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Scuba Diver Cartoon

How to Manage that AIR Supply


For a diver to effectively calculate the air requirements for any given dive they must understand that their lungs work on a principle of volume and not pressure. Divers rely on a submersible pressure gauge (spg) to monitor their air supply, to their lungs this means very little. Their lungs will expand on inhalation and contract on exhalation equally until such time as there is insufficient air to fill them. This mechanical expansion and contraction will occur equally at the surface and at depth, a basic principle of Boyle’s Law suggests the gas in the lungs works differently. The scuba cylinder experiences little to no change of its physical dimensions subject to depth. 

From these points we will look at how a diver can best plan their dive in the safest manner. We will look at calculating a cylinder’s baseline reference rated fill pressure and how the diver can determine their breathing rate referenced to the surface. We will also look at how divers of differing breathing rates and cylinder volumes can plan to turn a dive together as a team unit ensuring to maximize their safety.

Calculating the Cylinders Baseline

Each cylinder has a base mark from which we can determine the cylinders volume. Essentially it is from the internal measurements and the amount of air or gas that is compressed into the cylinder that will determine its volume in cubic feet (cu.ft) or liters. The DOT/CTC rated pressures are set as a result of repeated fills and monitoring stresses on the cylinder’s wall material allowing for extended life of that cylinder and repeated safe usage. The cylinder’s baseline is the starting reference mark from which we base our air calculations. You increase or decrease the internal pressure of a cylinder and you effectively change its volume. We’ll look at this later. As per the table below cylinder baselines can be viewed as Pressure per Square Foot (Psi) over Cu.ft or as Cu.ft per every 100 Psi. You may find that the latter is better to work with for calculating purpose as it leads to more simple air requirements calculations.

Cylinder Volume Baselines:

Volume Cu.ft.

50 (48.5) AL
63 (61.1) AL
65 Gen
66 AA
72 (71.2) AA
80 (77.6) AA
80 Gen
80 (79.3) E
85 AA
95 AA
98 AA
100 (97.0) AL
100 Gen
100 (99.2) E
104 AA
112 AA
120 (119.0) E
120 Gen
125 AA
131 AA
DOT/CTC Pressure Rating (psig)

PSI / Cu. Ft.
Single Cylinder

Baseline CuFt/100 PSI
Single-Double Cylinder



1. Baseline pressures are calculated with cylinders filled to DOT/CTC pressure rating;
2. Steel Cylinders if permitted have been calculated to 10% overfill of rated pressure;
3. Cylinders are filled at nominal ambient temperature of 70 F and do not factor heat buildup during filling or cooling when submersed into water;
4. Baseline is calculated using cubic feet of cylinder as rounded to nearest whole unit. Example 80 cu.ft AL vice 77.6 cu.ft AL;
5. AL=Aluminum, AA=Steel Low Pressure, Gen = Steel Genesis High Pressure, E= E Series High Pressure.

The Method; It's Real Easy...

Method 1
Baseline [cu.ft/100 psi] = (Cylinders Rated cu.ft/ Rated Fill Pressure) x 100

Single Cylinder Example: (80 cu.ft/ 3000 psi) x 100 = 2.67 cu.ft/ 100 psi

For Double Cylinder just multiply Single Cylinder Baseline by 2;

Method 2:  
PSI / Cu. Ft. = Cylinder Rated Fill Pressure/ Cylinder Rated cu.ft

Single Cylinder Example: 3000 psi / 80 cu.ft = 38.6 psi/cu ft

For Double Cylinders divide Single Cylinder by 2

Respiratory per Minute Volume or Surface Air Consumption

Ok now that we have seen how easy it is to calculate the cylinders baseline let's look at two methods to determine how much of that air a diver breathes when they are standing on the surface getting ready to dive. Now you will need to average this out over several dives and dives made under varying conditions just so we can get a more conservative consideration in our dive planning. Some divers will attempt to do this all in one dive by working at various capacities. I believe it best to work it over several dives. Each dive will allow you to see a difference and will have variables from one dive to the next.

 To do this we use what is called Respiratory per Minute Volume and Surface Air Consumption Rate. Respiratory per Minute Volume (RMV) and Surface Air Consumption (SAC) are in fact the same thing and become interchangeable terms. They are a measure of calculating a diver’s breathing rate in cu.ft/ minute, and yes like all things in diving  RMV and SAC are referenced starting at the surface, remember that your lungs work on volume of air not pressure and as you descend to depth your breathing will consume more air volume with each breath taken.

We will show this breathing rate calculation using two methods and you can determine which is best for you.

Now you may want a calculator for this but let us start with an example:

Two divers plan to make a dive to 60 ft. on a reef in the Atlantic Ocean. The dive tables tell them they have a no-decompression time limit of 55 minutes. Each diver is using a single 80 cu.ft Al cylinder with a starting pressure of 3000 psig. They plan to conduct a 35 minute bottom time or begin to turn their dive to the surface at 1500 psig air pressure remaining, whatever limit they arrive at first, they will begin to surface. The dive goes well but the strong current causes one of them, diver A to reach the remaining 1500 psig in his tank sooner than the 35 minutes, in fact it is 30 minutes.  Diver B has 1800 psig at the same moment when they turn the dive. Both divers reach the surface following a safety stop and notice that they now have 500 psig for Diver A and 800 psig for diver B. The total dive time of which they were breathing the compressed air was 50 minutes

Now for our purposes we will look at the total dive time the divers were breathing compressed air (50 minutes). We could use the time where the diver’s turned the dive to come back to surface (30 minutes) either way the answer will be the same. It is important that you note your air and time at the same moment during your dive.

The Method

First the steps common to both methods:

You will need to know how deep you were in atmospheres absolute (Ata). Base this on your average depth as it is not likely you remained at an exact depth throughout the entire dive. For our purpose we’ll use the 60 ft. Also make a note if you were diving in salt water or fresh water. There may be very little difference between them but over a greater depth it can add up. Salt water = 33 ft. for 1 atmosphere of depth, fresh water = 34 ft. for 1 atmosphere of depth.

1. Our divers were diving in salt water, therefore: They were diving to a depth of 2.8 ata. How did we get this?

Ata = (depth / water type) + 1

Ata = (depth / 33) + 1

Example:  (60 / 33) + 1 = 2.8 ata

Note: Fresh water would yield (60 / 34) +1 = 2.7 ata

2. How much air did the divers breathe from the cylinders?

AIR start - AIR end = AIR breathed

Diver A, 3000 – 500 = 2500 psig

Diver B, 3000 – 800 = 2200 psig


Two Methods: Cu.ft / 100 Psi and Psi / Cu.ft

Now here is where we will see a difference in the two methods. We will use Diver A to show the methods.

Method 1: Lets Use the cu.ft/ 100 psi relationship

1. Calculate the cylinder baseline?

(Rated cu.ft / Rated fill Pressure) x 100 = cu.ft / 100 psi

Example: (80 / 3000) x 100 = 2.67 cu.ft / 100 psi

2. Convert the AIR breathed to cu.ft breathed?

(Baseline x AIR breathed) / 100 = cu.ft AIR breathed 

Example: (2.67 x 2500) / 100 = 66.75 cu.ft AIR breathed

3. Calculate how many cu.ft per minute? This will be at depth.

cu.ft AIR breathed / Dive time = cu.ft/minute 

Example: 66.75 / 50 = 1.34 cu. Ft/ minute 

4. Convert your breathing at depth to the surface for your RMV or SAC rate?

cu.ft/minute / Ata = RMV or SAC 

Example: 1.34 / 2.8 = 0.48 cu.ft/minute RMV or SAC Rate

Method 2: Here Lets use the psi/ cu.ft relationship

1. Calculate the cylinder baseline?

Rated fill Pressure / rated cu.ft = psi/cu.ft 

Example: 3000 / 80 = 37.5 psi/ cu.ft

2. Calculate how many PSI per minute? This will be at depth.

AIR breathed / Dive Time = psi/minute 

Example: 2500 / 50 = 50 psi/minute 

3. Convert your breathing at depth to the surface breathing rate in psi/minute?

psi/ minute / Ata = psi/minute @ surface 

Example: 50/ 2.8 = 17.86 Psi/minute

Now you should come back to a better volume unit:

Convert your breathing at the surface to your RMV or SAC rate?

(psi/minute) / psi/ cu.ft) = cu.ft/ minute 

Example: 17.86 / 37.5 = 0.48 cu.ft/ minute

Mathematically the "psi" cancels each other out and you end up with cu.ft/ minute.

Now as you can see the same RMV or SAC rate is calculated with whatever method we use. By the Way Diver B’s RMV/SAC Rate is 0.42 Cu.ft/minute. You will also notice that if we calculated the divers breathing rates based on their air usage at 30 minutes into the dive the RMV/SAC rates would have been the same.

Let's take a look....

Calculating Turn Pressures

Using what we know lets have our two divers make a more advanced dive. Assume this dive to be to 110 ft of fresh water. The No-decompression limit is 16 minutes and our divers plan for a 10 minute bottom time. But they also want to know at what air pressure they should turn the dive to head back towards the surface. They are using the same scuba cylinders 80 Cu. Ft filled to 3000 psig. We will consider the RMV/SAC rates of each diver as previously calculated and again use Diver A with a RMV/SAC = 0.48 Cu.ft/.minute. 

Method 1: Let's start with our cu.ft/ 100 psi relationship

Realistically I would turn at 2300 psig. This is due in fact to the resolution of reading an spg and more conservative. This would also give Diver A 1700 psi in which to make a multi-level computer assisted ascent and still have 500 psi remaining in the cylinder once at the surface or to assist Diver B in an all out air sharing emergency situation.

Method 2: Let's use the psi/ cu.ft relationship.

Again, realistically I would turn at 2300 psig. This is due in fact to the resolution of reading a spg and more conservative. This would also give Diver A 1700 Psi in which to make a multi-level computer assisted ascent and still have 500 psi remaining in the cylinder once at the surface or to assist Diver B in an all out air sharing emergency situation.

Don't Go Away, We Have More!

We can learn to do all this even while we are in the water ready to get going or to make adjustments under during the dive. It all just takes practice.

Cylinder Matching

Cylinder Matching is comparing divers with two different volumes of cylinders and matching their usable gas to each other. The diver with the lowest volume is the regulating diver. You may even take it further by using the diver with the highest SAC/RMV rate against the lower volume cylinder for more added safety.

It is here that the more detailed dive planning starts. But it really is not all that more difficult than what we have looked at already. Not all divers have the same RMV/SAC rate. Nor do all divers wear the same size scuba cylinders. A lady diver may only need a 63 cu.ft cylinder to do the same dive that a male diver does wearing a 80 cu.ft cylinder. The question is what if something goes wrong? Will the diver wearing the smaller cylinder have sufficient air to get both dive team members safely back to the surface? We need to plan our dives in a conservative manner. Depth, time and air are the limiting factors, when the air is gone the dive need be over. We need to plan our dives to the weakest link. Who has the smaller cylinder? Who has the higher RMV/SAC rate? Can both these divers plan a dive together and safely execute it, considering the possible situations of what can go wrong? 

What we are going to show here now is how to consider divers of differing RMV/SAC rates and wearing different size cylinders can safely plan and execute a dive together, it is called Cylinder matching. 

Consider this, overhead environment divers always plan their dives based on rules of third or sixths. We can see how this is a simple way of planning dives considering what might happen. Basically it is dividing your air supply into thirds. Divers use the first third to penetrate the overhead environment and the remaining two-thirds to exit and use as a reserve for those unplanned events that may occur. Even this is still a guideline and these divers will match their cylinders. 

Example:  1.A diver has 3000 psig starting air pressure; He would turn to exit the dive at 2000 psig; 

2. If the same diver had 2800 psig starting air pressure; he would turn the dive to exit at 1900 psig. You should have noticed that it was required to first round DOWN to a number evenly divisible by 3 to calculate the usable air for the dive and then to subtract that from the actual starting air pressure. This allows for a more conservative and safer dive plan. If you do not have the air to breathe you cannot count on it. With AIR ROUND DOWN 

Now if two divers of different cylinder sizes and RMV/SAC rates were making  a deep dive or a dive into an overhead environment or any dive for that matter, it would be in their best interest to plan for the difference in their RMV/SAC rates and cylinders. Just working on rules of thirds may not be good enough. 

Let’s see what I mean: Example, Diver A is using a double set of LP 98 cu.ft cylinders filled to 2640 psig. Diver B is using a set of double 120 E series cylinders filled to 3500 psig. Since Diver A appears to have the smaller volume cylinders we will calculate his thirds:

2640 rounded down to 2400 divided by 3 = 800 psig. Turn pressure at 1840 or 1900 psig 

So we see that Diver A has 800 psi of available air for the dive, converting this to cu.ft you would see that he can use 59.2 cu.ft of air. Remember he is using doubles so his baseline is multiplied by 2. His total air or gas by volume is 196 cu.ft of air of which 59.2 Cu.ft of usable air is just under a third. 

Now how much air can Diver B use? 

Diver B has a baseline of 6.9 cu.ft when using double 120 E series cylinders. We will use this to compare his cylinders back to Diver A’s using some simple math. Basically we are going to compare Diver A’s usable air to the baseline of Diver B to get his usable air and turn pressure. 

(Diver A usable Air cu.ft / Diver B baseline) x 100 = Diver B Usable Air 

(59.2 / 6.9) x 100 = 857.97 psig or 800 psig usable 

Therefore Diver B’s turn pressure is: 3500 – 800 = 2700 psig

Adding the RMV/SAC Factor to the Plan

At this point based on our above example we have Diver A with 59.2 cu.ft of usable air supply or 800 psig. Assume now that Diver A has a RMV/SAC of 0.48 cu.ft/minute. And the team is planning to make the dive to an average depth of 90 ft. in fresh water or 3.65 Ata. [(depth/34) + 1= Ata] 

With this in mind Diver A will have a RMV/SAC rate at depth of 1.75 Cu.ft which tells us that he will turn the dive at about 33.8 minutes or 34 minutes as rounded. All subject to diving conditions.

Diver B however with a RMV/SAC of 0.42 cu.ft/ minute and usable air of 59.2 cu.ft (from cylinder matching calculations) could go for 38.7 minutes or 39 minutes rounded. However both divers would turn before 34 minutes or sooner since the no-decompression limits for a 90 foot is 25 minutes, but let's run the calculation anyway.... 

cu.ft usable/ (Ata x RMV) = TURN Time 

Example: 59.2 / (3.65 x 0.42) = 38.7 minute

One More Point

Let’s say for one moment that Diver B has a RMV/SAC of 0.7 cu.ft/minute. How would we plan the dive he is going to make with Diver A using double 98 cu.ft cylinders and a RMV/SAC of 0.42 cu.ft/minute? Simple actually we always work to the weakest link. Here Diver A has the smaller cylinders and a lower RMV/SAC rate. Diver B has the larger cylinders and a higher RMV/SAC. The problem is what if there is an emergency? Can Diver A safely surface from the dive sharing air with Diver B? Let us look at it. 

Diver A’s cylinders when we apply the rule of thirds, yields 65.3 cu.ft of usable air (196 /3 = 65.3 cu.ft). Diver A only requires 59.2 cu.ft of air to do the dive leaving 136.8 Cu.ft of air in reserve, is it enough to get Diver B back to the surface with a breathing rate of  0.7 cu.ft/ minute? 

From our previous calculations we matched the different cylinders baseline. Diver B at 0.7 cu. ft/ minute RMV/SAC rate would have only 23.17 minutes before he would turn the dive team. Diver A would have used only 35.52 cu.ft of air leaving a remaining 160.48 cu.ft of air, more than enough to get the divers back to the surface. 

You could also look at it from Diver B with a RMV/SAC = 0.7 cu.ft/ minute with him wearing the Double 98 cu.ft cylinder and Diver A wearing the double 120 Cu.ft cylinders. Applying the rule of thirds Diver B would still have 65.3 Cu.ft of usable air and would turn the dive at 1800 psig (2640 / 3 = 880 psi, 2640 – 880 = 1760 psi, (round up 1800 psig be conservative on air, an extra 40 psi to use in this case) 

This would mean that Diver A now with the double 120 cu.ft cylinders would have a usable air supply of 942.03 psi rounded down to 900 psi. [(65 / 6.9) x 100 = 942.03 psi].  This also means that Diver A’s turn pressure would be 2600 psig. [3500 – 900 = 2600] 

In either case Diver A would be able to assist Diver B in an air sharing emergency to safely return to the surface 

Summary: IT Takes Practice and Dives

It is important that you always plan your dive with all safety factors considered. You should match cylinders and adjust for differences in RMV/SAC rates for the members of your dive team. We can effectively alter the volume of the air or gas supply we carry but we cannot adjust for information we do not have or for what we do not know will occur, train and plan with this in mind. Dive Safe!

Keep Diving .. Keep Training!