Rebreather working explained

What is a Rebreather and How does it Work?

Inefficiency of the conventional SCUBA (acronym for Self-Contained Underwater Breathing Apparatus) apparatus has led to the emergence of the Rebreather. To know more about this new apparatus and its working, go through this Buzzle article.
Did You Know?
Since the rebreather has a closed circuit, carbon dioxide is not released in the surroundings. Hence, there are no bubbles to alarm marine life, enhancing your underwater exploration.

To understand a rebreather and its working better, it is important to know a little about conventional SCUBA and its flaws. These flaws were instrumental in the creation of this new device. The traditional or open-circuit SCUBA comprises a compressed gas supply and a demand regulator from which the diver breathes. The name open-circuit is given to the device because the exhaled gas, i.e., carbon dioxide, is released in the surrounding water and not reused in any form. A scuba diving apparatus has three main components- an inhalation connection, a high-pressure tank, and a pressure regulator.

The main drawback of conventional scuba diving equipment is that when you exhale carbon dioxide it is discarded into the environment. Also, only a very small amount of total inhaled oxygen is used by the diver for metabolism, and a great amount of reusable oxygen is simply wasted in the whole process. Furthermore, as the diver goes deeper and deeper, there is an increase in the loss of reusable oxygen.

What is a Rebreather?

It is a type of diving apparatus that is different from the traditional scuba breathing gear. A rebreather recirculates the breathing gas exhaled by the diver. There are three types of rebreathers - oxygen rebreathers, semi-closed rebreathers, and closed-circuit rebreathers.

All theses devices work differently, but have some basic components in common, which are:

The Breathing Loop, which consists of:
A Mouthpiece
Breathing Hoses
Carbon Dioxide Scrubber
Counterlungs

The Gas Replenishment System, which consists of:
Compressed Gas Cylinder
Pressure Gage
Gas Addition Controls

Components of a Rebreather and Their Working

Although there are some differences in the functioning of each rebreather, the basic task that they perform is the same. Following are the parts of a rebreather and their functions:

A Mouthpiece

The main function of the mouthpiece is to enable the diver to inhale oxygen. There are two types of mouthpieces - a full-face mask and a mouthpiece. Although a full-face mask is preferred by most divers, it has some drawbacks, like it can cause discomfort in hot and humid climate, the seal between the face and the mask is completely air-proof, causing fogging of the face mask and obstructing the view. Therefore, a mouthpiece is a better option.

Breathing Hoses

The breathing loop comprises two breathing hoses, which connect the mouthpiece to the rest of the breathing circuit, specifically the scrubber and counterlungs. These hoses need to be flexible enough to make it more comfortable for the diver to inhale and exhale.

Carbon Dioxide Scrubber

A scrubber acts as a purification agent in the process. The exhaled gas is directed to the scrubber through the loop, where it is neutralized. Further, the neutralized gas is directed to the mouthpiece and here, the supply gas is introduced to the neutralized gas while refreshing it.

Counterlungs

It is one of the most essential components of a rebreather as it helps to inhale and exhale the gas. Counterlungs comprise two bags, one for inhalation and the other for exhalation of the gas. The pressure relief valves are designed to maintain the lung pressure below 40m bar. The valves can be overruled for expelling excessive water from the counterlungs underwater and for manual gas release during ascents. There are two types of counterlungs to choose from - over-the-shoulder counterlungs, and back-mounted counterlungs.

Compressed Gas Cylinder

It is the source of oxygen that is required to replenish the used oxygen by the diver. Depending upon the diver's rebreather design, the gas cylinder will carry pure oxygen or gas mixtures for the replenishment of the used gas. Since pure oxygen is not considered safe for diving deeper than six meters, the diver also carries a cylinder of diluent gas. This cylinder may be filled with compressed air or another gas such as nitrox or trimix. These diluent gases reduce the percentage of pure oxygen and allow the diver to explore greater depths.

Pressure Gage

The main function of a pressure gage is to monitor the pressure in the oxygen cylinder. It helps to determine the amount of oxygen still available and how long the diver can go with it. For easy access to the pressure gage there is a long and flexible tube that connects the gage to the oxygen cylinder.

Gas Addition Controls

The supply gas is introduced to the neutralized gas to make up for the missed volume of the gas through metabolism or due to its release in the environment. The gas is introduced in two ways - namely active addition and passive addition. While active addition means the continuous supply of oxygen, regardless of the diver's breathing rate, passive addition refers to the addition of the gas with respect to the diver's breathing rate.

Differences Between Oxygen, Semi-closed Circuit, and Closed-circuit Rebreathers

Oxygen Rebreathers

These carry pure oxygen in the cylinder, which is the only gas supply. This may increase the risk of oxygen toxicity and therefore, limit the diver to a certain depth.

Semi-closed Circuit Rebreathers

These, unlike oxygen rebreathers, use gas mixtures for the gas supply, which enhances the scope for greater depths. Also, there is no risk of oxygen toxicity with these rebreathers.

Closed-circuit rebreathers

These are equipped with both the pure oxygen gas and gas mixtures. They are more efficient than the other two types because of their capacity to maintain the oxygen concentration.

A rebreather is a better diving apparatus than conventional SCUBA because it offers better gas efficiency, is lighter in weight, and allows the diver to stay down longer than conventional apparatus.
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