One of the big decisions when shopping for your first rebreather is whether you’d prefer a manual closed-circuit rebreather (mCCR) or an electronic closed-circuit rebreather (eCCR). In general, the primary difference is that one relies on automating electronics. Okay, that was easy! Right?

Surprisingly, it’s not that simple. But before we understand the differences between manual and electronic closed-circuit rebreathers, note that this article is not meant as an end-all decision-maker. Instead, it is a starting point for understanding the two different systems. Before deciding, you should always research and talk with rebreather instructors and experienced CCR divers to determine which option best suits you and your diving goals.

Related Reading: My Introduction to Diving a Rebreather

Manual Closed-Circuit Rebreathers (mCCR)

Manual CCR units, (sometimes called mechanical closed-circuit rebreathers), are based on the principle that human oxygen metabolization while breathing is relatively fixed in ideal conditions. If all other factors remain the same, such as the activity level, body temperature, excitement and so on, the amount changes little, no matter the depth you are underwater. In other words, if we are doing a relaxed swim at 40 or 120 feet, our bodies metabolize the same amount of oxygen.

MCCR systems deliver oxygen to the breathing loop to match what your body is metabolizing in one of several ways. One is a steady stream of oxygen from a Constant Mass Flow orifice, with the diver able to add oxygen via a manual add valve (MAV) to maintain a predetermined, desired partial pressure of oxygen (ppO2) ) called the setpoint throughout the dive. For example, as we ascend, the ppO2 in the breathing loop will naturally drop below the setpoint as ambient pressure lessens.

To maintain the setpoint, divers add oxygen via the MAV when needed. They would similarly do this if activity, temperature or other factor increased their oxygen consumption rate. The beauty of mCCRs is that they are relatively simple mechanical machines with fewer potential failure points. Because divers manually control the system, this essentially forces them to constantly monitor their ppO2—which is a key habit and may help lower the potential for complacency. But, eCCRs have advantages, too.

Electronic Closed-Circuit Rebreathers (eCCRs)

ECCRs, via a computer read oxygen sensor data then electronically add oxygen to maintain the setpoint. Think of it like cruise control on a car: You set your desired setpoint, and the system maintains it. However, just like your car, you cannot sit back and nap. An eCCR must be monitored just as closely as an mCCR, not only because technologies can fail, but because different factors can affect how quickly an eCCR can adjust to ppO2 variation.

One of the most essential principles of rebreather diving is knowing your ppO2 at all times—and it should always be near (some variation is normal) your setpoint (typically ppO2 1.2 or 1.3). While this might sound daunting, features such as a heads-up display (HUD) make this task simple because HUDs either alert you to look at your handset if your ppO2 is too high or low, while others show you the ppO2 readout for all sensors. The HUD is always in view at the edge of your vision, often attached to the breathing loop.

Although you must monitor the system, the rebreather does most of the work maintaining your setpoint. But, this introduces slightly more complex technology and more potential failure points, depending on the unit’s design. Your training on an eCCR, therefore, includes how to handle such failures by "flying manually." Having learned this, some experienced rebreather divers run their eCCRs manually, giving them more control. A common practice is to set the eCCR for a slightly lower ppO2 setpoint and then manually add oxygen throughout the dive to maintain the "true" setpoint. The electronics act as a backup if they become distracted or otherwise cannot manually maintain setpoint.

This brings up an important point when choosing an mCCR versus an eCCR. Although you can operate an eCCR manually, doing so is not the same as mCCR diving. If an eCCR is fully functional, then manual operation is acceptable. But, if it is not fully functional, manual operation is only used to end a dive. Manual operation should never be used to make or continue a dive using an eCCR with a nonfunctional automated system.

Related Reading: How to Choose Between Rebreather Styles

Conclusion

Both are great systems, and it really comes down to which will work better for you. After a lot of thought and testing out both types, I decided to go with eCCR. To understand my decision, it helps to know what my rebreather goals were from the beginning. As many of you know, I am a commercial photographer and cinematographer. Having the ability to be completely silent underwater, therefore opening up new opportunities to experience marine wildlife on camera, was the driving force for getting into rebreather diving.

Filming underwater comes with increased task loading; I’m diving the rebreather, managing my camera and thinking of compositions simultaneously. The thought of having a system to help me better manage some of that task loading sounded like a significant benefit.

Plus, if my background in cave and technical diving has taught me anything, it is that humans are usually the weakest link in any system or plan, with errors or procedure violations the most common causes of dive incidents.

Jakub Šimánek of Divesoft once told me, “Machines can’t think, and we can’t do that for them. We have to think for ourselves, but we can entrust straightforward calculations to computers. They are much better at it than we are.”

Does this mean an eCCR is better than mCCR? Not at all. They’re both solid systems that offer different features that can adapt to various types of diving. So there isn’t a bad choice. What you choose will come down to which will better help you accomplish your goals.

About Jay Clue

Exploring Rebreathers is a column following Jay’s journey into the world of CCR diving, sharing the best tips and info he has learned along the way. Jay Clue is an accomplished photographer, educator, and conservationist. He’s an experienced technical diver and instructor with over 6000 dives in all sorts of environments, from the cold waters of the North Sea to the beautiful caves of Mexico and exploring the open oceans in search of epic encounters with marine megafauna. His passion for marine conservation led him to the world of photography and storytelling in hopes of bringing the beauty of our planet's waterways to more people around the globe. Follow him at @jayclue on Facebook and Instagram, where he’s always happy to answer your questions and chat about diving.