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Every diver has a different SAC rate (Surface Air Consumption rate). We all breathe in different amounts of air, and have different breathing rates. It also depends on how deep you're talking. Remember, for every 30-33 feet down you go, the pressure reduces the volume in half.

Answer you're probably looking for:

an aluminum 80 cubic foot tank, at about 30 to 50 feet will last me 45 to 60 minutes.

I just noticed the other question at the bottom of this:

The effect of increased water depth on a Scuba diver?

Now here's the fun question to answer. Breathing compressed air at depth can be DANGEROUS. Let's say you take a breath of air at 30 feet, and then hold your breath while you come up to the surface (NEVER DO THIS!). The volume of the air in your lungs would double. Odds are your lungs can not hold this much volume, and you can cause your lungs to explode (collapse)

Another effect of breathing air at depth is called "Nitrogen Narcosis". Air is basically 21% oxygen, 79% nitrogen. At the surface ... not a problem. BUT if you're breathing air at depth, the Nitrogen starts messing with you a bit.

The first way is Nitrogen Narcosis. It affects your ability to think. The best way I've ever heard this described is the "Martini Effect". For every 30 feet down you go, it's like having a vodka martini on an empty stomach. So if you're at 120 feet, it's like 4 martinis on an empty stomach. Kinda tipsy.

Now, for the DANGEROUS aspect of Nitrogen at depth. Your lungs take air, and put it into your blood stream. Again, at the surface, this is not a problem. And when you're at depth this isn't necessarily a problem. Where you run into a problem is surfacing. If you come up too fast, or miss your decompression stops which allow you to offgas the nitrogen, nitrogen can form bubbles in your blood stream. The severity of this ranges depending on how deep you are, and how long you were at depth. Scuba divers have computers and dive tables to figure these things out. If you have the nitrogen bubbles forming in your blood stream, it's called "Decompression Illness" or more commonly "The Benz". Treatment can be a range from doing nothing and letting your body to naturally offgas, to having to go into a decompression chamber which puts you back into pressure as if you were at depth and then bring you back slowly. This is of course assuming you survive to get to a hospital with a chamber. Some divers die before they hit the surface. The worst case I've heard of was where they attempted to draw blood from a diver, and the blood had so many bubbles in it ... it was a FOAM. (That diver lost his life before reaching the hospital).

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16y ago
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15y ago

A well-maintained, annually inspected, regularly used scuba tank could theoretically keep passing Federally required static pressure tests indefinately. Practically speaking, however, ten years is a good long life for a conventional, air-only tank.

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10y ago

First we need to clarify things a bit. The typical recreational scuba diver does not use oxygen as a breathing gas. Recreational divers use cylinders filled with either normal surface air or air that has had some additional oxygen added (Enriched Air Nitrox or EANx). More advanced "technical" dives are often made using customized exotic gas mixes such as Heliox or Trimix. Those dives are made by highly trained advanced technical or commercial divers. The vast majority of scuba dives are made using simple air.

Once we have established that we're talking about air rather than oxygen, we need to decide what the questioner means by "tank". Scuba tanks come in a variety of sizes, the most popular ranging between 50 and 120 cubic feet of gas at maximum fill pressure. The idea of what a typical scuba tanks is has changed over the years. At one point most divers used 72 CF steel cylinders pressurized to 2500 PSI. Today the majority of dives made in the world are made using 80 CF aluminum cylinders pressurized to 3000 PSI. This cylinder became the staple of most dive resorts beginning in the 1980s.

So... how long can you dive using an 80 cubic foot tank filled with air? Nothing is that easy. The answer depends on two things: The depth of the dive and the breathing style of the diver. Addressing the breathing style issue first: relaxed, experienced divers can consume less than half the air used by an anxious, inexperienced diver. A small framed person often uses significantly less air than a large person. There are, however, breathing rates that are considered typical for dive planning purposes.

The most import consideration is the depth of the dive. The 80 cubic feet of air we discussed earlier means 80 cubic feet of air at surface pressure. Surface pressure is approximately 14.7 PSI or 1 atmosphere (1 ATM). Since water is quite "heavy", it applies an additional 1 ATM for each 33 feet of sea water in which the diver descends. This means that, at 33 feet, our 80 cubic feet is equivalent to only 40 cubic feet at that depth. Everything else being equal, the tank delivers exactly half as many breaths of air at 33 feet as it does on the surface. At 66 feet, a diver has access to one third the volume of air from a tank as they did on the surface. At 99 feet the air delivered to the diver is 25% the volume it was on the surface.

Interestingly, the volume of oxygen actually metabolized by the diver remains unchanged throughout all of these pressure changes, but for open circuit scuba diving, that is unimportant. The important thing is that volume of air the diver needs in order to breathe is effectively four times greater at 99 feet as it is on the surface.

There are other factors besides air consumption that limit a divers dive time. Avoiding decompression illness is the primary one of these, though temperature and workload are also factors. A typical dive plan for an experienced diver might be 60 feet for 60 minutes. This is, in fact, at the edge of the maximum dive time allowed for a 60 foot dive by most no-decompression tables. Most divers get more that 60 minutes of air out of an 80 cubic foot tank at sixty feet. Those same divers might get two hours or more from the same tank on a shallow dive that doesn't exceed 30ft.

If you're looking for a simple answer, "about an hour on a sixty foot dive" is a good one.

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15y ago

99 feet under water is approximately 3 atmospheres or bars of extra pressure. That means that one breath of air at 99 feet actually contains 4 times as much air as the same breath at sea level. (1 bar for the atmosphere and 3 for the water pressure) So if you are breathing at the same rate underwater as on the surface, your tank will only last you 15 minutes at that depth.

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13y ago

That depends on a number of items such as your depth, the tank size, gasses used, how fast you're breathing, exercise, how long it'll take you to decompress, etc.

I know myself, using a standard aluminum tank starting with 3k psi air I can dive in the 30 foot range for about 30 minutes max when not exerting a lot of energy. It's not a linear scale though.

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11y ago

Not all scuba tanks are the same. They are different sizes and hold different pressures. A big tank filled to a high pressure will last longer than a smaller one filled to the same pressure or a big one is a lower pressure. You start off by multiplying the volume of air in a tank by its pressure in atmospheres or Bars. This tells you how much air you have. Once you know how much air is in your tank you need to know how big your lung volume is and how often you breathe. Often novices breathe really fast and get through their tank really quickly because they are nervous or not very relaxed. Also the harder you work by swimming fast etc, the faster you use up air. Generally we assume 12 breaths per minute.

If you knew your lung volume, breathed 12 times a minute and sat on the surface you would be able to calculate how long the cylinder would last, however when you go under water the pressure increases by one atmosphere per 33 feet. This means at 33 feet you need twice the pressure of air in your lungs to support your rig cage against the water pushing on it. This means if your cylinder lasted an hour on the surface, it would be 30 minutes at 33 feet. The deeper you go, the quicker you use up your air.

The time your cylinder will last will therefore depend on how big it is, what pressure it is filled to, how many you have, how deep you dive and how fast and hard you breathe. Generally you will also ensure that you have about a quarter of a tank left in case of emergency too. Divers estimate how much air they will need for a dive and also keep an eye on the contents gauge to make sure they don't run out.

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12y ago

Generally around 1 hour

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Q: If your scuba tank lasts 60 minutes on the surface how long will it last at 99 feet below the surface?
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