I was recently talking to a friend about our experiences with re-breathers and the different aspects of buoyancy we consider when diving them in the military. As a Clearance Diver one of the dive sets I used was a chest-mounted, closed circuit re-breather, which supplied one hundred percent oxygen. The breathing loop consisted of an oxygen addition valve, inhalation hose, two-way mouthpiece assembly, exhalation hose, a protosorb canister and a counter lung (artificial lung).
One aspect of buoyancy we had to consider when diving this set is the counter lung. Because it is a closed circuit re-breather the oxygen content remains within the breathing loop and after time the volume of oxygen in the breathing loop will reduce which will affect buoyancy.
How is this relevant? For this particular re-breather, “over the horizon/very shallow water” operations in the dark hours are its core role, conducting operations such as reconnaissance and beach surveys.
In planning a dive you make yourself aware of your navigational considerations such as tides, currents and bearings. and taking the water temperature into consideration you ensure your wear the correct rubber and weigh. Finning through the water at a depth of around eight meters your focus is alternating between your compass bearing and depth at all times. Loosing sight of your depth exposes you to the risk of oxygen toxicity and loosing sight of your bearing puts you off course, jeopardising the task.
Because the destination is usually kilometres away it is important make do a "check bearing" every 10 - 15 minutes to ensure your still on course, and if your not you can take a new bearing. So noting a volume of air at ten meters in depth will double in volume when traveled to the surface. This is an important consideration when time comes to make a check bearing. When traveling from eight meters to the surface ton a rebreather the volume of air in the breathing loop will all most double and because and because we cannot dump air out of the buoyancy compensator (causing bubbles to raise to the surface), we have to reduce the volume of air in the breathing loop before ascending to do a check bearing.
So minute or so prior to making a check bearing we will “breathe down the counter lung”. If we stop adding oxygen to the breathing loop from our cylinder the volume of air in the breathing loop/counter lung will slowly reduce. This allows us to breathe down the counter lung and become negatively buoyant. We can then slowly travel to the surface and make a check bearing without breaching the surface and exposing ourselves. In some occasions a buddy diver will remain around the five-meter mark and tender you by a line while you ascend and make a check bearing. This helps ensure you do not breech the surface and assists you in your descent once you have made your check bearing.
It takes some time to perfect diving this re-breather, Clearance Divers on course are required to complete a minimum of fifty hours diving the re-breather within a two-week period. The minimum distance covered in each dive during this period is two thousand yards. Most of these dives are in the dark hours but I always remember the Wednesday afternoons when the yachts were out and we would be moving through the water at eight meters in depth often hearing the keels of the yachts passing by. Two weeks of compass swimming around the clock sure did strengthen the ankles and get me use to having sticky ears.