An OX500 or OX500Plus mini scuba tank gives you up to 8 minutes of air, an OX1000 about 15 minutes. These figures are ceilings. As our comparison of the three models explains, you get them near the surface, breathing calmly. As soon as you go deeper, work harder or get cold, the tank empties faster.
Here is why, and how to estimate your own time underwater while keeping a safety margin.
A tank holds air, not minutes
All three OXSEA models have the same working pressure: 200 bar. What sets them apart is volume: 0.5 litres for the OX500 and the OX500Plus, 1 litre for the OX1000. At the same pressure, the OX1000 therefore holds twice as much air.
How long that air lasts then depends on how fast you breathe it. The stated minutes assume calm breathing near the surface. Your own consumption may well be higher.
What physics says: pressure and Boyle’s law
At the surface, you are under the pressure of the atmosphere, about 1 bar. Underwater, the weight of the water adds to it: 1 bar more for every 10 metres. The total pressure, called absolute pressure, is therefore 1.5 bar at 5 metres and 2 bar at 10 metres. It goes up by 1 bar for every 10 metres; it does not double at each step.
Boyle’s law states that at constant temperature, the volume of a gas is inversely proportional to the pressure on it (P × V = constant). Your regulator delivers air at ambient pressure. At 10 metres, each breath therefore draws twice as much air from the tank as at the surface. A physics course from the French underwater federation (FFESSM) gives the example of a diver who uses 20 litres a minute at the surface: the same diver uses 40 at 10 metres.
With a mini dive tank, you feel the effect from the very first metres:
| Depth | Absolute pressure | Air drawn with each breath | Share of surface air time |
|---|---|---|---|
| Surface | 1 bar | baseline | 100% |
| 1 m | 1.1 bar | +10% | about 91% |
| 2 m | 1.2 bar | +20% | about 83% |
| 3 m | 1.3 bar | +30% | about 77% |
| 5 m | 1.5 bar | +50% | about 67% |
This table is a theoretical calculation at a constant breathing rate. Effort, cold and stress then add to the effect of depth.
What cuts your air time
Effort. Finning against the current or clearing a propeller takes effort, and effort speeds up your breathing. Divers Alert Network (DAN), an organisation specialising in dive safety, says some physical activities underwater can use air five to ten times faster than usual. Poorly adjusted weights also make you work harder, which increases consumption.
Cold. Water conducts heat 20 to 27 times faster than air, DAN Europe notes: you lose heat quickly. An FFESSM training document on cold in diving lists higher air consumption among its effects, and points out that breathing cool compressed air also chills you. A wetsuit suited to the water temperature therefore protects your air time.
Stress. Using the tank for the first time, or murky water, can make you nervous, and anxiety speeds up breathing. DAN advises keeping a closer eye on your air at those moments. Make your first attempts in calm water where you can stand.
How you breathe. Everyone’s air consumption is different, and it varies from one day to the next. Breathe calmly and steadily, and never hold your breath to save air: that is a real risk on the way up (see below).
The starting fill. Going by P × V, a tank filled to 100 bar holds about half as much air as the same tank at 200 bar. If the refill did not go all the way, your air time drops in the same proportion.
Estimating your real time underwater
Two methods, best used together.
1. Working it out from depth. Start from the stated air time, which applies near the surface with calm breathing. Divide it by the absolute pressure at the depth where you will stay, that is 1 + (depth in metres ÷ 10). Example: with an OX500 at 2 metres, 8 ÷ 1.2 gives about 6 minutes 40 seconds. That is still a ceiling. Subtract your reserve (see below), and expect less if you are working hard or the water is cool.
2. Measuring your own consumption. In calm, shallow water, note the starting pressure on the gauge, then read it again after 2 minutes of normal activity. An example with hypothetical figures: if the needle goes from 200 to 150 bar in 2 minutes, you are using 25 bar per minute. On a full tank at 200 bar, keeping 50 bar in reserve, you have 150 bar available, or about 6 minutes at that rate. Repeat the measurement in your real conditions, with a real task to do: that is the figure that counts.
Reading the gauge and keeping a reserve
The pressure gauge shows the pressure left in the tank, and so how much air you have left. The OX500 has a built-in gauge, and the OX500Plus comes with an external one.
- Before you get in the water, check that the needle shows a full tank.
- While you use the tank, check the gauge often. According to DAN, running out of air is the most common trigger of diving accidents, and DAN advises checking your gauge regularly.
- Before you get in, decide when you will come up, and stick to it.
For the reserve, an FFESSM physics course calculates the air time of a full-size tank filled to 200 bar while keeping a 50 bar reserve, a quarter of the starting pressure. We advise applying the same logic to a mini scuba tank: when the needle gets close to 50 bar, come up calmly. The ascent takes air, and so does anything unexpected (a mask that floods, a line that snags).
Safety: keep breathing, stay near the surface
Boyle’s law also works the other way. As you rise, the pressure drops and the air in your lungs expands. If you hold your breath, that extra volume cannot escape and can injure your lungs: this is lung overexpansion injury. The MSD Manual states that a gas embolism can occur from as shallow as about 1 metre in someone breathing compressed air who comes up quickly while holding their breath.
Shallow water is therefore no protection. DAN Europe gives an example: in a sudden ascent from 5 metres to the surface, gas volume would increase by 50%, compared with 20% for an ascent over the same distance from 20 metres. Over the same distance, changes in volume are greater near the surface, which is exactly where you use a mini scuba tank.
The rules to remember:
- never hold your breath, especially on the way up;
- come up slowly, breathing normally;
- stay shallow;
- take an introductory session with a qualified instructor before you first use the tank: training is recommended, even for occasional use.
Our video tutorials help you get to grips with the equipment. They complement training but do not replace it.
Which model for the time you need
The OX500 (€199.99) suits short jobs: up to 8 minutes, held in your mouth with its silicone mouthpiece, under 1 kg underwater. It refills with the manual pump in about 20 minutes, without electricity.
The OX500Plus (€289.99) offers the same maximum air time. You can hold it in your mouth, or wear it on your back with a harness and an external regulator. Its external gauge is included.
The OX1000 (€299.99) holds twice as much air: about 15 minutes, worn on your back with harness and external regulator. It refills with the mini compressor (about 20 minutes) or with a transfer station from a full-size scuba tank (about 1 minute). The manual pump is not compatible: if you have no access to a full-size tank, plan on the 12V mini compressor, which runs off a car or boat battery as well as mains power.
To see the whole range, visit our mini scuba tank shop. If you have a specific question about your own use, see our support and FAQ page.
Frequently asked questions
How long does a mini scuba tank last?
Up to 8 minutes for the OX500 and the OX500Plus (0.5 L), about 15 minutes for the OX1000 (1 L). These times apply near the surface, breathing calmly. Deeper down, when you work hard or in cold water, expect less.
Is air time the same at 1 metre and at 3 metres?
No. Absolute pressure goes from 1.1 to 1.3 bar. At 3 metres, each breath draws about 18% more air than at 1 metre, and the tank lasts about 15% less, for the same effort.
Why did my tank run out faster than expected?
Several causes often add up: sustained effort, cold water, the stress of a first use, more depth than planned, or a tank that was not full to start with. Check the pressure every time before you get in the water.
Can I make the air last longer by holding my breath?
No, and it is dangerous. With compressed air, holding your breath on the way up exposes you to lung overexpansion injury, even in shallow water. Breathe normally, and come up when the gauge gets close to your reserve.