Juergensen Marine
Mark 15 Maintenance Manual
Chapter Four
Chapter Four : Operation
Lets talk for a minute about exactly how your rebreather works. This will be a less formal discussion of the operation of the system than the earlier description of the Mark 15 operation, which mimics closely the Mark 15 manual.
Let me stress to you at this early stage the following:
YOU ARE RESPONSIBLE FOR LEARNING EVERYTHING YOU CAN ABOUT HOW THIS UNIT WORKS - YOU ARE COUNTING ON THIS MACHINE KEEPING YOU ALIVE UNDERWATER, AND TO NOT ATTEMPT TO LEARN ALL ABOUT IT THAT YOU CAN IS REALLY STUPID.
Sorry for the harshness there, but I really want to make sure you all understand how critically important this point is. Early on in my rebreather career, I listened to a supposed “expert” who gave me some completely wrong information on how my rebreather functioned - this mis-information nearly killed me in 260 feet of water. Had I been given the correct information, I would have at least known what was going on inside my rig, and taken appropriate measures to keep myself from getting seriously hurt. Learn from my mistakes, and try not to make any yourself.
The Mark 15 rebreather works on a very simple concept: The air the diver exhales runs through a canister containing Carbon Dioxide absorbent. The CO2 is leached out of the gas, which then passes across 3 independent Oxygen sensors, where the partial pressure of Oxygen is analyzed. Should the partial pressure of Oxygen present in the exhaled gas be lower than the pre-set point determined by the diver during calibration, a electronic solenoid is triggered, which injects Oxygen into the counterlung area of the center section, which is then inhaled by the diver, starting the process all over again.
Remember that sequence: Exhaust hose - Canister - O2 Sensors - Counterlung - Inhale hose.
Since Oxygen is dumped directly into the Counterlung area of the Center Section, it must be first inhaled and exhaled by the diver before it can pass through the Scrubber Assembly, and back over the O2 sensors. This, in practice, works quite well, but due to the fact that pure O2 under pressure is being injected into the loop directly before your inhalation hose, running excessively high ppO2 Set-Points is not recommended.
The Absorbent pads are located at the very top and bottom of the Center Section, and serve to absorb moisture build-up caused by the temperature differential between the inside of the Center Section, and the outside water.
That is how the loop works.
One important factor to keep in mind: Since you know the “loop sequence” as outlined above, you must realize that any water which gets into your mouthpiece during the dive, will flow into the exhaust hose. If you “blow” the exhaust hose to clear it, you are essentially forcing the water into the Canister, which contains the CO2 scrubber.
Excessive water in the scrubber material will neutralize its ability to remove CO2 from your exhaled breath, and can lead to a proportional increase in the CO2 in your loop. This can result in you experiencing “hypercapnia,” which is a decidedly unpleasant experience. It can also cause you to pass out and drown. Integrity of your loop seals (including the seal formed by your lips and mouth over the DSV) is paramount.
NOTE: IF YOU FLOOD YOUR LOOP, FOR ANY REASON, YOU ALSO RUN THE RISK OF LIQUID MIXED WITH CALCIUM HYDROXIDE FLOWING BACK INTO YOUR INTAKE HOSE, GIVING YOU A “CAUSTIC COCKTAIL” THAT CAN BURN YOUR MOUTH, THROAT AND LUNGS. PAY SPECIAL ATTENTION TO THE INTEGRITY OF ALL SEALS IN YOUR ENTIRE LOOP.
Gas System
Gas is stored in two High Pressure Vessels, we all call “Spheres,” since they are round. (duh).
The spheres are pressurized to 3,000 psi, just like most other scuba tanks, however, special precautions are necessary in handling/filling the spheres:
1) DO NOT fill the spheres beyond their rated capacity. They could explode and kill you.
2) DO NOT fill the spheres rapidly. They are very precision-made vessels, and as such need to be treated more delicately than regular scuba tanks. You should keep the spheres immersed in a bucket of water as you fill them, and fill them very slowly, keeping your hand on the side of the sphere to check for excessive heat build-up. I have heard of Oxygen spheres blowing up when pressurized too fast, so be very careful.
3) DO NOT drop the spheres. I have been told that the Inconel spheres are especially vulnerable to damage or cracking if dropped, even on a wooden deck. Handle them carefully.
4) DO NOT leave them sitting out in the sun when filled. We all know what happens to gas when it is heated - it expands. Keep a towel, or better yet, your rebreather cover over the spheres when they are sitting outside.
5) DO NOT over tighten the valve on the spheres. You will only damage it, and have to do a rebuild later.
6) DO replace the “o” rings on a regular basis. This will prevent leaks that bleed gas from your system. Remember: You only have 17-21 cu. ft. of gas in these spheres, so it isn't like you can afford to waste much.
Gas flows from the spheres through a 60 micron filter located on either side of the rebreather, depending upon which gas we are talking about.
A word about the 60 micron filter:
No one I know pays much attention to the Gas Filters located on either side of the rig. This is a mistake. Like all filters, they are there for a simple reason: To filter out junk that can get into your gas lines. I have personally seen gobs of Teflon Tape stuck in these filters, and tons of dirt and particulate matter that you would never believe could come from supposedly “clean” air. The price of the filter elements located in the barrel assemblies is not very high, and can save you a lot of grief during your dive trips. I personally advocate replacing the filter elements every year or 100 hours of dive time.
If you ever experience very low flow from either your Diluent or Oxygen lines, I suggest you first take a look at the filter. After removing the element, if there is no apparent large objects blocking the pathway, you should replace the element with a fresh one, making sure it is seated properly. If you are still experiencing gas flow problems, you may have some other obstruction in your tubing system. Remove each piece of tubing and blow it out with an air compressor - you may be surprised by what you find. I once removed a 3 inch piece of Teflon Tape from a 1/8th inch tube in a friends rebreather. How it got there is anyone's guess, but it blocked the flow of Diluent Gas so much that the diver couldn't breathe on descent.
After their respective Filters, the different gasses take different courses:
Diluent:
The Diluent flows from the filter to a “T” where it is directed to two different places. The first, is the Manual Add Valve assembly. This allows you to manually add Diluent as necessary. The second is to the Center Section, where it enters the Center Section through a bulkhead fitting. A internal 1/8th hard stainless pipe then carries the gas to the center of the Center Section, where it connects to a Schrader Valve - this is the Automatic Add Valve.
As the Counterlung collapses under increased outside pressure (as in descent, or by exhaling gas outside the loop - such as out your nose), it depresses a button located on the end of the Schrader Valve, which activates it, and allows Diluent gas to flow directly into the Counterlung.
From the Manual Add Valve, the Diluent gas runs through a manifold, which combines the Oxygen and Diluent lines into a single line that runs to an orifice located on the Right side of the Center Section. When the Manual Add Valve is depressed, Diluent gas is injected through this port.
Oxygen:
Oxygen flows through its own filter, and goes through a “T” where it is directed to two different places. The first is the Manual Add Valve assembly, which works identically to the Diluent Manual Add Valve listed above.
The second place it is directed is to a 50cc Accumulator located behind the Solenoid. The purpose of this Accumulator is to act as a storage container for a larger volume of gas that will be injected into the loop when the Solenoid fires.
Once the Solenoid is triggered by the electronics, it opens, and the Oxygen from the Accumulator flows through the Manifold into the orifice located on the right side of the Center Section.
Both Diluent and Oxygen First Stages have High Pressure Ports that run to gauges worn on the chest of the diver.
Diluent can be added in two different ways:
“Automatic” Injection:
When the counterlung collapses, due to exhalation of gas by the diver into the water, or upon descent with the compression of gas, a plastic knob located in the center of the counterlung places pressure on a plastic shaft connected to a Schrader valve. This Schrader valve is pressurized directly from the diluent low pressure input located on the left side of the center section. The intermediate pressure of the 1st stage Diluent regulator is anywhere from 130 psi to 275 psi (depending upon whether you have a Mark 15 or Mark 16).
Manual Addition By The Diver:
Located on the left side of the rebreather is the manual addition valve. This valve can be actuated by the diver at any time to manually add Diluent. It is essentially a Schrader valve, just like the “automatic” add valve. In the Mark 15, the gas, however, is ported through a combiner block, and is added to the counterlung on the Right side of the center section - the same place the Oxygen is added during automatic or manual addition.
Oxygen can be added in two ways as well:
Automatic Addition:
Once the electronics determines that Oxygen is needed, it sends voltage to the Solenoid, which then opens for about 2-3 seconds, injecting Oxygen through the gas port located on the right side of the Center Section, as you are wearing the unit.
Manual Addition:
Located on the Right side of the rebreather is the Oxygen Manual Addition Valve. This valve works the same way as the Diluent Manual Addition Valve discussed above.
What is going on inside the electronics?
Here's a primer on exactly what that mysterious round canister is doing during your dive. I have learned all of this through my building of new electronic assemblies. I'll simplify here for you non-electronic types:
The electronics package consists of 3 different circuit boards - each one with multiple functions, and each one makes sense when you realize how they all inter-depend upon each other.
One part of the circuit takes in the voltage from each of the sensors, and amplifies it. This is necessary, since the Oxygen sensors only output millivolts - very tiny voltage. In order to process the information coming from the sensors, the signal must be boosted. That's what this first part does.
The second part of the first board in the circuit is a voltage regulator. This essentially makes the rest of the circuits work, even with batteries of different strengths - otherwise, you'd have to have a battery that is brand new before each dive.
Once the voltage from the Oxygen sensors has been amplified, it is then sent along to another set of circuits that analyze the voltage coming from the sensors, and compare it against the set-point established by the diver during calibration.
Once the analysis has been accomplished, two things happen - one is that part of the signal regarding the determination of this circuit is sent to the third board in the electronics package - this turns on and off the various lights in your primary display, giving you the status of the Oxygen in your loop - the second part only happens if it is determined that the Oxygen level has dropped below the pre-determined set-point. In that case, another circuit triggers a transistor to fire the Oxygen solenoid.
This analysis, reporting, and triggering of the solenoid goes on continuously - the electronics do not take a “break” - the circuit that fires the solenoid “waits” about 5 seconds between firings to give the injected Oxygen time to work its way through the loop before adding more.
Electronic Problems
Like anything electronic, there can be problems - but after working on these things for quite a while, I have to say that they are probably some of the most robust electronic assemblies I have ever seen. There are really a lot of things that can go wrong, considering the complexity of the circuit, but in reality, there are only about 3 things that can go wrong, and most of them are repairable by the average user:
1) Bad wires:
I can't tell you how many people have come to me with “burned out” electronics, and usually it is nothing more than a broken wire in the battery terminal connector assembly. This connector takes a lot of abuse, with batteries being put in and removed all the time - if even ONE wire breaks, your unit will not function, and your Primary Display will glow like a Christmas tree. Check the wires leading to the connector, then check the pins of the connector itself - these corrode very easily - especially if you aren't careful, and somehow get salt water or salt water spray on them. Then, check the base of the wires, where they connect to the gold connector at the base of the electronics pod battery compartment. If there is a break there, you'll have to dig some of the epoxy out to try and get enough wire to make a solder-joint.
2) Water in the Bendix Connectors:
This is a very bad thing, and means that either you a) didn't screw down the Bendix connector fully or properly, or b) you didn't properly maintain the “o” ring in the Bendix connector. These Bendix cables are like hen's teeth to find, and if you screw one up, you'll have one hell of a time finding a spare - treat them right, and they'll take care of you.
3) Blown Fuses:
If you have an electronics package with fuses, you will get two different indicators for which fuse is blown. NOTE: You can only blow a fuse when you short something out. DO NOT replace a blown fuse before determining what exactly has shorted. Otherwise, you may destroy a component on your electronic boards, and will have to buy a completely new set of electronics.
There are two different indicators of blown fuses: If you blow the fuse on the Positive side of the circuit (+) the “Alarm, “1” and “1” lights will go on in your Primary Display. The same will happen if you break your Positive wire from the battery.
If you blow the fuse on the Negative side of the circuit (-), the “L” “O” and “H” lights will go on in your Primary Display. The same will happen if you break your Negative wire from the battery.
If you begin your dive, and ALL the lights go on on your Primary display, and begin to flash in random patterns, you have most probably flooded your battery compartment. You should try to get out of the water (if you can) and remove the flooded battery. You should thoroughly clean the Electronic Pod battery terminal, and try to preserve it for as long as you can. I want you to understand that anytime you get salt water on any of the electrical terminals located in the Battery Compartment, they are pretty much toast. You can make them work for a while, but they will eventually need replacement.
4) Actual Electronic Failure
This is quite rare, but sometimes can happen. I have to report that I have come across one electronic pod that had the D44H8 transistor fail (the one that fires the solenoid) - this resulted in the solenoid sticking in the “open” position. This is potentially very, very bad if it happens underwater. Fortunately, out of thousands of these machines built, this is the only one I ever heard about that had this kind of failure.
The biggest reason for looking at your Primary Display during diving operations is that by far, it is the best indicator of the overall status of your electronic systems in the rebreather. Anything going wrong within your electronic subsystem will be displayed on your Primary. Naturally, this should always be confirmed with a look at your Secondary Display to verify loop ppO2, since that is what keeps you alive.
Chapter Two - Various Models of Rebreathers
Chapter Three - General Maintenance of Systems
Chapter Five - Keeping it Working
Chapter Six - What to Look Out For - (Warning Signs)
Chapter Seven - Personal Philosophy of Handling Emergency Situations
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