Apr 2015

How Long will my Air Last?

By Mike Waddington

‘How long will my air last’ is a frequently asked question by those who are new to the scuba diving sport, and it is one that most scuba professionals would have asked their instructors when they were learning. It is a fair question, especially from those who are just seeking a one-day dive experience on their holiday. The answer given is usually along the lines of “it depends”, which is not the concrete answer that the new diver was looking for, but it is the only honest answer, and you will never find out until you actually go diving because there are so many variables involved.

How Long Will My Air Last?

On average (this is all from my personal experience) a newly certified open water diver, diving with an 11-litre tank, to 12 metres in warm water, a tank will last roughly 40 – 50 minutes, without going into the caution zone (50 bar or less). As I said this is only an average, and the answer is different for everybody. I have had a student on Open Water Dive One use 150 bar in less than 20 minutes at 10 metres, and I have had students at the same level not even use 100 bar in 50 minutes. If you want to really find out how long a tank will last you, then, first of all, you need to work out your air consumption.

What is the Rate of Air Consumption?

The rate of air consumption by a diver is frequently referred to as ‘Surface Air Consumption’, SAC for short, or ‘Respiratory Minute Volume’, RMV. Usually larger divers, with larger lungs have a higher SAC than smaller divers, with smaller lungs. So, if you are smaller then you are in luck, as your air cylinder will last longer, giving you a longer bottom time. Your SAC can be affected by many factors. Experienced divers will use less air than inexperienced divers because they will typically feel more comfortable in the water, and have much greater control over their buoyancy. The best way for a diver to improve their air consumption is to focus on keeping relaxed, breathing slowly, and buoyancy precision.

Does Depth Change How Long My Air Lasts?

Depth also plays a huge factor in determining how long a cylinder will last. As we descend underwater the pressure around us increases. The air in the tank remains unaffected because it is in a non-flexible container. However, the pressure does compress the air that goes through our regulator hoses and into our lungs. Because at 10 metres the pressure is double what it is at the surface, it takes twice as much air to fill our lungs. This means that if a tank of air lasted 1 hour 30 minutes at the surface then it would only last 45 minutes at 10 metres, and this effect continues as we descend. The deeper you dive, the quicker you will use all the air.

Tank volume is also something that needs to be taken into account while considering how long you will be able to stay underwater. A common tank size encountered while diving, especially in the tropics, is the 11.4-litre aluminium cylinder. There are many different sizes and capacities available, so if you burn through a cylinder quicker than a Hummer goes through fuel, then maybe you should consider trying to use a bigger cylinder, such as a 15 litre. Likewise, if you seem to barely breathe underwater, then try to use a smaller tank, such as a 10 litre. It will weigh less and feel particularly freeing to the petite diver.

Calculate Air Consumption – The Formula

You can easily work out your personal air consumption with a basic maths formula. First of all, you will need to go diving and take note of the cylinder size, as this is important. Simply choose a depth, 10 metres is the best to go for. Take note of your exact cylinder pressure, and keep at the exact same depth, diving in a normal relaxed way. Do this for a set amount of time, 10 minutes is good, and take note of the exact air pressure once again. Now you have all the variables you need to accurately work out how much you breathe underwater.

Let’s say our diver started the exercise with 155 bar, and after 10 minutes they had 128 bar left, and they were diving using a 10-litre cylinder. The diver used 27 bar, out of a 10-litre cylinder. This means the diver used 270 litres of air. The diver kept at an average of 10 metres, so to work out how much the same diver would have used at the surface, we simply divide the amount of air used by the surrounding pressure, which at 10 metres is 2 bar. This leaves us with our diver using 135 litres in 10 minutes at surface pressure, which means the diver has a Surface Air Consumption of 13.5 litres/minute.

It is easy to work out, and you can regularly perform the calculation to see if your air consumption is improving over time:

(Air used in Bar x Cylinder volume) ÷ Surrounding pressure ÷ Time = Surface Air Consumption (SAC)

Can Different Diving Gases affect Air Time?

Yes, using different air when diving can have an effect on how long you can stay underwater. If you are using different diving gases (rather than air) this will definitely change the time you are able to dive, due to the different density and absorption rates of other gases such as Nitrox, Trimix, Helium,

So as you can see there is no simple answer to “how long will my air last?” the variables are simply too great, and we can never know for sure until you go diving. We can tell you though, that with patience and practice, your air consumption will improve over time, and it won’t be long until air consumption is not the main limiting factor on dive time anymore.

‘How Long will my Air Last?’ was written by Mike

Photo Credit: Scubanase.com

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Mike Waddington

I first discovered diving in 2008 after going snorkeling on the Great Barrier Reef in Australia. After trying diving at a flooded quarry in England I decided to head out to warmer more interesting waters in Thailand where I ended on the Island of Koh Tao completing my Open Water course. Instantly addicted with money to spend and plenty of time on my hands I decided to continue until I became a Divemaster so I could live what seemed as the perfect life.

After that I headed to the Caribbean to an island called Utila to complete my instructor course, I spent several months out there completing the MSDT internship, teaching students and leading dives. This is also where I discovered my interest in the technical side of diving, taking part in equipment repair courses and learning about blending gasses and running compressors.

With all my new qualifications it was time to head back to where it had all started, Back to Koh Tao where I intended on living the dream. Once I arrived I quickly found a job and started teaching straight away. During my time on Koh Tao I took part in all many technical diving courses, learning how to dive with re-breathers, in caves and even going down to 90m/300ft!

Qualifications:

PADI Master Scuba Diver Trainer
PADI/DSAT Tech Deep Instructor
PADI/DSAT Gas Trimix Gas Blender
PADI/DSAT Trimix Diver
TDI Intro to Cave Diver
TDI Advanced Wreck Diver
TDI Inspiration rebreather Decompression Procedures
PADI Professional Videographer
BSAC Compressor Operator
TDI Equipment Service Technician

Dream Dive Locations:

Silfra, Iceland
Cenotes, Mexico
Chuuk Lagoon, Micronesia
Ice Diving in Russia