressure, not volume of air, SAC rates are tank specific:

• 500 psi air in a standard 80-cubic-foot tank corresponds to 13 cubic feet of air.
• 500 psi of air in a low-pressure 130-cubic-foot tank corresponds to 27 cubic feet of air.

A diver who breathes 8 cubic feet of air per minute will have a SAC rate of 300 psi per minute when diving with a standard aluminum 80-cubic-foot tank but a SAC rate of 147 psi per minute when diving with a low-pressure 130-cubic-foot tank.

Respiratory Minute Volume Rate

Because SAC rates aren’t transferable between tanks of different sizes, a diver usually begins air consumption calculations using the RMV rate, which is independent of tank size. The diver then converts the RMV rate to a SAC rate based on the volume and working pressure of the tank to be used on the dive.

RMV rate is a measurement of the breathing gas that a diver consumes in one minute on the surface. RMV rates are expressed in cubic feet per minute (imperial) or liters per minute (metric).

Unlike a SAC rate, an RMV rate can be used for calculations with tanks of any volume. A diver who breathes 8 cubic feet of air per minute will always breathe 8 cubic feet of air per minute regardless of the size of the tank holding the air.

Consequently, most divers remember their air consumption rates in RMV format. Gas planning is typically worked through in RMV format and then converted to either psi or bar based on the type of tank to be used.

Measure Air Consumption Rate: Method 1

Every training manual lists a slightly different method for gathering the data to calculate a diver’s air consumption rate. Nonetheless, remember to enter the water and allow your tank to cool before beginning your data gathering. As your tank cools, the pressure shown on your submersible pressure gauge (SPG) might drop 100 or 200 psi. Failing to account for this drop in pressure will result in calculating an inaccurately high air consumption rate.

Method 1: Collect Data During Normal Fun Dives

1. Hop into the water and allow your tank to cool for a few minutes.
2. Note the starting pressure of your tank. It’s best to record the starting tank pressure on a slate or WetNotes.
3. On the surface after the dive, record the final pressure of your tank before the tank can warm up in the sun.
4. Use a dive computer to determine the average depth of the dive. Use this depth in your calculations.
5. Use a dive computer or watch to determine the total dive time in minutes.
6. Plug this information into either the SAC rate or RMV rate listed below.

Many divers prefer this method of calculating air consumption because it uses data from normal dives. However, because the resulting rate is based on an average depth for the entire dive, it is unlikely to be as accurate as the following method. But, if a diver calculates the air consumption rate using this method over many dives and averages the results, it should lead to a reasonable estimation of air consumption rate.

Measure Air Consumption Rate: Method 2

Method 2: Plan a Dive Dedicated to Determining Your Air Consumption Rate

1. Hop into the water and let your tank cool down.
2. Descend to a depth that you can accurately maintain for at least 10 minutes (10 meters / 33 feet in salt water works well).
3. Record your tank pressure before the test.
4. Swim at your normal swimming pace for a predetermined period of time (e.g., 10 minutes).
5. Record your tank pressure after the test. ( Optional: Perform the test while resting / hovering and while swimming at a quick pace to obtain data for “resting” and “working” states.)
6. Plug this information into the SAC rate or RMV rate formula.

This second method of measuring a diver’s air consumption rate is more likely to yield reproducible data because it is conducted under controlled conditions at a constant depth. However, reality will never perfectly match test data, and SAC and RMV rates gathered with either method should be used solely as guidelines. Therefore, plan your dives conservatively.

Calculate Surface Air Consumption Rate

Plug the data collected during your dives into the appropriate formula below:

Imperial SAC Rate Formula

[{(psi start – psi end) x 33} ÷ (depth + 33)] ÷ time in minutes = SAC rate in psi/min

Metric SAC Rate Formula

[{(bar start – bar end) x 10} ÷ (depth + 10)] ÷ time in minutes = SAC rate in bar/min

• “psi start” is tank pressure in psi at the beginning of the dive (method 1) or test period (method 2).
• “psi end” is the tank pressure in psi at the end of the dive (method 1) or the test period (method 2).
• “bar start” is the tank pressure in bar at the beginning of the dive (method 1) or test period (method 2).
• “bar end” is the tank pressure in bar at the end of the dive (method 1) or the test period (method 2).
• “time in minutes” is the total time of the dive (method 1) or the test period (method 2).
• “depth” is the average depth during the dive (method 1) or the depth maintained during the test period (method 2).

Calculate Respiratory Minute Volume Rate

Plug your SAC rate and other necessary information into the appropriate formula below. Metric RMV rate calculations are much simpler than imperial RMV rate calculations.

Imperial Method
Step 1: Calculate a tank conversion factor for the tank you used when gathering data. You will need the tank volume (in cubic feet) and the working pressure (in psi), which is stamped on the tank neck:

Tank volume in cubic feet ÷ working pressure in psi = tank conversion factor

Step 2: Multiply your imperial SAC rate by the tank conversion factor:

Tank conversion factor x SAC rate = RMV rate in cubic feet/minute

Example: A diver has a SAC rate of 25 psi/minute when diving with an 80-cubic-foot tank and a working pressure of 3,000 psi.

First, calculate the tank conversion factor:
80 cubic feet ÷ 3000 psi = 0.0267

Next, multiply the diver’s SAC rate by the tank conversion factor:
0.0267 x 25 = 0.67 cubic feet/minute

The diver’s RMV rate is 0.67 cubic feet/minute.

Metric Method

Multiply your metric SAC rate by the volume in liters of the tank you used when gathering data. This information is stamped on the tank neck:

Tank volume in liters x SAC rate = RMV rate

Example: A diver has a SAC rate of 1.7 bar/minute when diving with a 12-liter tank.

12 x 1.7 = 20.4 liters/minute

The diver’s RMV rate is 20.4 liters/minute.

Calculate How Long Your Air Supply Will Last (Imperial)

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

Determine your SAC rate for the tank you plan to use. If you’re using imperial units (psi), divide your RMV rate by the tank conversion factor (above) of your tank. This will give you the SAC rate for the tank you plan to use.

Imperial SAC rate = RMV rate ÷ tank conversion factor

Example: A diver has an RMV rate of 0.67 cubic feet/minute.

For an 80-cubic-foot tank with a 3,000 psi working pressure, the tank conversion factor is 0.0267:

_{0.67 ÷ 0.0267 = 25 psi/min SAC rate}

For a 130-cubic-foot tank with a 2,400 psi working pressure, the tank conversion factor is 0.054:

_{0.67 ÷ 0.054 = 12.4 psi/min SAC rate}

In salt water:

(Depth in feet ÷ 33) + 1 = Pressure

In freshwater:

(Depth in feet ÷ 34) + 1 = Pressure

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

_{(66 feet ÷ 33) + 1 = 3 atm}

SAC rate x pressure = air consumption rate at depth

Example: A diver with a SAC rate of 25 psi/min who descends to 66 feet will use:

_{25 psi/min x 3 = 75 psi/min}

Starting Pressure – Reserve Pressure = Available Pressure

Example: Your starting pressure is 2,900 psi and you want to begin your ascent with 700 psi:

_{2900 psi – 700 psi = 2200 psi available}

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

Example: If a diver has 2,200 psi available and an air consumption rate of 75 psi/min at his planned dive depth, his air will last:

_{2200 psi ÷ 75 psi/min = 29 minutes}

for his planned depth and his buddy’s air supply.

Calculate How Long Your Air Supply Will Last (Metric)

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

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 an RMV rate of 20 liters/min, his SAC rate calculation goes as follows:

For a 12-liter tank:

_{20 ÷ 12 = 1.7 bar/min SAC rate}

For an 18-liter tank:

_{20 ÷ 18 = 1.1 bar/min SAC rate}

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 20 meters will experience a pressure of:

_{(20 meters ÷ 10) + 1 = 3 atm}

SAC rate x pressure = air consumption rate at depth

Example: A diver with a SAC rate of 1.7 bar/min will dive to 20 meters. At that depth, he will use:

_{1.7 bar/min x 3 atm = 5.1 bar/min}

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}

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/min at his planned dive depth, his air will last:

_{150 bar ÷ 5.1 bar/min = 29 minutes}

for his planned depth and his buddy’s air supply.