Good gas management means that you can enjoy your dive more.  Knowing your RMV (Respiratory Minute Volume Rate) is important. Most recreational diving agencies don’t go into it and there isn’t much talk about it. However, ask a tech diver and they will tell you theirs and how important it is to know. Have you ever been on a dive and due to your buddy being a heavy breather you’ve had to end your dive early? I know this has happened to me on more than one occasion. Knowing your RMV will allow you to plan your dives more effectively and increase your bottom time.

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Your RMV rate is how quickly you consume the gas from your cylinder if you were moving at a moderate rate at the surface. Knowing this allows you to estimate how much gas you are going to use on a dive. Now if you remember your rule of thirds (I am not talking about photography, but the common practice of using one third of your gas supply to get there, one third to get back and one third is kept in reserve in case of emergencies). It’s quite easy to calculate your RMV rate and you should give it a go on your next dive.

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Why is it important? If everyone knows how much their RMV rate is, it is easier to buddy up divers so that they have similar RMV rates equating to similar dive times. When you are planning a dive you should plan at the worst rate in your buddy pair. Remember if your buddy has a failure and loses all his breathing gas he is going to have to use yours to get to the surface, it would be better to be conservative. It also allows you to choose the right cylinder for the dive.

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What is the difference between SAC and RMV?

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SAC is your surface air consumption. It is a very similar calculation to the RMV but it is only valid for the tank that you calculated it on. If you change tanks and repeat the experiment you would have a different SAC rate. RMV calculates the how much you are actually breathing regardless of the tank’s size. This is a much more usable value as you can easily compare that amongst dive team members. If everyone is diving the same size of tank (which makes all calculations easier) then SAC rate is fine to use but the golden standard is the RMV and you should really use that.

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How to calculate your RMV rate

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You need to stay at a constant depth for about 10 minutes. The easiest way to do this is find a nice dive site that has a flat bottom. Get neutrally buoyant. Check all your gauges (depth and air) and record them. Now swim around at a moderate rate for 10 minutes keeping your depth constant. Once the ten minutes are up record your remaining air. Once you have done this we can get into the calculations.

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Differing cylinder sizes.  I have dived with cylinders of varying sizes from aluminium 40s to steel 15s. So unless you are diving the same cylinders all the time then you need to have an easy way of finding out how long any cylinder will last at any given depth. 

 

Metric or imperial?  I am going to do the calculations in metric, this means BARs and meters. The calculations are so much easier in metric, but also, in South Africa we dive primarily with metric systems, thus focusing on metrics. I plan to go back and convert the calculations for imperial at a later date.  To calculate how many liters of usable air are in a cylinder take the liters that the cylinder holds and multiply it by the number of BAR in the cylinder. So a 12 liter cylinder with 200 BAR will contain 2400 liters of gas. So easy.

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The Numbers.  OK, so you have done your 10 minute dive and calculated how much air you used. Your constant depth for the dive was 10 meters and you used 30 BAR from your 12 liter cylinder.

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     Let’s work out how many liters we used in those 10 minutes. 30 x 12 = 360 liters.
     That means we used 360 ÷ 10 = 36 liters per minute at a depth of 10 meters.

 

     Now that is useful but it would be better knowing how much air we would be using at the surface. If we know that we can work out how much air we will           use at any depth.  The first step is to work out the ambient pressure at the depth you did your test at. Divide your depth by ten and then add one.  

     (10m ÷ 10 ) + 1 = 2 ata (Remember from Open Water - ata = atmospheres or Bar - 1 bar (ata) atmospheric pressure + 1 bar (ata) hydro  static pressure.

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     Now that we know the ambient pressure at 10 meters is 2 ata we can work out how many liters we will consume at the surface. That our breathing rate at       depth and divide it by the ambient pressure at that depth.  Therefore 36 liters per minute ÷ 2 ata = 18 liters per minute - We now have our RMV rate.

 

Gas Consumption at Depth.  The deeper we go, the more air we use. To calculate how much air we are using we need to multiply our RMV rate by the ambient pressure. So here is a little table of our RMV rates for different depths. The numbers in the right-hand column are in liters per minute.

     Surface Rate = 18 liters per minute

         10 m         36 l/min         (18 x 2 ata)

         20 m         54 l/min         (18 x 3 ata)

         30 m         72 l/min         (18 x 4 ata)

         40 m         90 l/min         (18 x 5 ata)

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Furthermore, this consumption rate will increase depending on the level of activity, i.e. strong currents, photography, or work done underwater.  This factor can be divided into four categories, i.e. (1) at rest (x1), (2) mild work (x1.5), (3) moderate work (x2), and (4) hard work (x3).  This implies that you must now multiply the previous rates with the factor associated with the activity level

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    Take for example at 10 meters your RMV is 36 l/min then and multiply it with the activity level of for example mild work (x1.5) = 36 x 1.5 = 54 l/min

    This implies that for a 30 minute dive your normal consumption would be - 1080 liters of air

    For a mild work rate dive your consumption would increase to - 1620 liters of air

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The question now is to evaluate what is considered to be the the categories of activity level.  This is not easily defined but could be seen as follows:

    (1) At rest - not swimming, very little finning or just hanging at a depth - with other words doing almost nothing.

    (2) Mild activity - struggling somewhat against the currents, photographing, or swimming faster than normal to someone.

    (3) Moderate activity - struggling heavily against the current, swimming very fast to someone, or helping someone with weights or BCD.

    (4) Hard work - towing i.e. an object underwater, lifting something to place a rope around - more for those doing search & recovery type of work.

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Planning a Dive.  So now that we have our RMV rate we can plan a dive. Let’s go to a 30 meters with a 15 liter cylinder that is filled at 200 BAR.

    The cylinder contains 200 BAR x 15 liters  = 3000 liters of gas.

    As we are going to use our rule of thirds that leaves us with 2000 liters of gas that we can use.  

    Total time we can stay is usable gas divided by our breathing rate at that depth.
    2000 liters ÷ 72 liters per minute = 27.8 minutes

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I’ve created a spreadsheet that calculates your RMV rate and if you like it will also calculate the maximum amount of time you can breathe from a specified cylinder at a specified depth. I have also included a conservative factor or work rate, which will multiply your RMV meaning that you can plan for more conservatism into your dive. I have also found an interesting group of calculators for you to play around with at the following website:                           https://www.divestock.com/calculators#eu4bt3i9c

You can also look at: https://www.thoughtco.com/air-consumption-rates-for-scuba-diving-2962942

Some more about air pressure and air consumption: https://www.simplyscuba.com/blogs/blog/gas-density-and-how-it-affects-your-body

See this: 

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Warning

Note that this is an estimate of your air consumption based on you moving at a moderate rate. If you are doing more strenuous activities then you might want to include a conservative factor into your calculations and state that your RMV rate is higher. If you are doing less strenuous activities you might find that you get more bottom time than this predicts. Use this as a guide but always check your gauge. I’ve mentioned it before but it is a good idea to guess the reading before you look as this starts to help you build a 6th sense about your gas consumption.