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.
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.
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.
What is the difference between SAC and RMV?
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.
How to calculate your RMV rate
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.
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 metres. The calculations are so much easier in metric. I plan to go back and convert the calculations for imperial at a later date.
To calculate how many litres of usable air are in a cylinder take the litres that the cylinder holds and multiply it by the number of BAR in the cylinder. So a 12 litre cylinder with 200 BAR will contain 2400 litres of gas. So easy.
OK, so you have done your 10 minute dive and calculated how much air you used. Your constant depth for the dive was 10 metres and you used 30 BAR from your 12 litre cylinder.
Let’s work out how many litres we used in those 10 minutes. 30 x 12 = 360 litres.
That means we used 360 ÷ 10 = 36 litres per minute at a depth of 10 metres.
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. (10 metres ÷ 10 ) + 1 = 2 ata.
Now that we know the ambient pressure at 10 metres is 2 ata we can work out how many litres we will consume at the surface. That our breathing rate at depth and divide it by the ambient pressure at that depth.
36 litres per minute ÷ 2 ata = 18 litres 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 ourRMV rates for different depths. The numbers in the right-hand column are in litres per minute.
Planning a Dive
So now that we have our RMV rate we can plan a dive. Let’s go to a 30 metres with a 15 litre cylinder that is filled at 200 BAR.
The cylinder contains 200 BAR x 15 litres = 3000 litres of gas. As we are going to use our rule of thirds that leaves us with 2000 litres of gas that we can use.
Total time we can stay is usable gas divided by our breathing rate at that depth.
2000 litres ÷ 72 litres per minute = 27.8 minutes
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 which will multiply your RMV meaning that you can plan for more conservatism into your dive. RMV-Calculator-and-Planner
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.
First and foremost let me state that you must receive formal training from one of the many dive organisations before attempting a dive. Without formal training you risk injuring yourself. Please get training before attempting anything you have read or seen in this post. I will not be held responsible for you attempting to use the methods contained in this post. The following post is for illustration purposes only.