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

Warning!!! This tutorial does include some (very simple) calculations. But don't be afraid - even if you are terrible at math, you should not have much difficulty using the simple formulas given in the following pages to calculate your air consumption rate. This tutorial is designed to walk you through basic information on air consumption rates in a logical order.

### Air Consumption Rate and Why It Is Useful in Scuba Diving

What Is an Air Consumption Rate?

An air consumption rate is the speed at which a diver uses his air. 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).

Three Reasons That Knowing Your Air Consumption Rate Is Useful in Scuba Diving

1. Dive planning:
Knowing his air consumption rates allows a diver to calculate how long he will be able to stay underwater at his planned depth, and to determine if he has enough breathing gas for the dive he plans to make.

Air consumption rates are also useful in determining the proper tank reserve pressure for a dive. Divers are often to surprised to find that for deeper dives, calculations frequently reveal that more than the standard 700-1000 psi of reserve pressure may be required to get a buddy team safely to the surface.

In some types of technical diving, such as decompression diving, air consumption rates are essential in determining how much gas to carry for decompression stops.

2. Gauging Comfort/Stress:
Air consumption rates are a useful tool to objectively assess a diver's stress or comfort level during a dive. If a diver typically uses 200 psi in five minutes of diving at 45 feet, and he notices that he has used 500 psi, his unusually high air consumption rate may be an indication that something is wrong.

3. Identifying Gear Problems
A diver who has a major leak may notice that he is using his breathing gas more quickly than he usually does, although he is breathing calmly. An elevated air consumption rate may also be an indication that a diver's regulator requires servicing, as breathing resistance (and therefore a diver's air consumption rate) may increase when a regulator requires servicing.

### "Normal" and "Good" Air Consumption Rates

"How much air did you surface with?" One of my divers asked everyone on the boat. She was proud of her air consumption rate, because she could stay underwater longer than most divers. This diver was a repeat client of ours, and I knew exactly what she was doing - she wanted to prove that she had more air left in her tank after the dive than anyone else, and thus assert her dominance as a better, more experienced diver. "I have 700 psi!" She boasted, "How much do you have?" Inadvertently, I glanced at my pressure gauge which read 1700 psi. "Enough." I replied.

Almost no-one breathes as little air as I do, but please don't assume that I am boasting. I just happen to be 4 foot, 11 inches tall, female, and relaxed in the water. I have tiny lungs, which means that I require less air to fill my lungs, and therefore use significantly less air than most divers. This does not make me a better diver than my clients! Physics is simply on my side. In fact, I imagine that many of my divers have much better breathing techniques than I do!

When learning about air consumption rates, keep in mind that there is no "normal" breathing rate among divers. Different divers physically require different quantities of air to properly oxygenate their bodies. A dive need only concern himself with calculating his own average breathing rate.

A diver to who tries to lower his air consumption rate to "match" or "beat" another diver may accumulate carbon dioxide or under-oxygenate his body, which can be dangerous. Instead, a diver should focus on slow, calm, full breaths that properly ventilate his lungs.

I didn't answer my client's question as to how much air I surfaced with because I didn't want challenge her to use less air. Air consumption rates should never be a point of competition between divers!

### Surface Air Consumption Rate (SAC Rate)

There Are Two Different Methods of Measuring Air Consumption in Scuba Diving:

Divers typically express air consumption using SAC Rates and RMV Rates. Both are necessary.

The Surface Air Consumption Rate (SAC Rate)

• A surface air consumption rate, or SAC Rate, is a measurement of the amount of air a diver uses in one minute on the surface. SAC Rates are given in units of pressure; either in psi (imperial, pounds per a square inch) or bar (metric).

• Because SAC Rates are given in terms of tank pressure, and not in terms of 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 whereas . . .

500 psi of air in a low pressure 130 cubic foot tank corresponds to 27 cubic feet of air.
And so . . .
A diver who breathes 8 cubic feet of air/minute will have a SAC Rate of 300 psi/minute when diving with a standard aluminum 80 cubic foot tank but a SAC Rate of 147 psi/minute when diving with a low pressure 130 cubic foot tank.
Because SAC Rates are not transferable between tanks of different sizes, a diver usually begins air consumption calculations using his RMV Rate (explained on the next page) which is independent of tank size. The diver then converts his RMV rate to a SAC Rate based on the volume and working pressure of the tank he plans to use on his dive.

### Respiratory Minute Volume Rate (RMV Rate)

A Respiratory Minute Volume Rate (RMV Rate) is a measurement of the volume of breathing gas that a diver consumes in one minute on the surface. RMV Rate are expressed in either cubic feet per a minute (imperial) or liters per a 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 a minute will always breathe 8 cubic feet of air a minute regardless of the size of the tank that the air is stored in.

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

### How to Measure Your Air Consumption Rate: Method 1 (The Easy Way)

Every training manual lists a slightly different method of gathering the data necessary to calculate a diver's air consumption rate. This article lists two of the various methods. Whichever one you choose, remember to hop in 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) may drop one or two hundred psi. Failing to account for this drop in pressure will result in the calculation of an inaccurately high air consumption rate.

Method #1 - Collect Your Data During Normal Fun Dives

1. Hop in 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. (Do this before the tank has a chance to warm up in the sun).
4. Use a dive computer to determine the average depth of the dive. This will be the depth used 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 formula (listed on the following pages).

Many divers prefer this method of calculating air consumption rates because it uses data from normal dives. However, because the resulting air consumption rate is based on an average depth of an entire dive, it is unlikely to be quite as accurate as the second method (listed on the next page). Still, if a diver calculates his air consumption rate using this method over many dives and averages the results, he should end up with a reasonable estimation of his air consumption rate.

### How to Measure Your Air Consumption Rate: Method 2

Plan a dive dedicated to determining your air consumption rate.

1. Hop in 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 of salt water works well).

3. Record your tank pressure before the test

4. Swim at your normal swimming pace for a predetermined amount of time (10 minutes, for example).

5. Record your tank pressure after the test.

( Optional: Repeat 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 formulas.

This method of measuring a diver's air consumption rate is more likely to create reproducible data because it is conducted under controlled conditions at a constant depth. However, reality will never exactly mimic test data, and SAC and RMV Rates data gathered using either method should be used only as a guideline. Plan your dives conservatively.

### Formula for Calculating Your Surface Air Consumption Rate (SAC 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
Confused?

If you working in Imperial format:
• "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).
If you are working in Metric format:
• "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 at the end of the dive (method 1) or the test period (method 2)
For both Metric and Imperial formulas:
• "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).

### Formula for Calculating Your Respiratory Minute Volume Rate (RMV Rate)

Plug your SAC Rate (calculated on the previous page) 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. To do this, you will need the tank volume (in cubic feet) and the working pressure (in psi) this information 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 who has a SAC Rate of 25 psi/min when diving with an 80 cubic foot tank with a working pressure of 3000 psi has an RMV Rate of . . .
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! Easy!
• Metric Method:

Simply multiply your Metric SAC Rate by the volume of the tank you used when gathering data in liters. This information is stamped on the tank neck.
Tank Volume in liters x SAC Rate = RMV Rate
- Example: A diver who has a SAC rate of 1.7 bar/minute when diving with a 12-liter tank has a RMV Rate of . . .
12 x 1.7 = 20.4 liters/minute

It's that easy!

### How to Figure Out How Long Your Air Supply Will Last on a Dive (Imperial)

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.

If you are using Imperial units (psi) divide your RMV Rate by the tank conversion factor (previous page) of your tank. This will give you your SAC Rate for the tank you plan to use.

Imperial SAC Rate = RMV Rate ÷ Tank Conversion Factor
Example: If a diver has a RMV Rate of 0.67 cubic feet/minute, his SAC Rate calculation goes as follows:
For an 80 cubic foot tank with a 3000 psi working pressure the tank conversion factor is 0.0267:
0.67 ÷ 0.0267 = 25 psi/min SAC Rate
For an 130 cubic foot tank with a 2400 psi working pressure the tank conversion factor is 0.054:
0.67 ÷ 0.054 = 12.4 psi/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 Feet ÷ 33) + 1 = Pressure
• In Fresh Water:
(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 ata

Use the following formula to determine your air consumption rate in bar/minute at your planned depth:
SAC Rate x Pressure = Air Consumption Rate at Depth
Example: A diver with a SAC Rate of 25 psi/ minute will descend to 66 feet. at 66 feet he will use . . .
25 psi/minute x 3 = 75 psi/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 2900 psi and you want to begin your ascent with 700 psi, so . . .
2900 psi - 700 psi = 2200 psi available.

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

Available Gas ÷ Air Consumption Rate at Depth = How Long Your Gas Will Last
Example: If a diver has 2200 psi available and an air consumption rate of 75 psi/minute at his planned dive depth his air will last:
2200 psi ÷ 75 psi/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.

### How to Figure out How Long Your Air Supply Will Last on a Dive (Metric)

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

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.