Juergensen Marine
Mark 15 Closed Circuit Mixed Gas Rebreather
Rebreathers Making a Comeback - with Recreational Divers!
Over the last ten years, recreational Scuba Diving has become one of the fastest growing sports in the world. The Scuba or "Open-Circuit" unit is currently the dominant underwater breathing apparatus. But did you know that rebreathers were among the first Scuba units? Rebreather units have always been the apparatus of choice for military diving and now they are making a comeback at the recreational diver level.
Juergensen Marine, always on the cutting edge of diving technology, has recently acquired several MK15, fully closed, mixed gas Rebreathers units. These units are completely refurbished to "AS NEW" Condition with many features exceeding even factory and military standards.
Part One
1) How does it work?
A Closed Circuit Rebreather, such as the MK15, operates on the principal of recirculating the air that a diver breathes underwater. As most people know, our bodies do not use all the Oxygen we inhale with each breath, only part of it. Exhaled air also contains Carbon Dioxide, which is released by our bodies as a waste product. Early in the history of diving, it was determined that if there was a way to absorb this CO2, and replace the small amount of Oxygen absorbed by the body, a diver could continue to RE-breathe the same air over and over.
What a Closed Circuit Rebreather does is exactly that. The air exhaled by a diver passes through an air-permeable container called a "scrubber" which contains a chemical compound that absorbs Carbon Dioxide. The air then passes across 3 Oxygen sensors (most CCR systems use 3 sensors for redundancy). The sensors react to the presence of Oxygen by emitting voltage - in essence, they are like batteries - the more Oxygen present, the higher voltage they output.
If the Oxygen Sensors put out voltage lower than the user-definable "Set-Point" (meaning there is less Oxygen in the loop than the diver established through the initial set-up procedure) then a signal is sent to a Oxygen Solenoid, which will inject a metered amount of additional Oxygen into the loop.
Given the remarkably low Oxygen uptake of the average adult, this means that even a working diver can stay underwater for up to 8 hours with only 21 cu. ft. of Oxygen.
2) But if it adds Oxygen, how come there are still no bubbles?
When the diver breathes the air in the closed loop, the Oxygen is actually absorbed by his body, and the exhaled CO2 is absorbed by the Scrubber material. This lowers the total volume of gas in the closed loop, so when Oxygen is added, it merely replaces the gas volume lost to both these systems (the human, and chemical system).
3) How long can I stay underwater with a CCR?
An average diver can remain underwater up to 8 hours with a Mark 15, provided that they begin the dive with a full O2 tank, and fresh Scrubber material. Not too many divers care to be in the water that long, however, but that duration is about the maximum.
4) How deep can I go, and for how long?
The MK15 is the direct descendant of the CCR-1000 built by BioMarine Industries. In the 1980's a diver for Shell used a CCR-1000 on a dive of 1,800 fsw out of a lockout chamber. Since Oxygen metabolism and CO2 production are not depth-determined, the same 8 hour duration of the rebreather applies, regardless of depth. However, a rebreather diver is still subject to the SAME decompression concerns of any other kind of diver, and any CCR dives below 100 fsw require extensive training before attempted.
There is also a body of opinion that believes that due to the diameter of some of the loop components in the MK15, that it should be restricted to depths no greater than 200 fsw.
5) How do I get trained to dive a MK15?
Training for a closed circuit rebreather is, by necessity, rather extensive. These units are complicated devices that require thorough knowledge and understanding before they can be dove safely. It has been said about the MK15 that the unit can keep a diver alive under numerous failure scenario's, but it can also injure a diver under those same scenario's. The only difference between success and failure is in the training and discipline of the individual diver.
Rebreathers are not for the average recreational diver. These units are complex machines requiring extensive routine maintenance. They are best catergorized as "Life Support Systems" and should be treated as such. Training on the MK15 generally runs about 7-10 days of intensive classroom, practical, and in-water drills. Every possible failure-mode should be practiced by the student-diver. Currently, courses are offered by IANTD, TDI, and soon, NAUI.
The current recommended rebreather training is conducted by a highly qualified IANTD (International Association of Nitrox and Technical Diving) instructor.
Once initial training is accomplished, the new rebreather diver should attain at least 100 hours of dives no deeper than 60-100 fsw on the unit before attempting to go deeper.
6) What are the benefits of diving a MK15?
Herein lies the heart of the question as to whether or not you should dive a closed circuit rebreather. The benefits of diving a CCR are numerous, and for most CCR divers, outweigh the additional requirements of training, maintenance, and set-up. The first thing that most new CCR divers come to realize is the feeling of "freedom" from the time limitations of Open Circuit. All OC divers have to keep a close watch on their gas comsumption, which changes with depth. The deeper you go, the less in-water time you have. With a CCR, your gas consumption is so low, that the concerns about gas usage disappear.
The second big benefit of diving a CCR is the sheer amount of time you can spend underwater. 8 hours is a very long time. In fact, most divers usually don't care to be in the water more than 2 to 3 hours at a time. If you are shallow diving, and not subjecting yourself to large amounts of decompression, then you can spend many hours underwater with a CCR.
The third benefit of CCR diving is the quiet. Diving a CCR is one of the most amazing experiences you can have underwater. With the silence that accompanies diving a CCR, you begin to hear a myriad of sounds that you didn't know existed underwater. Since you are releasing no bubbles, you only hear your own breath sounds, and the sounds of the ocean around you. Marine animals that normally shy away from the sound of loud bubbles begin to come closer to you. For photographers, a CCR enables them to get much closer to their subjects than ever before.
There are many other topics that are generally covered in a training course, such as bailout strategies, decompression benefits, etc.
7) How much does it cost to dive a MK15?
The cost of diving and maintaining a MK15 is considerably higher than regular open circuit, and should be considered when making a decision whether to buy one.
In addition to the cost of purchasing the unit initially, there is the additional cost of scrubber material and the cost of pure O2 for the rig. This cost varies from place to place. Then there is the maintenance cost - regularly you must replace "o" rings and seals in the unit in order to maintain it in top condition. There is also the cost of Oxygen Sensors. The Mark 15 uses 3 of these, and they cost around $95 each. They last about a year, so you have to factor in an operating cost of approximately $285 per year. Parts for the Mark 15 are hard to come by, and expensive when you find them, so you should be prepared for these costs as well.
8) How much maintenance is required?
Basic routine maintenance should be performed on the units prior to any dive. Once you have done this routine maintenance, not much else is required during your diving operations. Most CCR divers perform the maintenance prior to a dive trip, and then perform only minimal daily maintenance for up to 3 weeks during diving operations.
9) How does diving a CCR change my decompression?
Presuming you are diving your rebreather with a Set-Point of Oxygen at 1.2 ata, above 154 feet you will accrue less decompression obligations than regular scuba. Essentially, the unit is mixing Nitrox for you that is variable, according to depth. Below 154 feet, you will actually incur MORE decompression obligation than a regular OC scuba diver. However, once you ascend above 154 feet, you will decompress faster than an OC diver, due to the higher Oxygen level provided by the CCR.
10) Is diving a CCR dangerous?
Yes, if you don't know what you are doing. To safely dive a CCR you need proper training, and you need to adhere to a strict maintenance schedule. These are fantastic machines that will allow you to do some amazing things underwater, and increase your diving enjoyment tremendously - but they aren't for everyone. If you are strictly a "recreational" diver, you really have no need for a CCR. But if you want to expand your diving into technical areas, such as Wreck diving, or Trimix diving, then a CCR may prove to be a valuable tool for you to use.
Part Two
The recent development of Digital Electronic Control for the MK15 Series Closed Circuit Rebreathers by Will Smithers and Kevin Juergensen adds a new dimension of simplicity and accuracty to the operation of these units.
The original CCR-1000/Mark 15 Rebreather used Analog circuits to control the set-point of Oxygen, and the Primary Display. There were several revisions of these Analog circuits, culminating in the final "Revision G" electronics which were used on the Mark 15.5 rebreather.
Basically, the Electronic Circuits in the Mark 15 Series rebreathers are designed to monitor and control the Oxygen level in the divers breathing loop to an established set-point, which is pre-programmed by the diver prior to the dive. Additionally, the Electronics control the Primary Display Unit to keep the diver informed as to the condition of the Oxygen Content in the breathing loop.
Analog electronics basically rely upon resistor arrays, and voltage dividers to control the various sub-systems of the rebreather. What this means, in practical terms, is that the entire electronic circuit must be "balanced" in all areas by use of different resistors, potentiometers, diodes, and small transistors known as "Operational Amplifiers".
Digital electronics, by comparison, take all the data presented to them by the Oxygen Sensors and convert this data into a digital signal, which is then processed in the digital environment.
Additional benefits of the new Digital system include a more dynamic determination of Set-Point, faster response, and variable "Alarm" readings.
VOTING LOGIC
The new Digital Electronics from Smithers/Juergensen incorporates what is called "Full Voting Logic."
The original Analog electronics used a system known as "Averaging" to determine the actual Oxygen content of the breathing loop. Essentially, the voltage output of the 3 sensors was added together, and divided by 3, then compared to the set-point determined by the diver in pre-dive set up. If the "averaged" output of these 3 sensors is below the set-point established by the diver, then the electronics will send a signal to the Oxygen Solenoid to open, injecting pure O2 into the loop.
Should a sensor fail during a dive, the Analog Electronics is designed to artificially assume that it is reading 47% of the set-point. It will then average the three sensors, to come up with a reading that determines O2 concentration of the breathing loop.
In the new Digital electronics, however, the system uses what is called "True Voting Logic" to analyze the input from the 3 Oxygen sensors. Essentially, the electronics are programmed to look at the output of the 3 sensors independently, and compare them against the others. Should one sensor fall outside a given parameter, established by the program in the chip (at this stage, that parameter is +/- 20%) the output of that sensor is IGNORED by the electronics, and the other 2 sensors are then averaged together.
What this does for the diver is protect them from exposure to excess Oxygen in the event of a failure of an Oxygen Sensor. It also works in the opposite direction - if a Sensor reads too HIGH, it will also "vote" that sensor out, and compare the other two.
The system in the Smithers/Juergensen electronics is dynamic, in that should a Sensor that has been "voted out" return to within the established parameters, it will then be "voted back in" and used to determine the overall Set-Point of the system.
This ensures that an otherwise accurate sensor that simply lags behind the others will still be able to be used in the analysis system of the electronics.
Additional benefits of the new Digital system include a more dynamic determination of Set-Point, faster response, and variable "Alarm" readings.
If you would like more information or if you have questions regarding the Digital Electronics of the MK15, email us with your questions!
Juergensen Marine can arrange training in Hawaii or in the mainland United States. Training involves a very intensive seven day course designed to familiarize the user with every aspect of the rebreather unit including maintenance, pre and post dive preparation of the unit, and diving the unit both electronically and manually. If you are interested in this course or would like more information email Juergensen Marine or call (814) 395-9509
Rebreather Modifications