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Rebreather Classification

Rebreather Classification
What are rebreathers – now they know if not all, then many. Unfortunately, in Russia they still know mostly by hearsay.

There are many legends about rebreathers, their features and design that are true and not very true. Therefore, the main goal of this article and the entire section is to try to “sort through” all these features, advantages and disadvantages of rebreathers.

Now on the Internet there are many articles devoted to rebreathers, even in Russian, so do not be surprised if, when reading, it seems to you that you have already seen this somewhere.

This article is intended for beginners as well as advanced technical divers who decide which rebreather to buy.

So let’s get started …

What is a rebreather?
A rebreather is a recirculating breathing apparatus, that is, a device in which, unlike scuba gear (SCUBA), when you exhale, the breathing mixture is not completely removed or completely removed. Instead, the spent mixture is processed to re-breathe it (re-breathe). To do this, remove carbon dioxide (carbon dioxide) from the mixture and add oxygen to the mixture.
The first task is solved in all rebreathers in the same way – they always include a container (absorption canister) in the breathing circuit, which is filled with a chemical substance that actively absorbs carbon dioxide.

The second task – adding oxygen to the mixture – is solved differently in different types of rebreathers. Let’s take a closer look at this …
What are rebreathers?
All rebreathers according to the principle of action can be divided into two large groups:

semi-closed rebreathers
fully enclosed rebreathers.
In closed rebreathers (CCR – Closed Circuit Rebreathers), the exhaled mixture is completely recycled, and after removal of carbon dioxide, pure oxygen is added to it. This is not to say that the mixture of these types of rebreathers does not get etched into the water at all; rather, it does not get etched when swimming at a constant depth. Upon surfacing, that is, with a decrease in external pressure, the respiratory mixture expands and its excess is removed into the water through an etching valve.

Semi-closed rebreathers (SCR – Semi Closed Rebreathers) differ from closed ones in that the mixture is removed from the breathing circuit even when swimming at a constant depth, but the amount of mixture removed is much less than that of a conventional scuba gear. Removing part of the mixture is necessary because in order to maintain the necessary level of oxygen in the breathing mixture, not pure oxygen is used here, but artificial breathing mixtures like Nitrox, Trimix and Heliox. Therefore, it is necessary to remove the excess of neutral gases: nitrogen and helium.

In turn, closed and semi-closed rebreathers can be of several types according to the principle that supports the optimal composition of the respiratory mixture.

Closed rebreathers:
Oxygen rebreathers (CCOR – Closed Circuit Oxygen Rebreather) operate on pure oxygen, i.e. the diver breathes pure oxygen without the admixture of any neutral gases. This principle simplifies the design and reduces the size, but also introduces its limitations. We know that oxygen becomes toxic when the partial pressure rises above 0.5 bar. In this case, toxicity manifests itself in two forms: pulmonary (calculated in OTU – Oxygen Tolerance Units) and convulsive (calculated by the effect on the central nervous system CNS – Central Nervous System). The maximum safe partial oxygen pressure for divers is considered to be 1.6 bar (usually 1.4 for long exposures) and only in urgent cases it can be briefly increased to 2.0 bar (3.0 in the French and Russian Navy). Considering that there is still a little neutral gas in the breathing circuit of the device, the maximum immersion depth in such devices is limited to 7 meters (10 meters in case of emergency).
Another negative factor in the action of pure oxygen is that it “feeds” any manifestations of caries or other diseases of the oral cavity. Therefore, when using such devices, do not forget to regularly visit the dentist (which, by the way, is recommended for all divers), and you will not have problems with your teeth.
Due to their small size, great autonomy and, most importantly, the absence of exhaled bubbles, such devices are very popular among military and underwater biologists.
The most famous representatives of this type: Draeger LAR VI and OMG Castoro C-96.
Oxygen rebreathers with chemical regeneration of the respiratory mixture (CCCR – Closed Circuit Chemical Rebreather). They are similar in design to the rebreathers of the previous type, but differ in the principle of renewing the oxygen content in the mixture. The fact is that, unlike an absorption substance that simply absorbs carbon dioxide, a regenerating substance is charged into the canisters of such devices, which, when it absorbs 1 liter of carbon dioxide, emits about 1 liter of oxygen.

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