Rebreathers and Buoyancy

The ability to maintain  neutral buoyancy is a key contributor to enjoyable diving.

Maintaining a neutral buoyancy allows us to effortlessly move below the surface without either sinking towards the bottom  or floating upwards towards the surface.  This effortless or buoyant state not only saves energy and uses less oxygen consumption, but it is one of the most enjoyable aspects of diving.

Archimedes’ principle explains that buoyancy (upward force) is achieved by displacing water.

Archimedes‘ principle indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. Archimedes‘ principle is a law of physics fundamental to fluid mechanics. – wikipedia.org

As divers we must balance or make adjustments for the total amount of water displaced by our bodies as well as our equipment throughout  a dive to achieve and maintain neutral buoyancy.

The main difference between OC diving and rebreather diving is that rebreather divers don’t have to worry as much about buoyancy compensation due to  air consumption.  This is because the air in the diver’s lungs are considered part of the rebreather-loop which means a fixed volume of gas is maintained throughout the dive. This is buoyancy neutral.  In fact, rebreather divers can start their dives correctly weighted because of this.

 

Read on below to see how open-circuit and rebreather divers compensate for buoyancy differently.

The diver’s lungs are essentially part of the rebreather loop and this maintains a fixed volume of gas buoyancy remains unchanged during normal operation.

Every new rebreather diver spends some time getting used to this concept. They swim towards an object, take a deeper than usual breath expecting to rise gently and bump straight into the object that they were trying to avoid.

In addition to remembering that on a rebreather, depth of breathing does not control buoyancy, there are two related things worth noting.

The first is being correctly weighted and NOT over-weighted. Using conventional scuba, every breath exhaled into the environment makes the diver and her kit slightly less heavy. Quite apart from the buoyant effects of the lungful of gas disappearing as a stream of bubbles on their way to the surface, all gas has some mass. A litre of air weighs a more than a gram and at a depth of 30 metres (around 100 feet) it’s not unusual for a diver to “consume” 50 to 60 litres every minute. Over the course of an hour’s dive, the weight of gas consumed by an open-circuit diver can make a considerable difference to the balance between the forces of buoyancy and gravity. On a technical dive, it is not unusual for a diver to use several kilos worth of gas. Consequently, the ballast they carry has to help compensate for this “Buoyancy Shift.”

Many OC divers, essentially begin their dives over-weighted. A rebreather diver does not have to consider or account for much buoyancy shift and therefore, should begin the dive correctly weighted. This will help with control throughout all phases of the dive.

Secondly, with the potential to have to manage gas volumes in the wing, drysuit and rebreather loop, it’s recommended to maintain just enough gas in the loop for a full breath and no more. This one-breath volume is the simplest to maintain and control. Having more gas than is required for a single breath adds complexity to an already complex management skill.

In simple terms, if a rebreather diver feels the slightest tug of resistance drawing a full breath from the loop, the loop volume is probably optimal!

Read the entire article for further dttails here at tdisdi.com