OX500 mini scuba tank: the half-liter tank you hold in your mouth

The OX500 is the smallest tank OXSEA makes, and the one most people start with. It is an aluminum cylinder with a silicone mouthpiece on top and a pressure gauge built into the head. You bite the mouthpiece, breathe normally and go under. There is no harness to adjust, no hose to route and no buoyancy jacket. It lives in a locker on the boat until the day you need it.

What it is for

The OX500 is a tool for short jobs right under the boat, the kind that used to mean calling a diver or waiting for the next haul-out.

• A line around the prop. Crab pot and lobster pot lines, a stray dock line, a tow rope pulled in by the wash. You go down, see what you are dealing with, cut it free and come back up. Our guide on clearing a rope from a propeller walks through the job.
• The zincs. Shaft, trim tab and outdrive zincs wear away, and it is hard to judge them from the dock. A look underwater tells you whether they can wait or need replacing. More on that in how often to change boat anodes.
• A fouled anchor. When the anchor has dug in under a rock or a ledge and will not break out, a minute at the bottom beats cutting the rode.
• Through-hulls and the speed paddle wheel. Weed or a plastic bag over a raw-water intake, or growth on the paddle wheel, clears in seconds from the outside.
• First breaths underwater. In a pool or in water you can stand up in, before you ever use it under a hull.

How long the air lasts

A 0.5 liter tank at 200 bar, about 2,900 psi, holds roughly 100 liters of air once it expands. That is around 3.5 cubic feet. A standard aluminum 80 dive cylinder holds about 80 cubic feet, which gives you an idea of the difference: the OX500 is made for a few minutes, not for a dive.

Near the surface and breathing calmly, it gives up to 8 minutes. Hard work, cold water and stress make you breathe faster, so a job like scraping or cutting rope uses it up sooner. Set yourself a time to head back up well before the maximum and keep an eye on the gauge.

Refilling it

The tank ships empty. You fill it before the first use, then after every dive, with one of two systems:

• The hand pump, about 20 minutes of pumping per fill, with no battery and no outlet. It suits the dock, the deck of a sailboat or the backyard. See the hand pump.
• The 12 V / 220 V compressor, about 12 minutes per fill, from the boat's house battery, a car battery or the mains. See the 12 V compressor.

Both packs on oxsea.shop put the tank and its refill system in one box, so you can dive and refill without buying anything else.

What it is not made for

• Long jobs with both hands busy. You hold the OX500 in your mouth, and often with one hand. If you plan to scrape a hull or change zincs with tools in both hands, the OX500Plus carries the same tank on a harness, with the regulator on a hose.
• Real dives. It does not replace scuba gear or scuba training. It is made for short stays at shallow depth, close to the surface and close to the boat.
• Going deep. Pressure changes fastest near the surface, which is where you will be working anyway. Stay shallow.

The rules that do not change

Breathe continuously, never hold your breath on the way up, and ascend slowly. Before anyone goes under a boat, the engine is off and the keys are out. Fly a dive flag: in Florida, the law counts you as a diver the moment you are in the water with a mask, and other states have their own distances. Our guide to dive flag rules in the US covers them, and the safety guide on oxsea.shop goes through the rest.

OX500 or a bigger tank?

If you mostly...	Look at
Check the prop, the zincs or the anchor, a few minutes at a time	OX500
Need both hands free, or want to wear the tank on your back	OX500Plus
Run out of air before the job is done	OX1000
Work under a big boat or swim a reef for 20 minutes or more	OX2000

The side-by-side comparison puts all four tanks in one table.