How much gas did you have left after your last dive? Do you know the precise number as a volume? Do you know how much your average consumption rate is? While It’s extremely common for divers to ask each other how much gas they had left after a dive many people express what was left in terms of PSI or BAR. And while that’s helpful if both you and your buddy were using the same tank it becomes tricky if you’re using different cylinders.
The most precise way to know is using your RMV. If you are serious about diving and improving your diving then you need to know this number. There is a famous business quote from Peter Drucker “If you can’t measure it, you can’t manage it”. This is certainly true of your breathing rate. If you’ve ever wondered about how to measure, use and calculate your breathing rate you’re going to want to read on.
What is RMV?
RMV stands for Respiratory Minute Volume. It is a way divers express how much gas was consumed during a dive. RMV is always expressed as a volume. In the Imperial system, we use cubic feet per minute to express RMV. In the Metric system, RMV is expressed in Liters per minute. RMV is usually standardized as a measurement on the surface. This means that if, for example, you have an RMV of 1 cubic foot per minute (28 liters per minute) on the surface you’ll use 2 cubic feet (56 liters) at 33 feet (10 meters), 3 cubic feet (84 liters) at 66 feet (20 meters), etc.. because of Boyles Law and the increase in atmospheric pressure. The illustration below shows how the increase in depth/pressure also affects your breathing rate.
By knowing the amount of gas you consume on a dive you will know:
- If you consumed more or less than on your average dive
- If your breathing rate is getting better or worse
- How much gas you need to do a particular dive
- What tank capacity is best suited for you
RMV Vs. SAC
SAC stands for Surface Air Consumption (rate). Many divers use the terms RMV and SAC interchangeably. Strictly speaking, however, SAC is supposed to be measured in PSI per minute or BAR per minute. Because it is a measurement of pressure, SAC is specific to the tank that it was measured on. This means your SAC rate will be different when you use a different-sized tank. We did a separate article on SAC rates here if you are interested in delving into this topic.
The important thing to understand is that If you look at the online calculators, apps that are available for smartphones, and even the software used for dive computers you’ll see that it’s difficult to get a consensus on whether SAC should always be measured as a pressure or a volume. RMV however, is always measured as a volume. For this reason, RMV is the unit of measurement I like to use when teaching how to measure your breathing rate. If you are unsure, just keep reading on. RMV is probably the better choice.
Ultimately, It doesn’t matter if you measure your breathing rate as cubic feet per minute, liters per minute, BAR per minute, or PSI per minute as long as you understand what you are measuring and how to use your measurements. There are many online debates about this very topic, but for our purposes, we’ll treat RMV as a volume and SAC as a measurement of pressure.
RMV Calculator
Now that you understand what RMV is, let’s look at what you need to measure to figure out your surface Respiratory Minute Volume. Because the information you need is slightly different in the imperial system than it is in the metric system, we’ll go over both below.
Imperial System
To figure out your RMV you’ll need to know the capacity of the tank you used for a given dive as well as its service pressure. You’ll then go on a dive and for a minimum of 10 minutes (longer is better) and make a note of your average depth, your starting and ending pressure in PSI during your timed RMV test and the amount of time you ran your test for. We’ll give you tips later in this article on best practices for getting these numbers, but for now, you just need to know the information you’ll need to collect. With these numbers on hand, you can plug them into the calculator below to get your surface RMV.
RMV Calculator (Imperial)
Metric System
To figure out your RMV in the metric system you’ll need to know the capacity of the tank you used for a given dive. Then you’ll then go on a dive and for a minimum of 10 minutes (longer is better) and make a take note of your average depth, your starting and ending pressure in BAR during your timed RMV test, and the amount of time you ran your test for. We’ll give you tips later in this article on best practices for getting these numbers, but for now, you just need to know the information you’ll need to collect. With these numbers on hand, you can plug them into the calculator below to get your surface RMV.
RMV Calculator (Metric)
Best Practices for Measuring Your RMV
To get the most accurate RMV possible you’ll want to follow the following 3 tips”
- Pick a dive where you won’t be exerting yourself excessively because of currents or any other conditions.
- Use a slate to write down all your information
- Pick a dive where your depth will be relatively constant.
With your slate ready, and calm dive where you’ll maintain a relatively constant depth you’ll be ready to begin your RMV test.
Once you’ve descended and ready to begin measuring, you’ll take a look at your submersible pressure gauge (SPG) and write down your pressure in PSI or BAR. You’ll also want to make a note of the time. Then simply go about your dive as usual. You’ll want to measure for at least 10 minutes (the longer the better). After 10 or more minutes have elapsed make a note of how much gas is left as measured by your SPG and also how long you were diving for. You’ll also need to make a note of average depth during the test. With these numbers handy you can now calculate your RMV either manually or using the online RMV calculator.
What is a good RMV?
A lot of factors go into determining your RMV rate. The single biggest factor is probably your body size. Below is a chart with average RMV rates for an experienced diver using an Aluminum 80 (11.2 liter) tank. This is for a dive where the diver is not exerting themselves and in tropical waters. It is only a guide to get a rough idea of where your RMV could be. You may be higher or lower than what’s listed on the table.
What Affects your RMV Rate?
Many factors influence the rate at which you consume gas from your SCUBA tank. Some of those factors include:
- Your physical size: The larger you are the larger your lungs are and the more gas you will consume as you dive.
- Your exertion level throughout the dive: Swimming fast or against a current are examples of things that will cause you to use more gas.
- Your buoyancy and trim throughout the dive: If your buoyancy is off or you’re not properly trimmed out in the water, you will likely exert yourself more and consume more gas throughout the dive.
- Water Temperature: If you do not have proper thermal protection, your body may begin to shiver and you’ll consume more gas as a consequence of your body trying to stay warm.
- Mental State / Anxiety: If you are anxious, nervous, or not confident in your diving ability this increased anxiousness will often cause you to breathe faster
- Your level of physical fitness: The more cardiovascularly fit you are the more efficient your breathing will be. If you become winded from climbing a flight of stairs, you’ll also likely consume more gas while diving than someone who regularly runs marathons.
How Can I Decrease My SAC Rate?
A good RMV rate is part of being a great diver! There are many things you can do to improve your RMV rate. Many of these suggestions are topics on to themselves and we even have an entire course, much of which is devoted to helping you achieve a lower RMV rate. Below are the top 4 things you can do to decrease your RMV rate.
Master your Buoyancy
If you are struggling to stay down because you don’t have enough ballast or have too much ballast and are kicking just to stay off the bottom, then you are surely wasting energy and driving your RMV rate up. Doing a proper buoyancy check, learning where your dump valves are, and mastering buoyancy are the first steps toward a lower RMV rate.
Learn How to Trim Your Body Out properly.
If you are not trimmed out properly in the water, you’ll consume more energy throughout the dive. Notice in the illustration below how the diver to the left encounters more resistance as she tries to kick. This is because she isn’t trimmed out efficiently.
Use the Frog Kick or the Modified Frog Kick More
The frog kick and the modified frog kick require less energy than the flutter kick when you are diving. Because these kicks have a resting phase built-in you end up kicking less to cover the same distance underwater. Mastering the frog kick and the modified frog kick is one of the things you can do to help to lower your breathing rate.
Become Comfortable Underwater
Your mental state plays a larger role in your breathing rate than most people realize. If you are not completely comfortable underwater it’s likely that your heightened state of anxiety will also raise your breathing rate. Being comfortable with all the basics of SCUBA diving including mask removal and replacement and buddy breathing, in case of emergency, are just the beginning.
Using Your RMV to Estimate the Gas Needed For A Dive
Once you know what your RMV, you can use it to estimate the amount of gas you’ll need at depth. Remember that in salt water you’ll encounter an additional atmosphere of pressure every 33 feet (10 meters. Because of this, the deeper you go the more gas you’ll consume. You can use the calculator below to estimate how much gas you’ll need at a given depth and time.
Using Your RMV To Choose A SCUBA Cylinder
Similar to estimating the gas you’ll need for a dive you can also use your RMV to help you choose the best SCUBA tank for given dive. There are a couple of things you need to keep in mind when doing this. First, remember that you always need to leave an amount of gas in the tank for emergencies. You never want to completely drain a SCUBA tank. Second, if you are purchasing a tank, you want to purchase one that you can use for the deepest dives you plan to do.
With this in mind, you can look at a dive table to determine what the no-decompression limit is for the depth you plan to dive to. From here, knowing your RMV you can determine the amount of gas you’d need to complete the dive. Add in your reserve gas and you should have a good idea of the capacity of the tank that you need. There’s a bit more to choosing a tank, but at least you have an idea of the capacity of the cylinder you should get. For more information on choosing the best SCUBA tank for you check out this article.
Calculating Your RMV Manually
Imperial System
To manually calculate your RMV you’ll need to know the capacity and working pressure of the cylinder you used during your RMV test. You’ll also need to know your starting PSI, your ending PSI, your average depth, and the time you did the test for.
Step 1: Calculate your “Tank Factor”. The “Tank Factor” is a name given to the value in cubic feet that 1 PSI represents. Calculating your tank factor is simple. Just divide the capacity of the tank by its service pressure. So, as an example, if you are using aluminum 80 for the test. You can look at the tank's specifications (LINK) and determine that an aluminum 80 has 77.4 cubic feet when filled to 3000 PSI. That means that the tank factor is (77.4 / 3000) = .0258.
Step 2: Determine how much gas was used during the test in cubic feet. Let’s assume during your RMV test you started with 2700 PSI and ended the test with 1900 PSI. This means you consumed 800 PSI during the test (2700 PSI – 1900 PSI) = 800 PSI. You need to convert 800 PSI to cubic feet, so now you’ll multiply the PSI consumed by the tank factor found in step 1. So (800 * .0258) = 20.64 Cubic Feet.
Step 3: Calculate your depth in Atmospheres Absolute. RMV is expressed at the surface. This means you need to account for the atmospheric pressure and how it compresses the gas you breathe underwater. Let’s assume you did your test at an average depth of 50 feet. To convert 50 feet to ATA you’ll divide it by 33 and then add 1. This is assuming your dive was in saltwater. For freshwater, you would divide by 34 and add 1. This is because salt water is slightly denser than freshwater. So ((50/33)+1) = 2.515 Atmospheres absolute.
Step 4: Divide the total gas in cubic feet by your ATA. So (20.64 / 2.515) = 8.2 Cubic Feet.
Step 5: Divide your result from step 4 by the total amount of time the test was conducted for. Let’s assume we ran our test for 15 minutes. This means (8.2 / 15) = .547.
For this test, the average RMV is .547. We can round this up to .55. This means the diver breathes about .55 cubic feet each minute on the surface.
Metric System
To manually calculate your RMV you’ll need to know the capacity of the cylinder you used during your RMV test. You’ll also need to know your starting BAR, your ending BAR, your average depth, and the time you did the test for.
Step 1: Determine how much gas was used during the test in Liters. Let’s assume you completed your RMV test with an 11.1-liter tank. During your RMV test, you started with 180 BAR and ended the test with 110 BAR. This means you consumed 70 BAR during the test (180 BAR – 110 BAR) = 70 BAR. You need to convert 70 BAR to liters, so now you’ll multiply the BAR consumed by the capacity of the cylinder. So (11.1 * 60) = 777. This means you consumed 777 liters of gas during the test
Step 2: Calculate your depth in Atmospheres Absolute. RMV is expressed at the surface. This means you need to account for the atmospheric pressure and how it compresses the gas you breathe underwater. Let’s assume you did your test at an average depth of 15 meters. To convert 15 meters to ATA you’ll divide it by 10 and then add 1. So ((15/10)+1) = 2.5 Atmospheres absolute.
Step 3: Divide the total gas in liters by your ATA. So (777 / 2.5) = 310.8 liters. This is the amount of gas you consumed when adjusted for atmospheric pressure.
Step 4: Divide your result from step 3 by the total amount of time the test was conducted for. Let’s assume we ran our test for 20 minutes. This means (310.8 / 20) = 15.54.
For this test, the average RMV is .15.54 liters per minute. We can round this up to 15.5. This means the diver breathes about 15.5 liters each minute on the surface.
Knowing your RMV rate, how to calculate it and how to use it is one of the most powerful tools you can have in your diving arsenal. What’s your RMV rate and how do you use it? Let us know in the comments below.