By itself the Optima CM is a fully functional, electronic closed circuit oxygen rebreather that can be readily fastened to most types of scuba harnesses like it is here on Jared Hires’ Nomad Side Mount system.

Dive Rite’s O2PTIMA CM Rebreather

“Incredibly versatile, self-contained, and travel ready.”  That’s how Dive Rite describes their O2ptima CM rebreather. It is an electronic, fully-closed system, and in case you are wondering, CM stands for chest mount. Compact and tipping the scales at just 14.4 pounds —noy including scrubber material or oxygen cylinder —the O2ptima CM is a truly unique design. 

While I am no stranger to closed circuit diving, having dove various systems for the better part of 18 years, I have never before encountered a fully closed chest-mounted design like this. And so, I was eager for the chance to evaluate O2ptima CM when Lamar and Jared Hires offered me the opportunity to try the unit in North-central Florida’s spring country.

On initial examination, several features sparked my interest. The first was the breathing loop, which is reported to be the shortest of any rebreather on the market. The standard configuration includes a solidly built Dive Surface Valve (DSV) joined to the unit’s counter lung by two 12-inch-long (30 cm) loop hoses. These hoses have threaded loop connections for easy removal and cleaning. 

Unlike military design chest mount oxygen rebreathers like the Draeger Lar V, the Dive Rite O2ptima CM doesn’t have a large single counter lung, instead utilizes two separate 3-liter lungs. The counter lung covers are made from Rhinotek, which is highly abrasion-resistant material that is still relatively flexible. The twin counter lungs actual working volume is closer to 5.5 liters (2.25 liters per counter lung) due to the counter lung covers, but when you add in the gas present in the CM’s scrubber, the total loop volume is brought back to 6 liters. Something to consider if you are an individual with a large tidal lung volume. 

What is a bit of a surprise given the CM’s compact design is that it shares the same horizontal cylindrical housing for its scrubber and Shearwater DiveCAN electronics system used in Dive Rite’s larger back-mount O2ptima CCR models. Mounted horizontally below the counter lungs, the CM’s scrubber is designed to accept the same Micropore ExtendAir SR-081C cartridges as its big brother. You can also utilize a self-pack scrubber that holds five pounds. (2.2kg) of 812 mesh granular CO2 absorbent. 

The CM’s electronic oxygen solenoid and four AI R22 oxygen sensors are governed by one of two devices. One configuration utilizes a Shearwater Petrel 2 controller paired with a basic triple LED HUD. The second configuration includes a Shearwater NERD 2 model controller mounted securely atop the CM’s DSV, paired with same HUD system. Because I am a big NERD 2 fan (having one on my personal CCR), I feel that this option is the way to go, as it makes the CM less cluttered by eliminating that cable running down your wrist to the Petrel controller.  

In addition to the Shearwater controller, the CM is fitted with an Oxygen Manual Add Valve (MAV) located on the right side of the counter lung, and an Automatic Diluent Valve on the left side. I found both of these to be conveniently located for easy activation with a press of forefinger or thumb. 

The CM featured on Dive Rite’s website is pictured with a 13 cubic-foot aluminum oxygen cylinder which is attached below the scrubber with two 1.5-inch cam straps. This configuration allows for a wide range of side or back -mound diluent tank configurations. The CM is also the most travel-friendly fully closed-circuit rebreather on the market, as it has the uniquely ingenious advantage of being a true add-on rebreather. By that I mean you don’t necessarily have to change out your current open-circuit equipment to enter the world of silent diving. 

All that is required of your existing harness/BCD (be it single tank, doubles, or side mount), is that the shoulder straps can accommodate a single D-ring atop each shoulder, with a second set on each side of the waist strap. From there, CM’s four bolt-snaps are able to securely fasten it to your harness with the quick disconnect for the CM’s dil making the fifth and final attachment.

My test rig utilized a standard Dive Rite Transpac XT harness with a EXP 25 -pound Travel Wing and 120 cubic-foot back-mount tank. Jared’s unit was attached to his Nomad LS side mount harness, while Lamar slung his CM on a Transpac XT harness with a larger 45-pound Rec Wing and a set of small doubles.

Having the ability to easily clip or unclip the CM either in or out of the water opens a diver’s range of possibilities. For example, in a cave diving scenario, where a diver needs to pass through a tight restriction, he could undo the CM’s two waist clips and swing the rebreather out front to move through a given opening without going off the loop. In a recreational line where a diver is on a trip in the Florida Keys, you could use the CM for the first dive on a deep wreck such as the Spiegel Grove or the Duane, then leave it in the boat while using the same tank they used on the first dive for the second dive on a shallow reef. 

Post Dive Thoughts and Impressions 

In a fully closed-circuit rebreather, the counter lungs are positioned one of three ways – on the back, over the shoulder, or on the chest. The positioning of the lunges has a direct effect user comfort and the work of breathing (WOB) – meaning how much effort is involved when breathing through the loop while in a prone position. 

A rebreather with back-mounted counter lungs is highly desirable to some as it frees up the diver’s chest. With this configuration, the counter lungs are above the diver’s own lungs when in a prone position, requiring more effort during the inhalation side of a breath cycle than during the exhalation side.  

With a chest mount-system, the reverse is generally true, as the diver can expect some resistance during exhalation. This occurs because the counter lung is below the diver’s body, and the gas in the lungs wants to rise. This can create enough back pressure to cause what most CCR divers call “chipmunk cheeks.” Even properly positioned over the shoulder counter lungs, which some attest as being the best of both worlds, can generate the same effect. 

Given its chest-mounted counter lungs, the work of breathing on the CM was not what I expected. The dual counter lung design, which I should mention feature dual water traps, actually provided a very comfortable WOB with just a touch of back pressure when resting in a prone position. Even when rolling all the way over to an inverted position CM’s breathing comfort was still good.

One niggling issue I have with all electronic CCR’s is that when you are at depths shallower than 30 feet, the oxygen solenoid generally fires more often to retaining a high constant volume in the loop. As a result, the counter lungs may already be filled before you try to exhale, requiring you to vent off that excess gas through the nose or out the corner of your mouth. One work around to this phenomenon is to place the controller’s set point around .5 or .6, shut off the dil, dump everything from the counter lung and replace it with pure oxygen, which will cause the solenoid to take a break. This was something I forgot to do during our trial dive, as I was focused on photographing Lamar and Jared demonstrating their CM’s in the shallower depths of Blue Grotto’s spring basin. 

On a final note, although the CM is compatible with a number of oxygen cylinders sizes all the way up something as hefty as a HP steel 32, but I would say not. I am what would be described as average height and build (5’ 9” at 175 pounds) and not at all too happy with the position of the AL 13. Despite it being the recommended cylinder size for streamlining, it added enough length to the unit’s overall profile to make it annoying when trying to take my fins on and off, or simply climbing a set of stairs or ladder to exit the water. Dive Rite recommends that people with a shorter frame might want to reposition that cylinder (or one larger) off board, perhaps under their right arm inside mount fashion. If it was my personal system, that would be exactly what I would do, thus reducing its overall profile underneath. 

Because the Dive Rite O2ptima CM is not dependent on one particular size cylinder for oxygen or diluent, and is void of a hard frame or backplate, it can become a really small carry-on for airline travel. Place a Micropore ExtendAir SR-081C cartridges in the scrubber, with two in the check bags and you’ll have 12 plus hours of diving. Alternately, you could include the self-pack scrubber for whatever granular absorbent is provided at your destination, and configure your diluent based on preference and tank availability. Ah, the possibilities.