Decompression Ascents - Some Secret Sauce

Let me start this month's blog with a couple of caveats; this post is predominantly for open circuit (OC) divers. If you dive a "breather" (Closed Circuit Rebreather or CCR), then some of the techniques I will explain involving breathing control won't work for you. Equally, if you have not mastered "relaxed static stability" then, with all the will in the world, you are not going to be able to complete a safe, controlled ascent as I will describe in this post. Take a look at some of my other blog posts on buoyancy control, stability, and trim, and master these skills first.

If you are still with me, then I assume that you are stable in the water, can hold trim and are diving open circuit or if on CCR, maybe you are thinking "what happens when I bail out to OC during an ascent?" What follows is some of the "secret sauce" that I have learnt and derived over the past few years of conducting decompression dives and guiding/instructing or acting as safety diver on such dives. If you employ and master these techniques you should be able to safely complete a team decompression ascent, "in the blue", meaning, with no visual reference other than your team-mates.

Secret Sauce 1 - Preparation

It is a well-worn adage but nonetheless true, that your preparation will determine your execution. Here, I am not referring to dive planning and your decompression profile even though this is a vital step. Nor am I referring to equipment or gas preparation, another key ingredient. I am referring to rehearsing your dive and ascent profile with your team. Rehearsal ensures that everyone in the team knows how the ascent will run, what will happen, when, and the role of each team member in safely executing the ascent. 

To perform your rehearsal, find a clear space on dry land and stand as a team in the formation you expect during your ascent. Generally this will be facing each other but in some circumstances such as currents this may occur on a line, all facing into the current. Maybe you practice both formations. Now run through the ascent, rehearsing communication, each stop, gas switches, and so on. At each step iron out any miscommunication or uncertainties.  Discuss contingency plans such as breaking depth, extended bottom time, lost gas and so on. At the end of your rehearsal, each team member should be clear and ready to perform the dive. Note, this aspect may also apply to tasks that are to be completed during the bottom phase of the dive.

 Sidemount divers performing surface equipment matching and checks before a decompression dive 

Sidemount divers performing surface equipment matching and checks before a decompression dive 

SECRET SAUCE 2 - Communication

Communication is key to good team execution. If team members do not understand each other or misread signals then problems, potentially serious, will result. Agree on communications before the dive and practice these visually during your rehearsal. I have seen ascent and deco signals done a few ways and the specifics are not as important as the team all using the same signals. The format I use employs Tech Diving numerical hand signals and goes as follows:

  • -1 minute - Deco Captain signals 1 minute to go on runtime. Divers prepare for their ascents and take up team formation at the agreed ascent point e.g. line.
  • Ascend time - Deco Captain signals; ascend, the new depth, and the deco time for the new depth. As prearranged, the team repeat the signals back or give an OK, and the team ascend.
  • Gas switches - Deco Captain signals gas switch. As prearranged, each team member performs their switch protocol in turn while another team member checks and OKs each step. If in a team of three, this can be done in rotation, with one team member switching, one checking and the third team member holding depth and providing a visual reference for the other divers. Only when all divers have performed their switches should gases be changed on the diver's computers.

SECRET SAUCE 3 - Personal Execution

You cannot be an effective team member if your own dive skills are not up to the required level. Firstly, you need to be able to execute your ascent at the correct ascent rate and stop at the correct depth. Decompression tables assume specific ascent rates usually 9-10m/min. Ascend faster than this rate and you run the risk of bubble formation and DCS. Slower, and you are on-gassing more than you planned, meaning longer decompression and eating into your gas contingency.

To begin your ascent, the goal is to become positively buoyant without becoming too much so. You have a few options to achieve this, you might start with a few upward fin cycles which get you started until positive buoyancy results from expansion of gas in your BCD and drysuit or expansion of your wetsuit. If in a drysuit, open your exhaust valve so that you can easily vent gas from the suit as it expands. Another option to start your ascent, is to use breathing control. To do this, breathe in deeply, expanding your lung volume and breathe out as you begin to ascend. Then take another deep breath to continue. Note: I am not for a moment suggesting breath holding here. Just deeper breathing and holding a greater lung volume to increase your buoyancy and promote your ascent.

As you ascend, your buoyancy will increase, accelerating your ascent rate. Most computers will visually display your ascent rate so monitor your rate adjusting your buoyancy via your breathing control and/or dumping gas from your BCD and/or drysuit. Use breathing control in the first instance by breathing either more deeply or more shallow depending on whether you wish to accelerate or slow your ascent. When you dump gas from your BCD, do so in short bursts, testing the impact on your buoyancy and ascent rate before making any further adjustments. By carefully managing your gas release from your BCD you will avoid overshooting and dumping too much gas.

 DIve Team on a Decompression Ascent on a Line

DIve Team on a Decompression Ascent on a Line

As you approach your target depth, say -1 to 2m, start to breathe out fully. This will reduce your buoyancy and slow your ascent. Take a shallow breath in, watch your depth and see how you respond. Generally, especially as you approach your shallower stops, you will need to dump gas from your BCD and drysuit to allow you to comfortably hold your new depth. After dumping gas, test your buoyancy via couple of breathing cycles. If you start to ascend past your stop, breathe out fully. See if you stop or descend. If so you may be Ok or only require a small adjustment to your BCD. If when you breathe out fully you continue to ascend, dump some gas from your BCD and/or drysuit. Then complete the breathing cycle test again. Repeat until you are stable at your target stop depth.

Now that we are at our target depth we need to hold this for the duration of our stop. I mentioned relaxed static stability earlier. That is the ability to hold a specific depth, in trim without undue movement especially hand movements or finning. You can't be swimming in circles to stay in formation. I have found that possessing a back-kick really helps when holding formation.

SECRET SAUCE 4 - Team execution

Given each team member is in control of their ascent, stable and able to complete ascent tasks, now we need to put this together as a team. A lot has been written on ideal team sizes and three seems to be the consensus regarding effectiveness versus size. Obviously standard buddy teams of two and teams of four will also work fine. Greater than four, you will want to consider splitting into smaller teams.

Within the team, each member should have a primary role and be able to backup the roles of the other team members if required. Generally on decompression dives the roles might be; deco captain (controlling the ascent and decompression plan), navigation, and depth/bottom time. Unless part of a dive course, the dive plan should be limited to the experience level of the weakest team member. In this way you are avoiding "trust me" dives where a team member might be taken beyond their limits. You also should consider making the weakest team member the deco captain, thereby allowing them to gain leadership experience surrounded by more capable divers.

It's worth repeating the part above about gas switching. This is a really good opportunity for each team member to play a role in assuring a successful and safe gas switch.

 Dive Team Completing a Blue Water Decompression Ascent

Dive Team Completing a Blue Water Decompression Ascent

SUMMARY

Executing your ascent is now a matter of pulling each part of this together; preparation, communication, personal execution and team execution. 

Hope this makes sense. Drop me a note if you have questions or comments.

Steve