Newly certified divers are often excited to get in the water and explore the vast world beneath the surface. Each new environment brings with it new underwater ecosystems, different species of sea creatures, and even wrecks to explore.
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However, it is normal for the new diver to have many questions when planning their first dives. One of the most frequent questions is how long a scuba tank lasts when diving.
After all, your air source is your lifeline underwater. Understanding how long you can safely be beneath the surface is key to being able to plan dives and have confidence. So, exactly how long does a scuba tank last when diving?
While there isn’t an exact answer, beginning divers on a typical dive will find that their scuba tanks provide approximately 45 to 60 minutes of underwater time while still providing a safe reserve of air. Factors that affect this include maximum depth, breathing rate, lung capacity, and tank size.
Obviously, it is not possible to state with certainty exactly how long a tank will last for each diver. This is why checking your air levels during dives and communicating effectively with your dive buddy are important.
However, understanding the factors that influence your time underwater will help you learn to better plan dives.
A Typical Dive – What Makes it Last 45 Minutes
Anyone who has a handful of dives in their logbook knows that each dive is different. So what do we mean by a “typical dive?” For a typical dive, let’s consider the dives most frequently undertaken by beginners with an open water certification.
Open water certification means that you can dive up to 60 feet. However, most of your first dives will be a bit shallower, usually in the range of 30-40 feet.
At this range, you are more likely to attain the 45 minute underwater mark.
This is because from 30 feet to 60 feet, air consumption increases substantially (we’ll discuss why later).
Your typical dive also assumes some information about your equipment. The most common rental tank used is an 80 cubic foot aluminum version. This usually contains 3,000 pounds per square inch (PSI) of compressed air.
This air, under conditions of a typical open water dive, will allow most divers to hit the 45 minute mark.
Another thing to consider is the water temperature. We’ll assume you’re diving in warm or fairly temperate water. While there are popular dive spots around the globe, the most frequented ones – particularly for beginners – are in warmer climates.
Finally, let’s make some assumptions about you because individual differences impact air usage as well.
Let’s say you’re a man that is 5’10” and 200 pounds or a woman that is 5’4” and 160 pounds. We’ll also assume you are of average fitness and engaging in a relaxing dive without strenuous effort.
During this “typical” dive, you’ll spend a few minutes descending to depth.
This will vary a bit depending upon how easy or difficult you find it to equalize on the way down. Once at depth, you will spend most of your dive exploring the environment, taking in the sights of coral and marine life.
You will spend up to half an hour at this depth, slowly cruising around the reef. Obviously, checking your pressure gauge periodically is important during this time.
The last ten minutes of your time underwater will be reserved for your ascent. This includes the important safety stop at about 15 to 20 feet that will last for three to five minutes.
A typical diver on this typical dive will reach the surface after approximately 45 minutes with a safe reserve of air in the tank.
However, what if you are not the typical diver on a typical dive? Let’s look at the factors that can affect your tank times.
What Factors Affect the Length of a Dive?
If you aren’t engaging in the “typical” dive, you’ll want to know the different factors that influence how long your scuba tank lasts.
This will help you better plan the length of your dive. There are a variety of factors; however, the basic ones are depth, exertion, breathing rate, tank size, and lung capacity.
Depth is one of the easiest factors to understand.
Deeper depths result in more rapid use of your air supply. Remember from your training that pressure increases as you dive.
The surface has a pressure of one atmosphere, which increases by one every 33 feet. Thus, changing a dive from 33 feet (2 atmospheres) to 99 feet (4 atmospheres) doubles the pressure.
This is important because it means that greater pressure is being applied to both you and the air you breathe, meaning even at the same rate of breathing, you are consuming more air from your tank.
Thus, the deeper you dive, the more quickly you use your air supply.
This is one of the easier factors to predict because it is relatively stable mathematically. If you know how long a tank lasts for you at 30 feet, you can predict how long it will last at different depths.
This is simple to understand, but harder to predict.
Essentially, you burn through air more quickly when you exert yourself more underwater. This is why scuba divers tend to move rather slowly and deliberately underwater. It decreases breathing, providing more time at depth.
There are different things that will require more exertion.
For example, if you are diving in current, you will exert yourself more. However, divers can enhance their air supply in general by practicing more fluid movements and improving their buoyancy.
The easiest thing a diver can do to increase their time underwater is to practice their breathing. Slow, full breaths will fill the lungs (improving efficiency) and allow for all carbon dioxide to be exhaled (increasing the time before the brain sends a signal to inhale again).
Temperature also plays a role here. During colder dives, we naturally breathe more rapidly as our body seeks to produce more heat to compensate for the heat we lose to the water.
Aside from depth, tank size is perhaps the easiest factor to calculate. If you use a tank with more air, you will be able to stay underwater longer. So why not just use a giant tank on every dive?
As you learned in your training, time at depth is limited not just by the air supply but also by nitrogen absorption (which also varies by depth). Thus, the standard tank will be suitable for most beginning divers.
Lung Capacity & General Biology
Your biology also plays an important role.
If you are relatively large, your lungs will likely be large as well. This means you will use more air with each breath than a smaller diver.
Fitness levels play a role here too as someone in poor fitness will breathe more while someone in good fitness will breathe less.
In fact, improving your fitness and practicing your breathing are two ways to help compensate if you have a larger lung capacity.
Do Twin Tanks Double Your Dive Time?
You may have seen people dive with twin tanks and wondered if they are more efficient with air usage.
The basic answer is yes.
There are two reasons for this. First, twin tanks can be set up to simply hold more air for a dive by having an increased total tank size. This means more time underwater.
Additionally, there is a functional benefit to the design.
Twin tanks have two connected but independent regulators. Let’s say one diver takes two single tanks of 3,000 PSI for two afternoon dives.
Another diver takes twin tanks, each with 3,000 PSI per cylinder. When the first diver gets back to the boat after their initial dive, he has air left in the tank that is discarded when he switches tanks.
Meanwhile, the twin tank diver doesn’t need to switch tanks and doesn’t discard this air. It can be used on his second dive.
Twin tanks may be an effective solution if you find that you use your air more frequently than a typical diver.
It is also a good option for deeper dives as a safety feature.
Ultimately, most divers will find that a standard rental tank of 3,000 PSI will last 45 to 60 minutes during a dive at the depth of 30 feet.
With greater fitness and lower breathing rates, time underwater will increase.
Meanwhile, more exertion and diving at greater depth will cause your time underwater to decrease.
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Divers who find they use their air supply faster than the norm can practice lowering breathing rates, improve their fitness, opt for a larger tank, or consider a twin tank setup.