How long does an average tank of air last while scuba diving?
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you can be underwater for days!! just try it once
roughly 30-40 Min’s but it all depends on depth and where your diving
It depends on your tank but most are about 30mins
Good Luck !
Unfortunately there is no definitive answer to your question. This depends on 3 factors, two ‘fixed’ and one ‘variable’:
(1) Tank capacity
This is calculated as follows (note that this only works in metric; for Imperial measures, further calculations are necessary):
Capacity (L) = tank volume (L) x fill pressure (b)
i.e. a 12-L tank at 200 bars holds 2400 L of air
The ‘standard’ aluminium resort tank holds approx. 2200 L (80 cu.ft.) when charged to 200 bars (3000 psi). Indeed, Americans often refer to them as ’80 cu.ft. tanks’–although you only get this amount if the tank is charged to its maximum working pressure. Europeans tend to measure tank capacity by actual tank volume, i.e. how much water a tank would hold if you removed the valve and applied a hosepipe (not recommended!)–an ’80-allie’ is about 11.3 L
(2) Dive depth
This affects the pressure at which that air is delivered to the diver: if the pressure is doubled, the ‘air-time’ will be halved.
Ambient pressure (bar) = (Depth (m) / 10) + 1
e.g. at 27 m, AP = 3.7 bar
NB You may hear ambient pressure being measured in ‘atmospheres (atm)’ or ‘atmospheres absolute (ata)’. 1 bar is approx. = 1 ata, so the terms are effectively interchangeable, at least for our purposes.
(3) Diver breathing rate
This is dependent on (among other things) age/body size, experience, fitness, workload, and comfort level (both physical and psychological). An extreme example would be a fit and highly-experienced instructor in a tailored suit vs. an unfit novice in a poor-fitting rental suit: the novice is likely to have emptied his tank (long) before the instructor has even reached the halfway mark.
Knowing your own breathing rate will allow you to predict how much air you personally are likely to use on any given dive, and thus how long you can stay down.
However, if you prefer to avoid maths, you can skip the rest, and stick to ending your dive when the needle hits the red zone
You can work out your breathing rate by measuring your air consumption at a given depth for a known amount of time, provided you know your tank volume. Some dive computers (and/or their subsidiary logbook software) can actually do this for you, after the fact. Breathing rate is also called the ‘surface consumption rate’ (SCR), i.e. it measures how fast the diver would breathe at the surface.
SCR (L per min) = Air used (L) / ambient pressure (bar) x time (min)
e.g. A diver uses 50 bar from a 12-L tank in 15 min at 10 m (2 bar):
diver’s SCR = 50 x 12 / 2 x 15 = 600 / 30 = 20 L per min
Once you know your SCR, it’s simple maths to calculate your tank capacity and thus ‘air-time’ at your planned dive depth. The SCR equation needs to be rearranged, to give
Tank duration (min) = air used (i.e. tank capacity) / SCR x ambient pressure
So the diver in the above example, at 20 m (3 bar) with a 12-L tank charged to 200 bar, would have 12 x 200 / 20 x 3 = 2400 / 60 = 40 min–but at that point he would run out of air!
As you (should have) learnt on your basic diving course, a diver and his buddy should decide before the dive what their ‘ascent pressure’ will be. A conservative measure would be the ‘rule of thirds’ (one-third out, one-third return, one-third ascent/reserve), i.e. 65 bars (33%) of a 200-bar fill. Most sport divers use 30 to 50 bars as a convenient reserve, i.e. 17 to 25% of a 200-bar fill. The remainder is your ‘usable’ air. The ‘duration equation’ then becomes
Dive time = tank capacity x (1 – proportion reserve) / SCR x ambient pressure
If our exemplary diver keeps 50 bars in reserve (i.e. 25% of his tank capacity), he then has 12 x 200 x (1 – 0.25) / 20 x 3 = 2400 x 0.75 / 60 = 1800 / 60 = 30 min for his dive. If he wants more time, he either needs to get a bigger tank, or work on lowering his SCR.
If you have any further questions, please feel free to email me through Answers.
In the western hemisphere, the “average” scuba tank is an AL80. This is the one that is most popular at resorts and dive destinations. AL 80 means that it is an Aluminum tank that holds roughly 80 cubic feet of air (72 actually) when filled to 3,000 psi and at 72 degrees F.
On a dive that I call a “typical tourist dive”, the tank will last roughly 45 minutes. A typicaly tourist dive has the following traits:
–warm water, around 80F
–average dive depth of 40-45 feet
–the diver might dive 10 to 20 times per year
–little to no current on the dive
As others have pointed out, depth and the diver make a huge impact on how long the tank will last.