Introduction to Rebreather Systems

Scuba diving is an exhilarating adventure that plunges you into a whole different world beneath the sea. One of the key pieces of equipment that makes this possible is the rebreather system. This system allows divers to recycle their exhaled gas, maximizing their underwater duration while minimizing the disturbance to marine life due to fewer bubbles.

Among the various types of rebreather systems available to divers, one of the most popular and frequently used is the Semi-Closed Circuit Rebreather (SCCR). This system is loved for its simplicity, efficiency, and moderate cost. Unlike a closed-circuit rebreather that recycles all the gas, a SCCR vents off a small part of the exhaled gas and replenishes it with fresh mix from the cylinder.

Anatomy of a Semi-Closed Circuit Rebreather

An SCCR consists of several components working harmoniously to enable a safe and sustainable breathing environment underwater. These components include the breathing loop (comprising of mouthpiece, hoses, and counter-lungs), gas supply, and the essential aspect we’ll be focusing on – the gas addition system.

A semi-closed circuit rebreather uses a minimalistic approach towards its design, with two gas cylinders typically attached: one containing a breathing gas mixture (commonly Nitrox) and the other, a smaller cylinder filled with pure oxygen for emergency use. The SCCR maintains a constant volume within the breathing loop by venting excess gas, which is replaced from the Nitrox cylinder to keep the oxygen concentration within breathable limits.

Understanding the Active Addition System

One crucial aspect of the SCCR is the gas addition system, which keeps the breathing loop filled with the optimal gas mix. Here, we discuss the active addition system, a type of gas addition mechanism that plays an integral role in maintaining a safe and breathable environment for the diver.

The active addition system, as the name suggests, actively adds gas into the breathing loop regardless of its current volume. This proactive approach is contrasted with passive addition systems, where gas is only added when the volume in the breathing loop falls below a certain threshold.

Working Mechanism of the Active Addition System

The active addition system operates on the principle of maintaining a constant supply of fresh gas. It involves a dedicated mechanism that continuously adds a fixed amount of gas into the breathing loop during each breathing cycle.

In most designs, this mechanism is typically a mechanically-driven valve connected to the gas supply, which opens for a set duration during each exhalation phase. This constant flow system ensures that the diver always has a sufficient volume of fresh, breathable gas in the loop.

To prevent over-pressurization, the excess gas from the breathing loop is vented out into the surrounding water through an over-pressure valve. This venting action also helps maintain the loop volume within specified limits, ensuring that the diver doesn’t have to exert extra effort to breathe.

Advantages and Disadvantages of the Active Addition System


The active addition system’s primary advantage is its simplicity. The absence of sophisticated electronics and sensors makes it easier to maintain and reduces the risk of technical failures.

Its proactive gas addition mechanism ensures a steady supply of fresh gas, reducing the likelihood of hypoxia, a condition that could occur in passive systems if the diver’s breathing rate suddenly increases. It also offers consistent performance, unaffected by variations in depth, breathing rate, or changes in ambient pressure.


On the flip side, the active addition system’s constant flow nature might result in a quicker consumption of

the gas supply compared to passive systems, thereby slightly reducing the overall dive duration. Also, due to its active venting mechanism, it creates more bubbles than a closed-circuit rebreather, which could be a concern for underwater photographers, cave divers, or divers who wish to avoid disturbing marine life.

Additionally, while the system is less prone to electronic malfunctions, mechanical issues can still arise. Regular maintenance and checks are essential to ensure the proper functioning of the active addition system.

Comparing Active and Passive Addition Systems

While the active addition system is a favored option among many, it’s always enlightening to understand the distinctions and similarities it shares with its sibling – the passive addition system.

Active Addition vs. Passive Addition: Functionality

Both active and passive addition systems serve the same purpose – maintaining an optimal gas volume in the breathing loop of an SCCR. However, the mechanisms they employ to achieve this are fundamentally different.

While the active addition system maintains a constant supply of fresh gas into the breathing loop, the passive system operates on a demand basis. The passive system uses a one-way mushroom valve that only opens when the volume in the breathing loop falls below a certain threshold, thereby adding fresh gas from the supply.

Active Addition vs. Passive Addition: Efficiency and Suitability


The choice between active and passive addition systems often boils down to the intended use. The active addition system, with its constant flow mechanism, might consume gas quicker but offers a steady supply of fresh gas, reducing the risk of hypoxia and offering consistent performance across a range of diving conditions.

Conversely, a passive addition system, which only adds gas when necessary, can be more efficient in terms of gas usage, potentially leading to longer dive times. However, it is more sensitive to changes in depth, ambient pressure, and the diver’s breathing rate.


The suitability of each system also depends on the nature of the dive. For instance, an active addition system, despite producing more bubbles, is often preferred for recreational and commercial diving due to its simplicity and reliability. On the other hand, a passive addition system might be a better fit for technical and deep divers, as its efficiency can extend bottom time.

Safety Considerations and Best Practices

In the realm of scuba diving, safety is paramount. This dictum extends to the use of semi-closed circuit rebreathers and, by extension, active addition systems.

Safety Precautions

Before diving, it is essential to thoroughly inspect and test the rebreather system. For the active addition system, check that the addition valve operates smoothly and that the over-pressure valve is clean and functioning correctly. Remember to verify the correct set-up and the quality of the gas supply.

Underwater, monitor your gas supply regularly. The active addition system’s higher gas consumption rate necessitates more frequent checks compared to passive addition systems.

Training and Best Practices

SCCRs with active addition systems, while simple in design, require dedicated training to use safely. Courses offered by recognized scuba training agencies can equip divers with the necessary skills and knowledge.

Ensure you fully understand how the active addition system works, how to respond to malfunctions, and how to manually add gas to the loop if necessary. Practice buoyancy control, as the continuous addition and venting of gas can affect your buoyancy differently compared to open-circuit scuba systems or passive addition SCCR systems.

In essence, the active addition semi-closed circuit rebreather system brings together the efficiency and eco-friendliness of rebreathers with the simplicity and consistency of active addition mechanisms. It indeed holds its charm for a vast range of diving enthusiasts, from recreational divers enjoying colorful coral reefs to commercial divers working underwater.

While it might have certain limitations, like a somewhat higher gas consumption rate and bubble output compared to other systems, the active addition SCCR is undeniably a valuable asset in the diver’s equipment arsenal. Its inherent simplicity and reliability combined with a steady supply of fresh gas make it a dependable companion beneath the waves.

Impact on Diving and Future Developments

The introduction and advancement of semi-closed circuit rebreathers, particularly active addition systems, have greatly influenced diving practices and possibilities.

Influence on Scuba Diving

Active addition SCCR systems have made extended dives more accessible, particularly for recreational and commercial divers. They’ve opened doors to extended exploration without the significant weight, cost, and training commitments that come with closed-circuit rebreathers.

Also, by producing fewer bubbles than traditional open-circuit systems, they’ve allowed divers to approach marine life more closely, reducing disturbances and improving underwater experiences for divers and underwater photographers alike.

Future Prospects

As we move forward, active addition semi-closed circuit rebreathers are likely to continue evolving. Manufacturers are continually refining and innovating their designs, making these systems even more reliable, efficient, and user-friendly.

Areas of focus for future development might include enhancing mechanical reliability, reducing gas consumption, and even integrating smart technologies for monitoring and safety purposes. The goal is to make this remarkable technology even more accessible and enjoyable for divers across the spectrum.

The active addition semi-closed circuit rebreather is more than just a piece of diving gear. It represents a commitment to extending our exploration of the underwater world, achieving this with a thoughtful balance between complexity, efficiency, and impact on the environment. It’s a testament to our never-ending quest for adventure and discovery, ensuring the way forward is filled with many more breathtaking moments beneath the sea.

Maintenance and Troubleshooting

Proper maintenance is critical for the longevity and reliability of any piece of scuba diving equipment, and active addition semi-closed circuit rebreathers are no exception. Regularly caring for your system will ensure optimal performance, enhance safety, and prevent unnecessary and costly repairs.

Routine Maintenance

Regular maintenance for active addition SCCR systems revolves around cleaning, inspection, and parts replacement. Post-dive, the rebreather should be thoroughly rinsed with fresh water to remove salt, sand, and other debris that could damage the system. The counter-lungs and breathing hoses should also be flushed to eliminate any moisture and contaminants.

Inspection should focus on the integrity of the hoses, mouthpiece, and valves. Check the condition of O-rings and seals, and replace any that are worn or damaged. Similarly, the addition and over-pressure valves in the active addition system should be inspected for signs of wear or corrosion and serviced or replaced as necessary.

Troubleshooting Common Issues

While an active addition system is robust and relatively simple, problems can occur. For instance, the addition valve could become stuck open or closed. A stuck-open valve could cause rapid over-pressurization in the loop and excessive gas consumption, while a stuck-closed valve could lead to a deficient gas volume in the loop. Both issues require immediate attention.

Regular inspection, cleaning, and lubrication can prevent most valve-related problems. However, if a problem occurs during a dive, knowing how to manually add gas to the loop and vent excess gas can be life-saving.

The Future of Scuba Diving with Active Addition Systems

While rebreathers have been in use for several decades, the improvements and innovations in design, particularly the active addition systems, have significantly expanded their accessibility and appeal. Divers who once considered rebreathers to be too complex or expensive are now adopting these units for their recreational dives.

The Rise of Recreational Rebreathers

Recreational divers have been slow to adopt rebreathers, primarily due to their complexity and the extensive training required to operate them safely. However, active addition semi-closed circuit rebreathers, with their simplicity and affordability, are changing this landscape.

More and more recreational divers are seeing the benefits of these systems, including longer dive times, fewer bubbles to disturb marine life, and a warmer and moister breathing gas, which makes diving more comfortable, particularly in colder waters.

Sustainable Diving

The shift towards active addition semi-closed circuit rebreathers also underscores the diving community’s commitment to minimizing its impact on the marine environment. The reduced bubble output of these systems not only allows divers to get closer to marine life but also decreases the noise pollution in the underwater environment.

Moreover, since semi-closed rebreathers use less gas than traditional open-circuit scuba systems, they also help reduce the carbon footprint of diving operations by requiring fewer tanks to be filled and transported.

Ultimately, the active addition semi-closed circuit rebreather is a reflection of scuba diving’s past, present, and future. Born from the spirit of exploration and refined by years of innovation, it stands today as a powerful tool for discovery, enabling us to delve deeper, stay longer, and tread more lightly in the incredible world beneath the waves.

Pioneers and Innovations in Active Addition Systems

The world of rebreathers, and specifically active addition systems, wouldn’t be where it is today without the tireless efforts of several pioneers and innovators in the field. Their contributions have not only refined the technology but have also elevated the safety and efficiency standards of these systems.

Key Innovators and Their Contributions

Diving equipment manufacturers have played a significant role in the evolution of active addition semi-closed circuit rebreathers. Companies like Dräger, Poseidon, and Hollis have developed and refined their own versions of SCCR systems, incorporating active addition mechanisms that have become benchmarks in the industry.

For instance, Dräger’s Dolphin rebreather, introduced in the late 1990s, is a prime example of an active addition SCCR. Its simplicity and affordability made it an instant hit among recreational divers, setting a precedent for future developments in this area.

New Innovations on the Horizon

As we look towards the future, new advancements are continually emerging in the field of active addition systems. Companies are focusing on enhancing the efficiency and reliability of these systems, integrating digital technology for real-time monitoring and improving their ergonomics and ease-of-use for a wider range of divers.

One such example of innovation is the integration of computer-controlled active addition mechanisms. These systems use sophisticated algorithms to control the addition of gas based on real-time monitoring of the diver’s breathing pattern and depth. This can result in even more efficient gas usage and improved safety, potentially marking the next step in the evolution of active addition semi-closed circuit rebreathers.

Key Takeaways

The exploration of the underwater world requires both curiosity and respect for our marine environment. Active addition semi-closed circuit rebreathers have proven to be a valuable tool in facilitating this exploration, offering extended dive times, minimal disturbance to marine life, and a sustainable diving practice.

Making an Informed Choice

The choice between active and passive addition systems, or even between semi-closed and closed-circuit rebreathers, largely depends on individual needs, preferences, and diving objectives. It’s crucial to understand the operation, benefits, and limitations of these systems before making a decision.

The Importance of Proper Training

Regardless of the system you choose, proper training is essential. Understanding the functionality, maintenance, and troubleshooting of your SCCR system, particularly the active addition mechanism, can enhance your diving experience and safety.

In the end, the allure of active addition semi-closed circuit rebreathers lies in their simplicity, reliability, and ecological benefits. As we continue to innovate and refine these systems, they will undoubtedly remain a vital tool for divers, allowing us to extend our explorations while preserving the beauty and integrity of our underwater world.