SMB stands for surface marker buoy and technically refers to any device that would mark your position visually on the surface. A delayed SMB or DSMB is a device that is inflated underwater and sent to the surface to mark your location. You would use this to make a safe ascent, help you stay safe on the surface, and make sure you are visible on the surface. This article refers to tips for buying a DSMB.
- Open or Closed.
DSMBs split into two main categories; open-ended or sealed. An open-ended DSMB is like a bag, sealed on three sides with an open end to allow you to fill it with air, typically from your alternate air source. Using your alternate to inflate a DSMB is by far the easiest method to use and also means that the task can take place at arm’s length. This approach minimizes the risk of getting caught up which can cause an uncontrolled ascent.
The simplest version of this type of DSMB will not have any means of sealing the buoy once air has been added. If it doesn’t seal, once the buoy arrives at the surface it can easily spill air; a poorly inflated DSMB has little visibility on the surface. Spend a bit more and look for a device that has a one-way valve so spillage can’t occur. Importantly, it should have an overpressure release valve so that it doesn’t pop either.
Sealed DSMBs are filled using a valve that you inflate by blowing into it; this obviously means you need to remove your regulator from your mouth. Some can be inflated via your LPI, and yet others come with a mini scuba tank which you can use to fill it. Sealed DSMBs should also have an overpressure release valve. - Size and colour.
Most DSMBs are tall narrow tubes of various lengths. Although a smaller DSMB is going to be less bulky, it will offer little visibility over even the smallest wave height. Further, if current were to sweep you away then the larger your DSMB, the greater distance you can be seen from. Some DSMB have a reflective strip on them which again increases visibility. DSMBs are orange. You will notice that it is possible to purchase them in yellow. Typically yellow DSMBs are secondary devices used by decompression divers to signal that they require emergency assistance. - Reel
Many DSMS come with a string, strap or lanyard which is 5m long but the conditions in which this would be effective are rare, and the chance that it doesn’t get tangled is infinitely less likely. If you deploy your DSMB from 5m depth in perfectly still waters, then your buoy will rise directly to the surface. More likely is that current, surface wind or chop will pull at the line which will mean you will need more than 5m of line to get your DSMB to the surface.
A reel safely stores more line and reduces the risk of tangles. Reels come with many options but do remember that the less moving parts something has, the less chance there is of something going wrong. A finger reel is a simple spool from which you can deploy a DSMB; these can spin on an axis created with your finger, or finger and thumb or can be released to unspool. They are weighed so that they should spin where you release them.
Further consideration
It is far easier to deploy your DSMB from deeper than 5m as you need less air to send it on its way. It’s also simpler to control your buoyancy when you are deeper, so sending your DSMB up before you ascend to shallower water to make your safety stop makes sense. This method also means you have a visual reference to assist your ascent and that your position is marked before you are in shallow water.
