mCCR versus eCCR

When you look into purchasing or diving a closed-circuit rebreather (CCR), you will quickly encounter two primary designs. These are the manual closed-circuit rebreather (mCCR) and the electronic closed-circuit rebreather (eCCR).

Both types of rebreathers use electronics to monitor your breathing loop. The fundamental difference between them is how they add oxygen to the system. Understanding these differences will help you make an informed choice for your technical diving.

What Do the Terms Actually Mean?

A common misconception is that an mCCR is completely mechanical and does not use any electronics. This is not true. Both systems rely on electronics to monitor the oxygen sensors to show you the partial pressure of oxygen (PO2) in your breathing loop.

The real difference lies in the mechanism of oxygen injection:

• mCCR (Manual/Mechanical Closed-Circuit Rebreather): These systems inject oxygen at a continuous, slow rate. They do this through a mechanical device called a constant mass flow orifice or a needle valve.
• eCCR (Electronic Closed-Circuit Rebreather): These systems inject oxygen using an electronically controlled valve called a solenoid.

A true “manual” rebreather would require you to press a manual addition valve every time you need to add oxygen. Because constantly pressing a button to stay alive is difficult during a dive, true manual units are very rare. Most modern mCCRs actually use a mechanical constant flow system. The term “manual” is largely a historical holdover.

How Oxygen Addition Controls Your System

Because their methods of adding gas differ, mCCRs and eCCRs rely on different electronic setups.

An eCCR requires a digital controller to decide when the solenoid should open to inject oxygen. This controller is often a digital computer system, such as a Shearwater controller running on a DiveCAN network.

An mCCR does not require a digital controller to add oxygen because the mechanical orifice does the work continuously. Consequently, many mCCRs use simpler analog electronics just to display your oxygen levels.

The Debate Over Electronic Complexity

You may hear some divers claim that mCCRs are more reliable because they use fewer electronic parts. While it is true that mCCRs have less digital complexity, they are not free from failure points. Mechanical mass flow orifices can still become clogged or fail.

Additionally, analog systems have their own unique failure modes. They are susceptible to electrical interference, wire corruption, and voltage drops. Your oxygen sensors produce a very low voltage, typically between 50 and 60 millivolts. When you deal with signals this small, even a minor voltage drop in your analog wiring can corrupt the reading on your display. Digital systems convert this signal to a digital value, which reduces the chance of signal corruption. A digital failure is off, an analog failure could still display values but they’re incorrect.

Managing Your Buoyancy

The way these two systems add gas also affects your buoyancy control:

• The mCCR Flow: Because an mCCR constantly trickles oxygen into the loop, it slowly replaces the gas your body consumes. Many divers prefer this continuous flow because it keeps loop volume stable and makes buoyancy very easy to manage.
• The eCCR Injection: An eCCR adds oxygen in periodic bursts when the solenoid fires. This can cause minor, sudden changes in your loop volume.

However, once you understand how the solenoid operates, you can easily anticipate and account for these volume changes.

Furthermore, solenoids offer a smarter method of gas addition. If you experience a high PO2 situation, a solenoid will stop injecting oxygen. A mechanical mass flow orifice on an mCCR will continue to trickle oxygen into your loop regardless of how high your PO2 rises. You must manually manage the system to stop the flow in that scenario.

Overcoming Historical Fears

In the early days of technical rebreather diving, solenoids were prone to failure. This history created a fear of electronic control systems that some divers still hold today.

However, modern solenoids and digital controllers are reliable. The most important rule of rebreather diving remains the same regardless of your unit: you must never blindly trust any automated oxygen addition system. You must always monitor your PO2 actively throughout the dive.

A major benefit of modern digital eCCR systems is that they make it easy to connect secondary monitoring devices, such as Head-Up Displays (HUDs). This digital integration makes it easier for you to keep your eyes on your oxygen levels.

Is One System Superior?

Neither system eliminates the inherent risks of technical diving. Both mCCRs and eCCRs have different failure modes that you must understand and manage. Your choice should depend on your diving style, your comfort with digital versus mechanical systems, and your commitment to monitoring your breathing loop.

Choosing the right rebreather configuration is a personal decision that requires careful thought. If you want to discuss the differences between mechanical and electronic oxygen addition, or if you are ready to begin your rebreather training, please contact me today. We can help you select and master the system that best fits your technical diving goals.