Scuba tank: mini or full-size, which one do you need?

It comes down to what you want to do underwater. To explore a reef or a wreck on a real dive, you need a full-size scuba tank of 10 to 15 liters, along with training and a dive shop to fill it. For a short job near the surface, under a hull or at anchor, a small scuba tank of 0.5 to 2.3 liters is enough: up to 8 minutes of air from 0.5 liters, 20 to 25 minutes from the largest, and you refill it yourself, at home or on board. One does not replace the other.

Both hold compressed air and both can be refilled. They differ in how much air they carry, how deep they belong, how you fill them and what training they call for.

Full-size scuba tanks: sizes and pressures

The tank a dive center hands you usually has an internal volume of 10 to 15 liters (US tanks are often named for their air capacity in cubic feet). Depending on the model, it is generally filled to 200 or 232 bar, about 2,900 to 3,400 psi. It rides on your back, strapped to a buoyancy compensator (BC), with a regulator and weights.

Air follows pressure times volume: in theory, a 12 liter tank at 200 bar holds about 2,400 liters of air at surface pressure, roughly 85 cubic feet. Enough for a full dive, which gets shorter the deeper you go, since each breath then draws more from the tank.

A full-size tank comes after training. An entry-level course such as the FFESSM's Level 1 (in French), recognized internationally as a CMAS one-star certification, is taught at a club, requires a medical certificate and then lets you dive to 20 meters (66 feet) under supervision. Dive shops set their own rules, and most ask to see a certification card before they rent you a tank or fill one.

Small scuba tanks: 0.5 to 2.3 liters, near the surface

A refillable mini scuba tank, often called a small or portable scuba tank, carries far less air, for a different job. At OXSEA, the OX500 and OX500Plus hold 0.5 liters: the first is held in the mouth with a silicone mouthpiece, the second can be held in the mouth or worn on the back. The OX1000 (1 liter) and the OX2000 (2.3 liters) are worn on the back. The OX500, OX500Plus and OX1000 are filled to 200 bar, about 2,900 psi.

By the same math, a full OX500 holds about 100 liters of air and an OX1000 about 200. That gives you up to 8 minutes from 0.5 liters, about 15 minutes from the OX1000 and 20 to 25 minutes from the OX2000, near the surface with calm breathing. Depth, effort and cold all cut those times.

It is the tool for short jobs in shallow water: cutting a line off the prop, checking a zinc, freeing an anchor in a shallow anchorage, recovering something dropped overboard. It also works for snorkeling, or in a pool to get used to breathing underwater. No certification is required to buy one, but an introductory session is still recommended. Our main guide explains how a mini scuba tank works.

Full-size or mini scuba tank: the comparison table

Refilling: a fill station or your own gear

Every scuba tank can be refilled. What changes is where.

A full-size tank is filled at a dive shop or club, on a high-pressure compressor, provided its inspections are current. DAN, the reference organization for diver safety, warns in its Alert Diver magazine that a fill station may turn away a cylinder that is out of date, and advises using only authorized fill stations that test their air regularly.

A mini tank is refilled wherever you are.

• Manual pump: about 20 minutes for 0.5 liters, no power needed. It fills the OX500 and OX500Plus, not the OX1000 or OX2000.
• 12 V / 220 V compressor: on a 12 V boat or car battery, or on 220 V mains power, about 12 minutes for 0.5 liters and 20 minutes for 1 liter. The OX2000 takes longer.
• Transfer from a full-size tank: a DIN or yoke transfer station fills the mini tank from a full scuba tank, in about a minute for the OX1000.

The air you compress is the air you will breathe: refill outdoors, away from any engine or generator exhaust, and replace the filters, which wear out. Our guide on how to refill a mini scuba tank compares the three methods, with times and budgets.

Weight and bulk

A 12 liter tank weighs more than 10 kg (22 lb) on its own. Add the BC, the regulator and the weights, and the kit fills a dive bag.

A mini tank fits in a boat locker: the OX500 weighs 1.55 kg (3.4 lb) out of the water, the OX1000 3.15 kg (6.9 lb). The pump or compressor goes wherever the tank goes, on board or ashore, so you no longer depend on a fill station.

Care and inspections

The basics are the same for both. After a day at sea, rinse the tank and regulator with fresh water, let them dry in the shade, then store them away from heat and knocks. Before every fill, look for dents, corrosion, a damaged thread or a split seal.

A full-size tank also needs periodic professional inspections (visual and hydrostatic), on a schedule set by each country's rules. For a mini tank, we advise a periodic check by a professional, as often as your use calls for. Seals, the safety disc and the filters wear and get replaced: the OXSEA School videos show each step.

Full-size or mini: it depends on the dive

A full-size tank is the right call if you want to explore the bottom and stay down for a long time. You work your way up through certification levels and plan each dive. For going deep, no mini tank replaces it.

A mini tank makes sense when the work is near the surface and short. It also helps when no dive shop is nearby, since you refill it yourself. Already a certified diver? You can fill it from your own tank and keep it on board for quick jobs.

Which mini scuba tank for which job

• A look at the prop, something to fish out, a boat with no power: the OX500, held in the mouth, and the manual pump, sold together in the OX500 and manual pump pack.
• A line to cut, both hands busy: the OX500Plus, worn on the back, sold in packs with the pump or the compressor.
• A lap around the hull, a zinc to check: the OX1000 and its quarter hour or so of air, in the OX1000 and 12 V / 220 V compressor pack.
• Sessions longer than a quarter hour: the OX2000 and its 2.3 liters, sold with the compressor in the OX2000 pack.
• Learning to breathe underwater in a pool: 0.5 liters is enough.

The comparison of the four mini tanks sets them side by side, criterion by criterion, and each product page shows the pack price. Tanks and refill systems come with a 24-month warranty and 30 days to change your mind.

Safety rules, whatever the tank

A mini tank holds the same compressed air as a full-size one. The basic rules are the same, from the very first meter.

• Never hold your breath on the way up. The air in your lungs expands as the pressure drops. According to DAN's Alert Diver, an ascent of about 1.5 meters (5 feet) with your breath held can be enough to cause an arterial gas embolism, and a lung overexpansion injury can happen even in a swimming pool.
• Stay close to the surface. A mini tank is made for shallow water, not for going deep.
• Never dive alone. In diving the basic unit is the pair, as France's ministry for the sea (in French) puts it. Keep someone at the surface watching you.
• Head up while there is still air. Watch the gauge and decide to surface well before your last breath.

The mini scuba tank safety guide goes through these rules one by one, along with the medical contraindications.