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The need for a BOV, short for Bailout Valve, remains a hot topic among CCR Divers. If you have one, congratulations. You have enhanced your underwater safety and don't need to keep reading. If your rebreather isn't fitted with a BOV, you may want to read on.
All rebreathers come equipped with a Dive Surface Valve (DSV), a special type of mouthpiece that the diver can manually open when they want to breath on the loop and close when they don’t. The primary purpose of this valve is to keep water from entering the unit when the mouthpiece is no longer in the mouth. Introducing water into the system can lead to very bad things.
The difference between a DSV and a BOV is that the latter incorporates a 2nd stage regulator into the valve. When the valve is put into the closed position (sealing off the loop), the passageway in the mouthpiece is rerouted to the 2nd stage regulator, which can be used by the diver as an alternative breathing apparatus.
The rationale for a BOV are quite simple. Should the rebreather be compromised to the point where the diver can no longer breathe from it safely, the 2nd stage can be activated with a quick flick of the wrist - either in the form of turning a knob or pulling down a lever in front of the BOV. This single motion seals the gas passage from the loop and simultaneously delivers a known breathing gas from either the onboard diluent supply or an off board tank. In an emergency situation, that single motion can greatly reduce stress, as it does not involve the removal of the mouthpiece to get to a breath, and switching to a known gas source can prevent the situation from getting out of control.
As an experienced (15 years) CCR diver, I am well aware of the risks more inherent to this piece of equipment. A flooded loop in one instance resulted in a severe injury from a caustic cocktail. (You can read about that here). If it weren’t for the fact that my rebreather had a BOV, the situation, which rendered my rebreather completely out of commission 90 feet underwater, could have been a whole lot worse.
So yes, I am a big advocate for having a BOV to the point that I will never dive a rebreather without one. I realize that this viewpoint is not universal within the rebreather community, as it becomes abundantly clear if you browse important technical diver forums like RebreatherWorld.com or CCRExplorers.com. Going through these sites, I am continuously floored by some of the pundits who argue that neither they nor anyone else need bother with a BOV. They offer all manner of flimsy reasonings, ranging from being too expensive (as if rebreather diving was cheap to begin with) to justifications such as “the manufacturer of my unit doesn’t really believe in them, and that is good enough for me.” In all seriousness, that has to be the worst reason I've ever heard for not adding a BOV.
To be fair, there are some slightly better arguments against BOVs, such as the idea that the work of breathing on a BOV is not as easy as it is on some DSV’s. This often comes into play on discussions of CO2 hits caused by hypercapnia. But in reality, most CO2 hits can be tied to user error, ranging from not packing a scrubber properly to overusing the sorb beyond effectiveness. Not providing proper maintenance on various parts of the loop such as the mushroom valves in the mouthpiece can also cause CO2 levels within the loop to become dangerously high.
But (and this can be a big hairy one), even when everything is done right, bad things can still happen. And should it involve a diver becoming so incapacitated through shortness of breath due to a CO2 hit, the additional time and effort needed to remove their DSV and change to an open circuit supply could prove lethal. Experienced rebreather divers who have actually been down that road have described the process as being one of the most difficult acts to perform, as the mind has difficulty processing the information, and each second the switch is delayed which further exacerbates the situation. In comparisons, the flick of a switch on a BOV becomes the most workable solution.
As a good friend of mine who knows the unpleasantness of a CO2 hit all too well told me “having that ability to make an instantaneous gas switch from CC to OC, likely did more to avoid a bad situation from becoming a fatal one.” He further adds, “poor breathing or not, as compared to the alternative of staying on the loop a second longer or attempting to breathe water, you will see that BOV is a thing of beauty, and your best friend.”
While you might be able to say that you never had an issue on CCR, even after 100’s of hours logged, there will come a time when the defecation hits the fan. At that point, all your prior training and experience can go right out the window. And the ability to flick a single lever and take in a life-giving safety breath makes a BOV seem like a really good option. And even if the event isn't life threatening, it's awfully convenient to be able to switch over for a few quick “sanity breaths” whenever something doesn't seem quite right. Finally, and contrary to what some might claim, BOVs are not difficult to maintain.
There are many arguments in favor of incorporating a BOV into your breathing loop. From personal experience I feel having one is the most import piece of safety equipment you can have on a rebeather. I encourage you to study the facts impartially, but at the end of the day, it's your choice.