How long will a tank last?
The big question that every new diver asks is: how long will the air in my tank last? The answer depends on how much air there is in the tank to begin with, at what depth it is going to be breathed in and how much air the diver is going to breathe.
In metric standard, the first two parts are easily identified. The shoulder of the tank has its fixed volume marked on it, and you can read from the pressure gauge how much it has been filled. Multiply one by the other: a 15-L cylinder filled to 230 bar has 3,450 L of air.
It is best to set aside a reserve of air, and conventional thinking suggests that one-quarter of the initial supply be kept aside.
This may be overcautious with a large tank, but divers have to make a judgment based on the circumstances they expect to encounter. Let’s assume we have a 10-L tank filled to 200 bar. Only 1,500 L of air is at our disposal with the rest (50 bar) held in reserve.
The next thing to identify is the depth at which the air is going to be breathed. At 30 m (100 feet) deep, the regulator delivers air at four times the pressure that it would at the surface. Thus, if we are diving at this depth, we have only 375 L of air to breathe.
How much air do you need?
The amount of air you need depends on several factors. Body size is important – a slender woman with small lungs will probably breathe a lot less than a heavyÂweight boxer, who will have large lungs to pump a huge amount of air. Activity and stress are also important. A relaxed man may breathe only 8 L (0.3 cubic feet) every minute, but increase his heart rate by increasing his workload or stress him in some way and this can leap to 30 L (1 cubic foot) per minute.
Fitness is not necessarily an indication of breathing rate. An older diver who has smoked all his life may not be very fit, but if he is relaxed – and often that comes with experience – he will use less gas than a young trained athlete who is working hard underwater. If you have to swim, you will consume more than if you are merely hovering in the water. Even thinking uses a lot of energy.
Training agency manuals usually use a figure of 25 L (0.9 cubic feet) per minute in their examples of how to calculate air consumption. At 30 m (100 feet), a 10-L tank (with 50 bar held in reserve) would last only 15 minutes.
Breathing underwater
Potential divers all want to know how it feels to breathe from a regulator underwater. It feels exactly the same underwater as it does if you try it on land in a dive shop. Either underwater or on land, as you breathe in there is a faint resistance as the valve pulls open. The mouthpiece floods with air that you inhale. It stops when you stop. When you exhale, there is slight resistance as the exhaust valve opens to allow the air to escape. The exhaled air then bubbles away.
