I know there are lies, damn lies and statistics, but I read this stat that divers using rebreathers are 900 times more likely to suffer a fatal diving accident that divers using open circuit (see page 3 of
http://www.deeplife.co.uk/files/How_Rebreathers_Kill_People.pdfhttp://www.deeplife.co.uk/files/How_Rebreathers_Kill_People.pdf).
Some statistics seem so skewed as to strike me as instictively "must be b****cks". But the rebreather world is complete alien to me. Does anyone with more knowledge have any views?
Couldn't access the link, but I've read enough similar statistics to have a pretty good idea of what it says. Either way, here's what I have to say on the subject.
I've lived in South Florida my entire life and have been diving for over 23 years. I was certified as a PADI Instructor in 1986...I was certified as a TDI Trimix Diver in 1995, and I've logged thousands of dives throughout the years. In September of 2007, I purchased a Dive Rite Optima Rebreather and completed the course required to attain my IANTD Optima Rebreather Diver certification. Since then, I've exclusively been diving my rebreather practically every weekend and have logged in excess of 80 hours underwater, none of which was logged on long dive trips where it's easier to rack up the hours. I've also unfortunately been on a dive boat this year when a rebreather related fatality occurred.
Please bear in mind that my experience has exclusively been using the Optima, which is a fully closed circuit electronic rebreather. However, I do regularly dive with several other rebreather divers who use different rebreathers. So here are my observations on the subject:
If you ask me whether I feel as though the probability of suffering a fatal accident while diving a rebreather is greater than while diving open circuit, I would have to unequivocally say yes. However, if I’m provided the opportunity to expand on my answer, you might be surprised at the answer.
Rebreathers are riskier, yet in many ways they are safer. Here's what I mean:
Traditional open circuit equipment provides the average diver with no emergency recourse in the event of a gas restricting emergency, other than their buddy's air supply or an emergency ascent. However, all rebreather divers I know enter the water with some type of independent open circuit gas supply or bailout system, exclusively dedicated for emergency gas restricting or gas altering emergencies. Furthermore, most divers have their bailout gas supply system directly plugged in to a b.o.v. (bailout out valve), which is built into the same mouthpiece they are breathing, and only need to flip a switch to be immediately connected to their bailout system, so is that particular difference less safer than a pure open circuit diver, I think not.
Open circuit divers are supplied with a gas source of consistent composition, which is very inefficient at most depths, and they either have it supplied to them or they do not. However, rebreather divers breathe on a loop which is composed of a multiple gases, one being pure oxygen and the other being a carefully selected diluent, often air. The diluent is injected manually into the loop by the user or can be set to be injected upon negative loop pressure or breathing demand via the ADV, which is usually what is done on the descent portion of the dive. The O2 can be injected manually by the user as well, or can be injected electronically, based on the constant partial pressure selected by the user within the controlling electronics of the rebreather. The effect of the injecting of these gases contributes to making up the composition of the breathing loop and are monitored via the use of multiple independent oxygen sensors, usually at least three, but sometimes more. The monitoring of the users oxygen sensors at all times is the number one rule for a rebreather diver, because oxygen is what sustains life, and by monitoring it closely, you will have ample time to deal with any problems which might lead to a either deprivation or excess of oxygen in the breathing loop, which can both be bad. Consequently, in the event of a oxygen depriving or excess oxygen emergency, which would normally be spotted by the user long before it ever became a problem, the user has several options for dealing with the situation (i.e., manual addition of O2, electronic addition of O2, manual addition of diluent, flush the loop out and replace it with diluent, bailout to open circuit, etc.) Furthermore, many rebreathers come with backup systems that will further alert the user to a problem or a life threatening drop or increase of the partial pressure of the oxygen within the breathing loop, by way of flashing lights and vibrating mouthpieces. In addition to that, many rebreathers will automatically inject oxygen into the breathing loop, if the partial pressure of the loop approaches non-life sustaining levels. So therefore, are rebreathers more complex than open circuit, sure they are, but also they do provide the user with multiple ways in which to deal with any drop or increase in the life sustaining oxygen content of the breathing loop, which if checked, maintained and monitored, is actually extremely safe.
Which brings me to what in my opinion is the cause of the data pointing out the statistical difference between fatalities in rebreathers as compared to open circuit. Here's the bottom line - EXISTING REBREATHERS ARE NOT FOR EVERYONE.
I say rebreathers are not for everyone, because all the advantages that come with the efficiency gained by using a rebreather, also come with a cost, and I'm absolutely not talking about a financial cost. The cost I'm referring to is the increased time and diligence that must be applied to the use of a rebreather. Most divers probably spend, on average, one hour preparing their rebreather each time they are going diving. That preparation time includes assembly of the unit, packing of the scrubber, lubing and testing the seals and o-rings associated with the breathing loop, calibrating oxygen sensors, changing and or testing batteries, adjusting the rebreather's computer(s), etc. Much of this preparation is performed at home and some is performed at the dock or on the boat. Also, most divers perform some manner of last minute testing or double checking of all systems, once they've donned the rebreather and are ready to hit the water. And as previously mentioned, underwater, the user is constantly monitoring his oxygen supply and adding gas to his loop. This level of attention and self-disciplined diligence that is required to safely dive a rebreather is not something that everyone possesses the ability to keep up on a consistent basis, which leads to short-cuts. Unfortunately, short-cuts with rebreathers can be life threatening, which is why the current generation of rebreathers should only be used by those that have the self-discipline and diligence to take all the required steps to ensure safe operation of their rebreather.
I think if you took a detailed look at each of the fatal accidents comprising the statistical data used to derive the comparison within the link you've provided, you'll most likely find that the use of the rebreather didn't lead to the fatality, but instead, the user's inability to approach the preparation and use of the rebreather in the diligent and detailed manner in which each user was trained was most likely the primary contributing factor causing the fatality.
It is my humble opinion that if the use of a rebreather is consistently approached according to the training each user receives from their respective instructor/training agency, then the probability of suffering a fatal accident while diving a rebreather is no greater, and in many cases even less than diving open circuit.
Just my 0.02 cents!
