To begin to answer that question lets list those things that can kill you on a CCR:
- High PPO2 resulting in possible cns seizures, followed by probable drowning
- Low PPO2 resulting in possible asphyxia, with death the most likely outcome
- Hypercapnia, possibly resulting in the divers inability to respond and rectify the situation, followed by death
- Full flood of the breathing loop, resulting in possible caustic burns or drowning
Are these potentially fatal dangers possible on open circuit?
- High FO2 improper choice for the target depth has resulted in an excessive PPO2 resulting in seizures and drowning
- Low FO2 mixtures are used in Trimix courses. A mixture below 19% is considered hypoxic and training reserved for divers meeting specific
prerequisites and dive experience. Not analyzing a bottle or incorrect gas switch has resulted in death.
- Hypercapnia can be experienced but is most often not fatal (as it often is in CCR use) but results in headache, nausea, fatigue and reduced
performance only.
- Full flood can also be experienced as the result of a damaged regulator diaphragm or damaged exhaust diaphragm.
If the incidences that occur and have caused death on CCR are also possible to occur on open circuit, WHY is the fractional incident rate appear to be much higher on ccr?High PO2
When diving open circuit PO2 changes according to the decent / ascent rate or in some cases at a gas switch. It takes 2 things to affect PO: TIME and MOTION. Time to travel through the water column, changing ATA, thus effecting PO2 change. On open circuit, the diver is breathing a “stable” preset mix. On a ccr the diver is breathing a dynamic mix, in other words; a constantly changing mix, the FO2 constantly changes. Following we will examine how TIME and MOTION can work against the CCR diver and compound an existing problem. This makes it vital for the CCR diver to monitor PO2.In ccr diving excessive oxygen can be moved into the loop due to a component failure and PO2 change quickly without ANY change of depth. But to alleviate unfounded fears, the diver does indeed have sufficient time to correct the malfunction AND has successfully accomplished this task many times during the CCR course.
Example: An
open circuit
diver at 100 fsw has an ATA of 4.03, if that diver descends 10 ft they will now have an ATA of 4.3If the diver is breathing .32 eanx the PO was; 1.28 at 100 fw and at 110 ft has gone to 1.37The change in the open circuit divers PO is unlikely to cause any symptoms what so ever, much less induce seizures, or death..Now examine the same 10 ft drop in depth with an unobservant diver on CCR that has a malfunction:Example:
A diver, again at 100 fsw but on CCR, erroneously does not monitor PPO2 readouts, which is key to safe CCR diving and hammered home repeatedly during every CCR course.PPO2 has risen to 1.5 over the course of the last 5 or 6 minutes. The diver drops 10 ft to 110 fsw the PPO2 is now above 1.6 and the cause of the PO2 spike was due to a runaway injection of O2 could very quickly reaches 1.9 or above. This potentially deadly incident, in my judgment, was
caused as much or more so by user error in failure to monitor the equipment than the failure of the component.
User error by failing to monitor, is
perhaps the most likely cause and absolute downfall in the majority of CCR incidents.
And surprisingly there is at least one additional physical sign presenting that any astute open water diver would notice IMMEDIATELY. This sign is taught and practiced in the basic ccr course also.True examples reporting indirect cause of death in CCR incidents: did not turn unit electronics on, was in habit of checking PO2 infrequently,
began dive with inadequate gas supply, failure to turn O2 on and many more like known causes. The pre-dive check covers these items as failure
to perform them can cause injury or death. However, even failure to perform a necessary task at set up is very unlikely to cause death IF the diver MONITORS the equipment. For instance; the diver misses turning on the O2 bottle, there are TWO readouts that will warn him, if he follows standard protocols: 1) check spg 2) check PO2 and even a 3rd stop gap: pre-breath the unit for 3 – 5 minutes while monitoring the microprocessor readouts.
This pre-breath is standard operating procedure. In essence:
the diver’s failure to follow training and operational protocols!Low PO2
Low PO2 happens slowly as the diver metabolizes the oxygen in the mix. IT TAKES TIME. There is absolutely NO reason for this to result in death, but it does. Reasons: # 1: diver does not monitor PO2. Other reasons include did not turn on oxygen, not a fatal error either, if one MONITORS the PO2 readout. Most rebreathers have 3 sensor readouts on 2 separate microprocessors. So there are 6 readouts of the divers PO2. That alone
should send an important message to the diver of importance to monitor,
still divers fail to do so
. Low PO2 often happens at or near the surface where ambient pressure is reduced and the diver is often working harder than while on the relaxing dive. Diver feels like ” the dive is over” stops
proper monitoring of PO2 metabolizes O2 in the loop and dies due to asphyxia.
Again failure by the user to follow known and established safety
protocols for operation of the system.
And there is at least one physical sign presenting that even an astute open water diver should notice IMMEDIATELY. This sign is taught and practiced in the basic ccr course also.Hypercapnia is rather easy to address; although it is possible to have a failure in the manufacturing process of scrubber material, I have never heard of one.In most cases that hypercapnia is suspect, there is a diver history of using the scrubber material much longer than the manufacturers
recommendation, some cases 2 x and 3 x longer.
Again failure by the user to follow known and established safety protocols for operation of the
system. Why would anyone spend $8,000. on a ccr and 1500. on training and die attempting to save $10?
Changing scrubber material is the simple and safe solution to hypercapnia.Full flood of the loop
The action required is quite simple: bailout to open circuit and if that was the end of the incident this would almost be a non issue. However, once again the diver due to ego, waste of gas, not wanting to end of dive or other reasons decides to continue the dive. The situation is a bit more complicated than that if the diver follows proper protocol and bails to open circuit. EVERY CCR diver is to know and understand rmv and what their personal rmv is. Dive planning can be an issue here. Was a dive plan devised and adequate gas with the diver for bailout in the worst possible point of the dive? If not, user failure played a key role in the incident.What if anything can be done to reduce or eliminate the incidence rate in CCR use?Training and experienceThe system almost “runs itself”, and IS indeed quite simple to use. HOWEVER, faithful monitoring, understanding what the machine is telling you and correct response to warnings is REQUIRED. Interpreting the microprocessor must become intuitive, this takes TIME AND experience on the unit.As my engineer dive buddy reported; ” learning ccr is a huge learning curve”. It is not however complicated or difficult. Once a diver has completed his course he has sufficient knowledge and skill to begin diving a unit safely at recreational levels. He does NOT have sufficient experience to dive his unit to the depths and in conditions he has experienced in open circuit diving. Time and time again, this has been a fatal error: “I’ve done it in open circuit, so I can do it on my ccr”. If a diver has been to 180 ft on open circuit SOME feel they should be capable on CCR. Other divers, open circuit trimix trained, feel they should be able to dive CCR Trimix. This thinking has proven fatal also and a portion of the basic CCR course addresses these issues and instructs the new CCR diver to gain experience SLOWLY and to follow up with advanced CCR training. There are changes in the information that must be processed and the responses to those conditions by the diver.This is where others have died thinking it did not apply to them.ALWAYS know your PO2: if you know your PO and maintain PO2 within normal and accepted CCR parameters, you have eliminated two of the most frequent causes of death on ccr; high and low FO2.Would you analyze a mix with an analyzer that had only a blank screen? Not checking your PO2 readouts is just that. It is a “dynamic” mix, mixing as your use it. It is IMPERATIVE that you check you microprocessor screen and assess the PO2 readouts per training protocols.Plan your Dive: I feel divers are lulled into complacency due to the bottom time that is afforded them on rebreathers along with the experience they have on the unit. Proper dive site assessment, team selection, communications, equipment maintenance and allocation is still mandatory for the safest possible dive. While on the dive it is mandatory to follow up with equipment checks during the dive; spg readouts along with PO’sContinue to practice and train: dream up new skills and drills with fellow CCR divers and Practice them along with those you’ve learned in your course. Remember the least learned skill will be the first to go in a stressful situation, make certain the one you MAY need is not the one you failed to practice.ComplacencyLast of all but as important as ANY other issue is complacency. Don’t get complacent. Continue to dive the unit, practice skills and train.NEVER forget that given the opportunity by YOU, the machine can kill you. Sadly enough it’s warning you that it’s doing such during the process, but no one is listening