How long does a scuba tank last? A simple question. However, there is no shortcut for the answer. Several factors will determine how long an air tank lasts. In this article, we will draw on physics and my personal experience as a diving instructor.
Let’s base the average on the most common scuba diving conducted around the world. New divers taking their try dives, certification courses, and fun divers with less than 30 dives, make up the largest portion of divers in my experience.
These divers are often still nervous and learning to control their air-consumption. They use standard equipment with a 80-cubic-foot cylinder filled with 200 bar of air. Dives at these levels are not deep at around 18 meters / 60 feet.
Based on my experience as an instructor these dives last between 35 and 45 minutes with 20% lasting as long as 60 minutes. This is surfacing with a minimum of 50 bar air-reserve still in the tank.
Let’s look at some determining factors.
Everyone’s favorite subject in school, physics. Don’t worry, diving physics at the recreational level is very simple and easy to understand. Most of it comes down to Boyle’s Law which states the linear relationship between depth, pressure, volume, and density.
Our atmosphere exerts 1 bar of pressure on us. This is where we can assume 1 bar of pressure on the surface of the ocean. Because water is almost 800 times as dense as air, it exerts vast amounts of pressure even in shallow depths.
Here’s a quick excerpt of Boyle’s Law at 20 meters:
This means that we are under 3 times the pressure at 20 meters depth compared to the surface. Under that pressure, air particles are three times closer together, therefore air is three times denser. To fill the same lung volume at that depth, three times the amount of air is required. In turn, if our air-consumption rate at the surface is x it becomes 3x at 20 meters. This means the deeper we go, under more pressure, the more gas we will use. There is nothing we can do to stop this. This is how physics affects how long our tanks last.
Air Consumption Rate
We can calculate gas consumption at various depths more accurately by using formulas based on a diver’s SAC rate (Surface Air Consumption rate) and Boyle’s Law. This will give us an expected air-consumption rate.
We need to figure out how long does a scuba tank last at the surface and then we can answer how long does a scuba tank last at 47 meters.
For simplicity’s sake, let’s assume our SAC rate translates to us using a full 80-cubic-foot tank on the surface in 3 hours. We can then use the density at 47 meters, which is 5.7, to calculate our consumption rate at that depth:
180 minutes / 5.7 density = 31 minutes
More experienced divers breathe slower and dive more relaxed and efficient, which reduces their air consumption. Mastering buoyancy control helps reducing air-consumption as well as breath-awareness.
The most widely used scuba tank size is 80 cubic feet or about 12 Liters. Other common scuba tank sizes used in diving are 8-, 10-, 15-, and 18-Liter cylinders. While there are many more sizes, in my experience, the most commonly used sizes besides the 12-Liter tanks are the 10-Liter for children and 15-Liter for air-hogs (divers with rapid air-consumption).
Next, it will depend on how much air the scuba tank compressor has filled the tank with. Air is so light, so why are these tanks so heavy? Many new divers ask me how much does a scuba tank weigh? The standard scuba tank weight in kg is about 16 or 35 pounds. A common misconception is that steel tanks are even heavier. In fact, a steel tank with a similar volume actually weighs in at 30 pounds or 13.5 kilograms.
Because steel is stronger than aluminium, steel tanks are a lot thinner. Aluminium tanks have an average wall thickness of 15.5 mm whereas steel is around 5 mm. Now even if steel is a heavier material, that’s a lot of extra aluminium needed to hold tank pressure.
Now, while size matters, it also matters how much pressure is in these tanks. An average male’s lung is about 6 Liters. So if we fill our standard 12-Liter tank with atmospheric pressure of 1 bar, that’s two breaths on the surface. 200 bar, however, is 200 x 12L = 2400L, so about 1200 breaths on the surface. Remember depth will use more as we established above.
Female’s often have smaller lung volumes, giving women an advantage in air-consumption rates when scuba diving.
Scuba cylinders are rated to maximum pressures that they can safely hold. This is why we can’t pump 500 bar into a standard scuba cylinder.
Don’t fancy lugging that much weight around? Maybe figure out how long does a pony bottle last, and take that instead.
Tips to Improve Your Air-Consumption Rate
This is a very common question I get asked. People want to stay down for longer, so they want to make their tank last longer. Experience is one of the factors that matter greatly. However, I often tell my students these tips to improve their air consumption:
- Relax yourself and your breathing
- Don’t use your arms to swim
- Find perfect buoyancy and kick efficiently, not constantly
- Move slowly and as little as possible
- Try to breathe out longer than a breath in
For more advanced divers, I also suggest finding a rhythm. Really experienced divers don’t simply breathe in and out and kick. They find a rhythm of breathing and moving that is comparable to meditation!
This is why scuba diving is so awesome!
Other Factors Limiting Your Time Underwater
Air is important for survival. But no matter whether you take your certification with PADI, SSI or RAID, you will learn about other limiting factors. Temperature can limit a dive if you are diving in Norway for example. Dive buddies in your dive team with faster air consumption than you can end a dive early. When diving at depth, no-decompression limits can also be your limiting factor.
How Long Does a Scuba Tank Last?
Now that we have established the answer is more complicated than the question, we know there isn’t a simple scuba tank time calculator we can use to calculate the exact time. It goes without saying that you should never push air limits to last a little longer underwater. Finished it in 20 minutes? No worries, make a safe ascent, and switch to a new tank. Air is free after all.
One thing I do know is that a scuba tank never lasts long enough!