Electronic components in a rebreather are essential for monitoring the partial pressure of oxygen (PO2) in the breathing loop. Modern rebreather electronics do much more than simply track loop PO2. For example, electronically controlled rebreathers can actively maintain a constant loop PO2. Additionally, these systems are capable of computing inert gas tissue loading through algorithms, track cylinder pressure, and in some cases, even monitor carbon dioxide levels or scrubber consumption.
Shearwater manufactures a variety of electronic rebreather components tailored to meet the diverse needs of rebreather manufacturers, including generic items compatible with multiple systems. This article aims to clarify the different types of Shearwater products applicable to rebreathers by breaking it down into three sections: communication protocols, functions, and connections.
Photo Courtesy of Dirty Dozen Expeditions
Communication Protocols
In this article, when we discuss communication in rebreathers, we’re referring to how oxygen sensors transmit their outputs to the diver. This communication can occur via analog or digital signals.
Analog signals are continuous and vary smoothly over time, representing data with infinite resolution (like a dimmer switch). Digital signals, on the other hand, use discrete values (usually zeros and ones) to represent information in binary form, making them more resistant to noise and easier to process by computers.
A galvanic oxygen sensor outputs an analog signal between about 0 mV and 100 mV. To convert these voltages into a more practical loop PO2 reading that can be read on a monitor display, the signal must be amplified and digitized.
In an analog monitor, this process occurs within the monitor itself, with the monitor connected directly to the oxygen cells. This setup is straightforward, as it involves only one device besides the cells. However, detecting failures can be problematic. The scale of the sensor signals is so small, minor amounts of noise can subtly impact PO2 readings, making issues hard to detect.
Conversely, a digital monitor relies on additional electronics located inside the rebreather, nearer to the oxygen sensors, to amplify and digitize sensor outputs before transmitting the information digitally to the monitor display. This approach minimizes the distance small analog signals travel prior to amplification and digitization, resulting in more robust communication of loop oxygen levels. While more complex and requiring additional electronics located within the rebreather, digital systems allow for easier error detection for the diver. Furthermore, these digital systems can transmit much more than just sensor readings.
For more information, please refer to the “What is DiveCAN® and Why Should I Care?” article.
Photo Courtesy of Dirty Dozen Expeditions
Functions
Shearwater’s rebreather products can also be categorized based on their functionality. Broadly, rebreather devices can be separated into monitors and controllers.
Monitors
The primary purpose of a monitor is to enable the diver to observe the PO2 in a rebreather loop. A monitor can either be the sole electronic component in a rebreather or it can work alongside a redundant monitor and/or a controller. Both Heads-Up Displays (HUDs) and wrist-mounted monitors connect to oxygen sensors via cables (described in the connections section below) and often include features beyond PO2 monitoring, such as decompression management.
HUDs may range from simple LED indicators sending color-coded signals to more advanced devices, like the NERD, which provide extensive data alongside PO2 readings. Typically, HUDs are mounted on the rebreather loop near the DSV for easy visibility. Wrist-mounted monitor computers (like the Petrel line) are sophisticated dive computers that also monitor loop PO2.
Shearwater produces both analog and digital DiveCAN monitors. Analog monitors (including Petrels and NERDs) are versatile and can fit multiple rebreathers, though there are exceptions for unit-specific products. Digital DiveCAN monitors generally feature a standard digital signal, with some models fitting various rebreathers. However, different connector types restrict some digital monitors to specific rebreather models.
Controllers
Shearwater DiveCAN controller systems are comprised of two components: “head” electronics located inside the rebreather and a “controller” unit such as a Petrel or a NERD. Shearwater has produced analog controllers that did not require head electronics in the past, however, Shearwater only produces digital DiveCAN systems today.
DiveCAN controller systems function by measuring the oxygen cells and directly managing solenoid control through the head electronics located inside the rebreather. A digital connection between the head electronics and a specialized computer known as a setpoint controller allows the diver to monitor PO2 and adjust the rebreather's setpoint. An advantage of this system is that if the setpoint controller becomes disconnected, the head electronics can continue to control the solenoid and maintain loop PO2 (provided there is oxygen available).
Shearwater DiveCAN controllers are designed for specific rebreathers, making them unit-specific.
Connections
A critical aspect of Shearwater rebreather electronic systems is how the components physically connect with one another. Specifically, how peripheral components (monitors and controllers) link to the rebreather. Connections can generally be categorized as hardwired - permanent cable connections - or plug-in, which allow users to easily and safely disconnect peripheral components.
Hardwired Connections
Hardwired systems are typically coupled through cable glands, establishing fixed connections. This setup means that both the "head" and the "computer" are permanently linked, requiring them to be transported together and sent for service as a unit. A drawback of hardwired systems is their complexity when diagnosing failures; isolating issues can be difficult because the components cannot be easily separated.
Plug-in Connections
In contrast, plug-in connections are more versatile, allowing components to be easily attached for diving and disconnected for cleaning, storage, or maintenance. They also facilitate troubleshooting since different peripherals can be swapped for testing. However, not all plug-in connectors are the same. Some, like Fischer connectors, may be used across analog monitor systems from different manufacturers, while others are exclusive proprietary connectors specific to certain rebreathers. Even if two different rebreathers use the same connector type, compatibility of peripheral components is often limited.
Analog monitors from Shearwater use one of three connection types: hardwired, Fischer, and 4-pin plug-ins. The 4-pin version is available in different cable lengths.
A common question among divers is: "Which connector should I choose?" The answer depends on the specific rebreather you plan to use with the monitor. Some manufacturers offer various cable options, while others may only provide one type. The best approach is to consult the rebreather manufacturer to determine the appropriate connector for your needs.
Most Shearwater digital monitors and controllers utilize a DiveCAN connector (also known as 5-pin), with some products featuring a proprietary x-connector. Digital controllers are always unit-specific. While many monitors are also unit-specific, some may share connector types but employ differing strain relief systems. Again, it’s advisable to verify with the rebreather manufacturer for the correct item.
For cable maintenance tips, please refer to “DiveCAN and AK 4-pin Cable Maintenance Guidelines.”
Understanding Shearwater’s rebreather electronics is crucial for divers aiming to select the right components for their systems. From communication protocols and functional roles to physical connection types, each aspect plays a critical part in performance, safety, and ease of maintenance. Whether choosing a monitor or controller, analog or digital, hardwired or plug-in, careful consideration ensures optimal compatibility and reliability underwater. By breaking down these fundamentals, divers can make informed decisions that enhance their rebreather experience.
We’re always happy to hear from our customers. If you have any questions about our rebreather products or would like to get in touch, please don’t hesitate to contact us.
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Photo by Jason Brown
Written by Jose Ventura, Shearwater Rebreather OEM Specialist
Cover Photo by DIVEWELLS | © Carolina Wells
