What is SMB in diving?

An SMB is a brightly coloured inflatable tube, usually 1.2–2 m long, that a diver sends to the surface on a thin line. It marks the team's position for the boat, for the shore crew and for any traffic overhead. Inflated on the surface and towed for the whole dive it is an SMB; carried rolled up and launched from depth at the start of the ascent it is a DSMB, a delayed surface marker buoy — and that is how most divers actually use one.

What is the difference between an SMB and a DSMB?

Nothing about the tube itself. The difference is when it goes up.

• SMB — inflated on the surface and towed on a reel for the entire dive, so the team is visible from the first minute. Standard practice where boat traffic is heavy.
• DSMB — stowed in a pocket and sent up from depth, normally as the ascent begins, so the boat knows where you will surface before you are there. "Delayed" describes the deployment, not the buoy.

Why send one up?

• Traffic. A diver's head in chop is almost invisible; a metre and a half of orange tube is not. This is the only thing keeping a propeller off your ascent.
• Pickup. The boat can start moving toward you while you are still hanging on the safety stop.
• A reference line. In blue water with no wall and no shot line, the taut line to the buoy gives you something to hold depth against, and the stop gets much calmer.
• Signalling. A yellow buoy is conventionally reserved for "diver has a problem"; the everyday orange one just says "divers here".

How do you inflate it?

• Orally, into a closed-bottom buoy. One breath through a non-return valve, and the buoy holds its air whatever it does at the surface. The most reliable method, and the one worth learning first.
• From a regulator's exhaust or a low-pressure hose, into an open-bottom buoy. Faster, but an open-bottom tube can dump its air if it tips over up top, and a hose means one more thing attached to you.

Either way the gas expands on the way up: a buoy you fill completely at 20 m arrives at the surface over-pressurised.

What is the safe way to deploy one?

DSMB deployment is a classic route to an uncontrolled ascent, and the mistakes are always the same ones.

• Never clip the spool or the buoy to yourself. If the line jams, the buoy takes you up with it. Hold the spool in your hand and be willing to let go — a lost buoy is cheap.
• Look up before you launch, and keep the line clear of your fins, hoses and valves.
• Let the spool run free, then take up the slack once the buoy is on the surface.
• Be neutral and stable before you start. Deployment takes both hands and your full attention, so buoyancy and trim have to be looking after themselves.

What do you need to send one up?

The buoy, a safety spool with enough line for the depth — 15 m covers recreational diving, 30 m and up if you go deeper — and a double-ender to clip the spool off when it is not in your hand.