While typing the lengthy response to Keith's 'Describe your equipment' I realised that using a DSMB is quite an involved process which deserved an equaly lengthy post. What makes things worse is that it is a procedure many UK divers do routinely and can (does) lead to major incidents. Maybe some discussion on techniques could help us all. Most of this is intended for wreck dives in the 30m range.

I came up with some reasons why this discussion could be useful;

1. Every year a number of divers are involved in incidents where deploying a DSMB has resulted in a rapid ascent.

2. Deploying a DSMB is not covered by the mainstream agencies I am familiar with (BSAC, PADI).

3. Decompression diving is approached as much more routine than it used to be. To support this, how many boats do you use that hang sets in the water, provide deco bars, seperate down/up lines and deco lines etc. How long do you spend planning your 30m dive? a couple of hours or a couple of days?

Why use a delayed SMB?

In a perfect world we would all find our way back to the shot line and not need a DSMB. However there are too many factors that prevent this. Poor visibility at the end of the dive often means that you can be less than a meter from the shot line but still not be able to see it. 10 divers doing stops on the shot line can lead to the line being pulled up and down, this is not ideal for doing your stops. A short tidal window may mean the tide has turned close to the end of the dive and swimming back to the shot will involve using a lot more air for the return. If you do make your stop on the shot in a current you will be blown about like a flag (not very comfortable and causes chilling). By making the decision that you will not regain the shot you remove the complication of having to make decisions at depth. Finally and importantly your DSMB provides a reference mark for the boat cover. So if you are going to use a DSMB you need to know about the different types and their applications.

Emergency Decompression Marker Bouy:

These are commonly open ended, provide limited bouyancy, and are designed to be deployed from a depth of 15m or less. A weighted line is dropped and the open ended tube inflated. The diver then does the stop drifting free under the marker. The biggest problem with these is making the ascent to 15m. OK if you come up a shot then fall off at your stop depth and send up the marker they work, but if you did not get to the shot you have to do an ascent with no surface reference. I have also seen problems with maintaining depth while deploying the marker. Finaly if the marker topples over all the air empties and the mark descends again.

Delayed Surface Marker Bouy:

These come in a variey of forms but generally include the following. Closed or self sealing end so they can not lose air once inflated. Over pressure valve to allow excess air to vent when the bouy ascends. Provides adequate bouyancy for two divers to hang off. Reel attachment strap. In use the bouy is attached to a reel with adequate line (1 1/2 times depth), inflated at depth and ascends to the surface. Divers can then use the line as an ascent reference (route to surface). At stop depth divers may hang on the line while completeing stops. The biggest problem with these is deploying them safely from depth. The bouy does not have to drag you up very far before expansion leads to an uncontrolled ascent. There is also the chance that the marker will hit a diver on the way up or get caught in a boat prop.

Why send from depth?

There seem to be 2 distinct camps in the DSMB arena those that deploy from the bottom and those that deploy from the shallows. I am firmly in the former camp. My reasons. Although I have done it in the past, making an ascent from 30m without a reference (except your depth gauge) is very difficult. It is too easy to find the ascent running away, or dumping air find yourself descending again. Also there is the question of how far off the wreck I have drifted while I ascend to 15m, will the boat lose me? A diver ascending is a bit (but not much) like a pilot landing, it is the most hazourdous part of the dive. I would like as few complications as possible. If I can safely deploy the bouy I get all the advantages of a shot line. A fixed reference with the surface. All I have to do is reel in and I will surface. I can stay a little negative and so bouyancy control is eased especially in a strong or rising current. My buddy can also ascend the line and will not drift off. Counting the frequency of the clicks as I reel in helps with ascent rate control. All this works for me during the ascent phase so I will carry on sending from the bottom.

OK I said safely deploy the bouy. Here is the technique I use. Before I go diving I reel off the amount of line required and reel the line back on evenly. I think this is essential to gaurd against the line tangling when you send the bouy up. I take a little time ensuring that the bouy is folded (rolled up) correctly attached to the line on the reel (using a shackle) and firmly secured to the handle of the reel using a velcro strap left over from a strobe. The strap is tied to the handle of the reel by a piece of yellow cord. The free end of the yellow cord goes under the end of the velcro so that I can find it in the dark. (Using a bit of bungee cord may be less complicated!) The package of reel and DSMB has no slack line or straps hanging around to get caught up. Prior to diving the reel is attached to a ring on the left shoulder strap of my stab. The lanyard is doubled back so the thing don't drift about. I have tried placing the reel elswhere but keep coming back to this as being the most convenient and least likely to get caught up.

Prior to ascent. I know it takes 2 minutes to deploy the bouy (I practiced it to death on shallower dives, starting in 6m, and constantly monitor how long I take). So 5 minutes before the end of the dive I start looking for a location to deploy the bouy from. There is no golden rule about where you send the bouy from but consider. Is there somewhere to tie the reel to while I send up the bouy. What is above me. Am I likely to hit another diver or a boat. Where is the current running, am I going to drift into the wreck as I start to ascend. If I find a good location before the end of our planned time, well we cut it short a couple of minutes.

My first choice of location is to swim off the wreck down tide 20 meters or so. This is not always possible (if it is a large wreck it could mean redescending to the sand, not an option). Next choice is the highest point of the wreck, or the bow, or the stern, or the side that is down tide. I hate having to send the bouy from the middle of the wreck or from the uptide side.

Having got a location I look up. Can I see green water above (or a greener shade of black). Pointing your torch down between your knees can help. Quite often I will turn the torch off as it stops me getting blinded while I am deploying the bouy. Next dump all my air from my suit so I am negative. Try and find something to wedge my feet under. Next unclip the reel from my stab ALWAYS. Find the end of the velcro and unroll the bouy. Face down tide so that any loose line is blown away from your body. Make sure your buddy is uptide (if he is feeling brave he may be holding on to you). Let out about 2' of line. If it is practical I drop the reel so that the line is kept tight. Check there is nothing to get caught up. Check again there is nothing to get caught up. Hold the end of the bouy open, and above the exhaust of my regulator. Breathe out twice (you can hear the air going in). Even with only half the exhaust of each breath going in to the bouy two breaths will be adequate and expansion ensures the bouy is fully inflated at the surface. At this point at 30m the bouy will not be rocketing to the surface you will be able to hold it down without much trouble. Keeping the bouy held low, take the reel and hold down tide at arms length. Check where the slack line is going. Press the trigger. check that the winder of the reel is not going to get caught, let go of the bouy. If your arm starts to get pulled upwards let go, do not think about it just do it. Watch the amount of line left on the reel, it is possible that a current will cause the reel to run out of line before the bouy breaks the surface.

Some of the more attentive readers may have noticed that my buddy has little involvement with all of this.

Once the bouy is up and the reel stops winding I regain neutral bouyancy (well almost, I tend to keep a little negative) then get my buddy to put his hand around the line and use it to keep contact during the ascent. He is not supposed to grab onto it but has the option. If we are in current or I am feeling tired I will clip the bouy back on. What, clip your bouy on during the ascent, thats dangerous! OK I have thought long and hard about this, the reason people tell me not to clip it on is that it may get pulled up by a boat. OK that may happen but at least I am on the surface, and how many times does that scenario feature in the incident reports (not many). On the other hand if I let go of the reel and lose it I am likely to end up back on the sea bed, or drifting without a mark, or ascending with no point of reference (read fast ascent). Now redescent and boat seperation do account for quite a few incidents each year. I do have a fastex clip on the lanyard which might allow me to ditch the reel before I became uncontrollably bouyant in the case of a boat pulling me, however I find this much more useful for detaching the reel when getting back into the dive boat. At the end of the day I consider my DSMB line to be my route to the surface, all I have to do is reel in and I will get back to safety, if there is the possibility of me dropping the reel there is the possibility of me losing my route to safety. Consider it and decide for yourself.

When starting to ascend I check with my buddy and then we let go of the wreck. Initially I wind the reel fairly quickly. With a lot of line out the diameter of the drum is reduced so you have to wind faster to recover the line. Also I am always keen to get above 20m it decreases our shallow deco time (on Cochrane and Aladin computers). Finaly in a current if you do not get the line in fast you are in danger of penduluming back to depth when you let go of the wreck. If the bouy was sent from uptide it is also handy to get above the wreck before you drift back into it.

OK so we are into the ascent and the reeling in has got less frantic. I use the frequency of clicks to indicate the ascent rate. My computer is on my right wrist and I can read it without having to let go of the reel. If I start to ascend too quickly (variable ascent rate on the computer) I just dip below the reel and air vents from the cuff dump. The line is marked at 9m with three black bands of permanent marker, 6m with 2 bands and you guessed it 3m with 1 band. The reason for the bands is so that I can recognise the depth without having to see any of the other marks. Stops is straight forward, clip on, dump air, hang about.

There are some parts of this that do not require discussion. This is a hazardous procedure and should be carried out carefully every time.

NEVER clip your reel on while deploying a DSMB.

Do not hesitate in dropping the reel if it gets jammed.

Make sure the equipment you use is suitable. Penetration reels are not for attaching to DSMBs.

Always make sure you have practiced in the shallows and you are completely capable and confident of the technique before using it at depth.

Deploying a DSMB is an advanced skill for suitably trained and qualified divers, blah blah. Don't blame me etc.

Do not take anything I write as gospel I am but a mere diving mortal with an internet connection.

Comments please, maybe we can make an ultimately unsafe procedure a bit safer.

Regards

Matt Stevens