Oceanic Alpha scuba diving regulator, $20.

I'm always on the lookout for scuba diving equipment, especially the regulators, whether new, old or anywhere in between. Scuba regulators confuse many people because they don't even know what they are, much less how to tell if they're worth bothering with. But used diving regulators sell very well, so it pays to learn more about them and what they do!

Although the early 'dual hose' style regulator (see the Gallery entry on the Cousteau Gagnan regulator) looks drastically different from the one in our example here, all contemporary scuba regulators you're likely to find will have the same look to them as this one, and will be made up of a combination of the following components: a first stage regulator, one (sometimes two) second stage regulator, a tank pressure gauge, which often includes a depth gauge and a compass, and hoses that connect everything together. There's also a hose that connects to the buoyancy compensator vest, so that it can be inflated from tank air.

The air pressure in a modern scuba tank is around 3,000 p.s.i., which is obviously way too much pressure to work with. To reduce this incredible pressure to something more manageable, a first stage regulator is screwed to the top of the air tank, bringing the pressure down to around 150 p.s.i. This lower pressure air is then fed out to the the second stage regulator, as well as to the hose used to inflate the buoyancy compensator vest. The hose that goes to the pressure gauge naturally bypasses the first stage regulator so that it can read actual tank pressure. Note that the hoses on our regulator are all rated at 250 p.s.i., except for the high-pressure hose that goes to the gauge, which is rated at 5000 p.s.i.

The purpose of the second stage regulator is to reduce the air pressure from 150 p.s.i. down to 'on demand' pressure, so that all the diver has to do is inhale slightly to cause a flow of air into his lungs. This doesn't seem like much of a trick, until you consider that the second stage regulator has to maintain this delicate pressure balance over a very wide range of atmospheric pressures, from the surface all the way down to many atmospheres of pressure underwater.

Unless you have a full scuba tank around, it's pretty difficult to actually test a regulator's functions, such as making sure that the secondary regulators don't leak and that the tank pressure gauge works. And even if you do have an air tank, and can test these various functions, there's no ironclad guarantee that the rig will work in the water. So in most cases, the only testing you'll be expected to do is to give everything a good visual inspection, and then tell your bidders that you can't test it any more thoroughly.

Actually, your bidders should understand completely why you can't guarantee a regulator you're selling, since only a certified dive shop should be allowed to certify a regulator's performance. And that's just what you tell your bidders: although this regulator looks fine to me, you should take it to a dive shop and have it checked out before using it.

So, as far as your visual inspection goes, you'll be primarily looking for breaks, cracks and other signs of abuse, as well as rust or corrosion from the user failing to rinse the regulator off after use in salt water. I normally avoid regulators that exhibit more than the minimum amount of corrosion. Also check for stiff or cracked hoses, mainly caused by age.

In the first photo of the regulator, notice the little black plug that is screwed into place over the air inlet hole. The purpose of this cap is to keep dust and dirt from getting into the regulator when it's not connected to a tank. If you peer into the first stage air inlet, you should see a sintered metal filter, ideally showing no signs of dirt or corrosion. Note that to keep dirt and dust out of the air fitting on the tank itself, a little vinyl cap is fitted over it when the regulator isn't mounted.

A final hint. You can't go into a dive shop and have a tank filled unless you're a licensed diver. However, Used tanks that are still full of air are relatively common at yard sales. If you pick up one of these tanks, you can use it to test the regulators you find.

This scuba regulator sold on eBay for $217.

Photo of Oceanic Alpha scuba diving regulator