The mCCR explained
The mCCR is often misunderstood by divers - even rebreather divers. When trying to explain it, a question we frequently hear - "Is a mCCR better?" I wouldn't say it's better. Like anything else in life, the mCCR has its advantages, and it's disadvantages. As we often tell prospective rebreather divers, "Get the unit that is right for you."
However in overhearing conversations on this topic for the better part of a decade, it's clear that many divers don't fully understand how they work. We have heard a lot of mis-information disseminated about the mCCR - by far the biggest myth is that they are a lot of work to dive.
For the layperson, there are 3 things to know when it comes to any rebreather:
Every human being metabolizes oxygen. No matter what we are doing, our bodies are taking in oxygen, metabolizing it and exchanging it for CO2.
Every rebreather must somehow add oxygen back into the breathing loop to compensate for what our bodies are using.
Every closed-circuit rebreather has the same components: a CO2 scrubber, a set of counterlungs, a PO2 monitoring system, a means to add oxygen, and a ‘loop’ to tie the whole system together. No matter the make or manufacturer, all rebreathers have these same 5 basic elements.
That said, all closed-circuit rebreathers in the market today generally fall into 2 categories. These would be eCCR's and mCCR's. (There are also hybrid units which combine the features of both eCCR and mCCR.
The term eCCR stands for Electronic Closed Circuit Rebreather, and it is by far the more popular type. The most simple explanation for how an eCCR works is, a computer ‘brain’ reads the PO2 of the breathing loop. When the PO2 falls below a user-defined threshold, the computer sends a signal to a solenoid. The solenoid is an electronic valve connected to your oxygen feed. The computer will open this valve, often in little pulses, to squirt the precisely required amount of oxygen back into the breathing loop. This causes your PO2 to rise back up to your desired threshold. This process repeats itself throughout your dive.
The term mCCR has traditionally stood for Manual Closed Circuit Rebreather, however I’ve always found the term Mechanical Closed Circuit Rebreather is more appropriate. Like the eCCR, the mCCR does have an automated way to add oxygen back to the breathing loop. Except this method instead of being electronic is, you guessed it - mechanical. Unlike the eCCR, there is no computer controlling PO2. The computer used to control the PO2 is the one between your ears. So what is the automated way the mCCR is adding oxygen to the loop? The mass flow orifice.
Several different angles of a mass flow orifice. Note the hex section is only 1/4" of an inch. If you magnify the image and look down the center, you can see the tiny .003" orifice.
The idea behind the mCCR is this mass flow orifice is connected to your oxygen tank. The orifice is a piece of brass with a very tiny hole drilled into it - about .003 inches. This orifice takes intermediate pressure oxygen from your 1st stage regulator, and converts it to a constant and steady 'trickle' of gas. The principal of how these valves work is based on sonic pressure waves, which is why they are sometimes called 'sonic valves' or 'leaky valves.' Once the oxygen tank is opened, a constant ‘leak’ of oxygen is released into the breathing loop. This leak, or trickle of oxygen is tuned (during your initial diver training) to match your body's basal oxygen metabolic rate.
Any automated flow of oxygen must be adjustable though. As we descend in the water column, diving physics increases our partial pressure of the oxygen, which means we need to add less oxygen into the loop. For the eCCR diver, the computer simply opens the solenoid valve less often. For the mCCR diver, the ‘trickle’ of oxygen is automatically limited based on the increase in water pressure. This is because the oxygen first stage regulator is a fixed intermediate pressure. Unlike almost every other 1st stage regulator in the world, these regs do not compensate for the water pressure change. The simple version is, as we go deeper the trickle of oxygen gradually decreases - a perfect balance of what we need and what it provides.
If you are reading this, it’s a good chance you are doing research on the mCCR. It's possible you’ve spoken to an eCCR diver who may not have understood the mass flow orifice concept completely. A diver who has only dived an eCCR's may not have been properly informed on its operation, so we will elaborate here:
During eCCR training, a diver is taught to ‘fly the unit manually.’ This simply means that instead of letting the computer control the PO2 of the breathing loop, the diver does this by manually actuating the o2 addition button on their rebreather. This is a very important skill to have, as the diver could conceivably need to take manual control in the event of a computer failure. Manually operating an eCCR also makes the diver more aware. But 'flying' an eCCR manually is certainly more work.
When the computer and solenoid are managing your PO2, the eCCR diver will hear the solenoid fire (little clicking sounds) every 5-10 seconds or so. When the diver assumes manual control, they can stretch this interval of actuating their manual addition button to 30 seconds, or even as much as a minute if they don’t mind larger fluctuations in their PO2. But this means at minimum, the diver must push the oxygen manual addition button at least once per minute.
Those who learned to dive an eCCR, often mistakenly associate flying their eCCR rebreather manually with the same workload a mCCR requires, and this isn’t the case. They forget (or don’t know) about the mass flow orifice, which is constantly trickling gas into the breathing loop. I mistakenly believed this as well - my first rebreather was an eCCR. Although I had seen KISS Classics on the dive boat, I only knew they were 'manual' rebreathers and didn't understand how they worked.
While an eCCR diver flying their unit manually means they need to add oxygen at least once per minute, a properly tuned mass flow orifice means that the mCCR diver only needs to add oxygen once every 10-15 minutes. This of course assumes the diver is not ascending, and is performing a normal workload.
A rendering of a KISS manual addition block. Oxygen flows into the port (2) and then through a pathway (not shown) down to the O2 button chamber (6.) From here, O2 can trickle through the orifice (5) and into the diluent return line (9.) When additional O2 is needed, pressing the O2 manual addition button (6) allows a greater flow of oxygen into the o2 return path (8.) The cap (7) serves as an access port so the orifice may be removed and cleaned as needed.
What happens when workload increases? Fighting current, flow, or otherwise working hard causes an increase in your metabolism, and hence requires more oxygen, In these cases, the eCCR diver will have their solenoid fire more often, and the mCCR diver will need to add oxygen more often as well. This increase though is relative. An eCCR diver using manual oxygen addition may need to increase their oxygen addition to every 20-40 seconds, while the mCCR diver may need to increase to every 5-8 minutes. This of course assumes the oxygen mass flow orifice has been properly tuned to the diver.
Can the mass flow orifice deliver too much oxygen? Not really. It's pretty much impossible for the orifice to increase size on its own. Since the mass flow orifice flow rate is directly proportional to the intermediate pressure of the oxygen first stage, first stage creep or a leaking manual addition valve can cause your PO2 to climb. First stage creep though - where the intermediate pressure of the first stage begins to climb due to a mechanical failure, is something that is not limited to mCCR’s. Creep, o-ring, and button leaks are failures would cause the PO2 to climb on an eCCR as well.
A cross section of a KISS single gas manual oxygen valve. Note the brass orifice which provides a constant 'trickle' of oxygen for the diver.
An exploded view of the KISS single gas manual oxygen valve
Are there downsides to a mCCR? The answer to this question will vary depending on the person who gives it. Some will argue that an mCCR requires more work. Others will say the diver needs to pay more attention to their rebreather. We actually believe this is a good thing, as the golden rule in rebreather diving is to always “know your PO2.” All divers, eCCR or mCCR need to pay attention. After all, if a failure were to occur on your eCCR, you should be paying attention to notice this.
All mCCR’s do have a limitation eCCR’s do not have, and that is a practical depth limit. As you dive deeper, the ambient pressure of the water of course increases. Eventually you will reach a depth where the water pressure equals the ambient pressure of the first stage regulator, and oxygen will no longer flow from it. At this depth, your mass flow orifice stops delivering gas. That depth depends on the intermediate pressure of your first stage regulator. On my personal unit, it occurs at 284 feet. That said mCCR divers certainly can (and do) go deeper, but they have alternate ways of adding oxygen deeper than these depths. If you are reading this article to better understand mCCR’s, you really shouldn’t be planning dives that deep just yet.
Lastly, as we previously said, the mass flow orifice is a tiny opening - .003 inches, so it can easily get clogged if impurities exist in the upstream oxygen flow. Avoiding saltwater intrusion is important on any rebreather though. While saltwater can block a mCCR orifice, it can also easily corrode the ferrous metals found inside an eCCR solenoid.
What are the advantages of mCCR’s? In a nutshell, it's their simplicity. While a mCCR has electronics to monitor the PO2 of the breathing loop, these are very simple. Unlike an eCCR, there are no electronics or wires connected to battery packs, or electronic solenoids inside the breathing loop.
Many divers consider depth limit we just spoke about to be a benefit. After all, when your depths approach 100 meters, you need so little oxygen to sustain you. Pure oxygen leaks into your loop at these depths could be extremely dangerous.
Any device, no matter how well constructed, can and will fail at some point. This is especially true of rebreathers, where the rigors of boat and cave diving can be very tough on any equipment. Add the corrosive nature of salt water into this equation, and at some point your rebreather will need maintenance or repair.
With an eCCR, depending on the model, you will need to send your head, or on some units, the entire rebreather, back to the manufacturer for repair. With mCCR’s like the KISS, with a complement of spare parts, repairs can be conducted on the dive deck with simple hand tools in just minutes.
While we love our eCCR’s and they certainly have their place in the diving world, there is certainly an advantage in the simplicity of the mechanically controlled rebreather.