SMB or DSMB is short for Surface Marker Buoy or Delayed Surface Marker Buoy.
Searching around the internet I found a lot of different opinions on this subject. All agree that it’s a surface marker, and that DSMB is deployed underwater. But an SMB can either be a big round buoy dragged in a line during the whole dive or a long inflatable tube used by divers to get visual attention. Here's Safer Diving: Situational Awareness. SMB’s are also often used to mark your position on the safety stop. You deploy your inflated tube from 5 meters so boats don’t pass above you on your safety stop of ascent.
SMB – Surface Marker Buoy
This device can apparently have many forms. The first is a long bright collared tube deployed in the surface after a dive. This is done to get the attention of the boat crew and to let other boats know you are there. This device is especially good to get attention if lost at sea or when an accident happens. Do you know The 4 Things To Bring So You Don’t Get Left Behind? Another shape of an SMB is small and round or typical buoy shaped, this is attached to a line and is dragged during the whole dive. This makes the diver visible to boats and others, during the entire dive. This way the boat crew can easily follow the divers.
DSMB – Delayed Surface Marker Buoy
This is also a bright collared long tube, but a DSMB is deployed underwater just before ascent. The DSMB has an over inflation valve, much like the one on your BCD shoulder, which releases air as it expands on its way to the surface. This way boats don’t sail over the divers on the way to the surface. The diver holds on to the DSMB with a line. Learn more How to Safely and Effectively Launch a DSMB. The length of the line varies from 5 meters and up. This way you will always be able to deploy it doing your safety stop. It is a good idea to get the line of a DSMB attached to a Spool or a Reel, this way you won’t risk the line getting entangled.
Sizes of a DSMB
The length of an SMD and DSMB varies quite a lot from less than one meter to more than 2.5 meters. Knowing this you need to choose a (D)SMB that you can use. You need one that is long enough to be seen from a distance, but at the same time it can’t take up too much space when rolled together for storage in the BCD pocket. Here's Stay Safe – 5 pieces of equipment to improve your safety while diving.
Choosing a DSMB
When it's time to buy a DSMB or any surface marker you'd like. It's important that it's robust and will handle to be deployed from a depth. So, make sure there's a vale to let out excess air as the marker shoots to the surface. And look at the stitches at the side, that's usually the weekst point.
Also, make sure you have a long line or reel, so you don't lose it.
I've used this one from Dive Smart, it's a fitting package for any diver.
Deploying a safety sausage
When you are on the surface and want to deploy the SMB, simply roll it out and blow it using your alternate air source. Deploying a DSMB is a bit trickier, but still quite easy. You roll out the DMSB and free it from all equipment so it has free passage to the surface. Using your alternate air source you fill up the DSMB, holding on to the Spool, reel or line, whatever you use to hold on to it when it’s at the surface.
It’s very important that you never ever attach the reel or line to your BCD when deploying a DSMB. If the line is entangled and stops in the reel, you will fly with the DSMB to the surface risking Decompression sickness A visual signaling device makes you visible from a distance, to be safe always bring it on your dives. Better safe than sorry.