Juergensen Marine's

MK 15 Manual

A Mark 15 Manual written by Kevin Juergensen.  Kevin owns and dives a MK15.5 in the course of making underwater films and he also remanufacturs MK15 rebreathers to like new (or better) condition.   See also: Kevin's mods and batteries.

Disclaimer...  This manual should not be viewed as a substitute for training from a qualified agency or individual.   Rebreathers can kill you, get trained.

-The Rebreather Website webmaster


The purpose of my writing this manual was to aid those who are starting to dive the Mark 15/CCR-1000 Closed Circuit Rebreathers. Over the years, I have come to realize three things: 1) There is a complete dearth of information out there on how one should properly maintain these machines, 2) How many divers overlook some of the important maintenance procedures necessary to keep the units functioning at top shape, and 3) The general level of MIS-information out there on how these systems actually work.

I decided, once I started refurbishing surplussed Mark 15's, to write this manual detailing many of the nuances of rebreather maintenance. The surplus rigs I have restored were maintained in such poor condition, that it is really surprising that most of these rigs didn't end up on the sea-floor attached to dead divers. The fact that they didn't, is more a testament to the ruggedness and reliability of the Mark 15 rebreather, than an endorsement of poor maintenance.

Bear in mind, that most of the procedures you'll read in this manual are confined to the Mark 15/CCR-1000 - it is not meant as a general rebreather primer.

It is my hope that this manual will aid those of you with these units to keep them running for a very long time, and to dive them safely. If you have any questions or comments, please feel free to contact me via e-mail.

Kevin W. Juergensen   

Table of Contents

1) The Mark 15 Description

2) Various models of CCR’s

3) General Maintenance of systems:

a) General Maintenance

b) Routine Maintenance

c) High Pressure Line Maintenance

d) Electronics Maintenance

4) Operation

a) How the Loop Works

b) How the Gas Transport Works

c) How the Sensors Work

d) How the Displays Work

e) How the Electronics Work

5) Keeping it Working

a) Preserving a sealed loop

b) Preserving Gas Transport Integrity

c) Storage and Handling of Sensors and Sensor Wires

d) Proper care of Displays

e) Proper care of Cables

6) What to Look Out For (Warning Signs)

a) Leaks in the loop

b) Leaks in the gas transport

c) Failure of Sensors and Sensor Wires

d) Failure of a Display

e) Failure of a Cable.

f) Failure of the Electronics

g) Failure of the Solenoid

7) Personal Philosophy of Handling Emergency Situations

a) Leak in the loop

b) Failure of a Sensor

c) Failure of a Display

d) Failure of the Electronics

e) Failure of the Solenoid

f) Failure of Gas Delivery System

g) Blown Lines

Mark 15 Maintenance and Operation

The Mark 15 Closed Circuit Mixed Gas Rebreather was built for the US Navy by a couple of companies, most notably BioMarine Industries, which later became known as Rexnord, Inc.

Essentially, the Mark 15 was used for EOD work in the Navy - that is, Explosive Ordinance Disposal. They wanted a device that was quiet, and emitted no bubbles. Later, they required a device that was non-magnetic - so they developed the Mark 15.5 and later, the Mark 16.

Chapter One : Description of Operation

The Mark 15 - Mod "0" is a closed-circuit, mixed gas rebreather which re-circulates the diver's respiratory gas. At each breath the exhaled carbon dioxide is removed. Oxygen is mixed with a diluent gas, to maintain a pre-selected absolute oxygen level, permitting operation at great depth.

Compressed air, Trimix or Heliox can be used as a diluent, depending on the depth of the dive, and the preferences of the diver.

A man generally consumes between 0.4 and 2.0 liters of oxygen per minute, depending on his activity level. Tests have shown that over an extended period of time, a working diver will consume an average of about 1.2 liters of O2 per minute. Therefore. for a six hour dive about 432 liters (or approx. 15 cubic feet) of oxygen is required to meet the respiratory needs of the average man.

The Mark 15 provides approximately 21 cubic feet of O2 at a charging pressure of 3000 psi. thereby providing adequate reserve for a six hour dive.

The Mark 15 controls the partial pressure of oxygen supplied to the diver to +/- 10% of the set point. This set point is selected during the calibration procedure and may vary from 0.5 to 1.3 atmospheres.

For most diving applications, an Oxygen partial pressure of approximately 1.2 is recommended.

Assuming appropriate decompression and support equipment is available, the unit can be used for up to 6 hours of diving, independent of depth. The record depth for diving a unit of this kind was set by a diver working out of a saturation lock-out bell at 1,800 feet.


Functional Description Of The Mark 15 (Refer to Schematic 1)

The diver exhales into his mouthpiece (1) and his exhaled breath passes though the exhalation hose (2) over a moisture trap (3) and then through a carbon dioxide absorbent bed (4) where the carbon dioxide is removed. The gas next passes over the oxygen sensor assembly (5) containing three sensors. The gas then passes into the diaphragm (6) and back to the diver via the inhalation hose (7).

The Primary Electronics (28) powered by the battery (8) and activated by the switch (27) computes the value of Oxygen in the breathing gas by averaging the limiting the output of the three sensors and then displays this value on the Primary Display (9).

Once every five seconds, the electronics compares the average sensor output with the set control point and, when required, opens a solenoid (10) in the oxygen supply line to admit a pulse of Oxygen to the system. This pulse of Oxygen flows from a small accumulator (11) which was pressurized via a flow control orifice (12) with gas at a regulated pressure of approximately 175 psi above ambient by the 1st Stage Regulator (16).

The Oxygen is stored in a spherical tank (13) fitted with a manual shut-off valve (14). In the event that the diver wishes to manually add Oxygen to the breathing gas, he can bypass the solenoid by depressing the Manual Add Valve (17). To allow the diver to monitor the supply pressure in his Oxygen tank, a gauge (18) is provided

In order to keep the diver’s breathing gas mixture at a pressure which is equivalent to his depth, the pressure of the breathing gas is referenced to the water by the CounterLung (6). As the diver descends, the increased water pressure operates a Diluent Automatic Add Valve (19) maintaining the total pressure within the system at ambient water pressure. As the diver ascends, the water pressure lessens, and the Overpressure Relief Valve (20) is opened, permitting breathing gasses to vent to the surrounding water.

The Diluent gas is contained in a spherical tank (21) with a manual shut-off valve (22) and 1st Stage regulator (23). A Manual Add Valve (24) allows direct addition of Diluent to the breathing mix. A Pressure Gauge (25) is provided to indicate the tank pressure.

The Secondary Display (26) does not require battery power to operate. The power required to read each sensor is derived directly from the sensor itself. The display also indicates the automatic control system battery voltage (plus and minus).


Chapter Two : Various Models of Rebreathers

What’s the difference?

MARK 15 - MOD 0 (Also Civilian CCR-1000)

The Mark 15 Mod 0 utilizes a stainless steel Center Section assembly, and stainless/plastic canister for CO2 scrubber. It also used flexible hoses for both the High Pressure, and Low Pressure lines running from the 1st stages. The 1st stages themselves were placed in the unit "upside down" - or with the tightening nut facing the back of the unit. To load the spheres (which were made of Teflon coated steel) you raised the 1st stages out of the unit, inserted the valves from the spheres, tightened them down, and placed both sphere and 1st stage back into the unit.

The Center Section of the Mark 15 also incorporates a rather low-tech method for gas sealing. Basically, it uses two "rubber bands" that are placed around 1) the Canister, and 2) the Cover. The first, isolated the "dirty" air that had to pass through the scrubber from the "clean" air in the counterlung. The second, sealed the Canister/Counterlung compartment from sea water.


MARK 15.5

With the introduction of the Mark 15.5, there were some notable improvements, the most notable being the incorporation of a newer, plastic Center Section. It was larger than its predecessor, and had a much more engineered sealing system, using large "o" rings instead of rubber bands. It also had larger bore Inhale and Exhale fittings to accommodate larger bore hoses.

The next improvement, was the elimination of the flexible hoses in the unit, which were replaced by hard tubing. The 1st stages were also "flipped over" and mounted to the unit, eliminating the need to remove them to install the spheres. The Spheres themselves were changed from Teflon-coated steel to Inconel - a non-magnetic metal.



The Mark 16 was the last (so far) to be developed for the Navy, and is still in use today. It had a larger, deeper case, was completely hard-plumbed, had separate O2 and Diluent Addition Ports in the center section (both ports are combined via a manifold on the 15 and 15.5), and also was the first of the units to have true "Voting-Logic" in the electronics.

The Navy also required that the "on/off" switches be removed from the Mark 16 - I have no idea to this day why they did that. The O2 solenoid was also removed, and in its place a piezoelectric solenoid was built in. This eliminated the need for the 50cc Oxygen Collector that is found in the 15 and 15.5’s.

The 1st stages were also modified to operate at higher intermediate pressures. The Mark 15 and 15.5 had intermediate pressures of 130 psi. The Mark 16 regulators were tuned to 275 psi nominal. This allowed the exclusive use of 1/8th inch tubing throughout the Mark 16.

The Mark 16 is also the only unit to have built-in external "knobs" for turning on and off the O2 and Diluent valves on the spheres.

Chapter Three : General Maintenance Of Systems

Section One : Maintenance Overview

It is not my intention to teach you how to dive one of these machines, only to give you some of the insight gleaned from diving, maintaining, and repairing these rigs over the last few years. You should pay very close attention to all the maintenance tips given in the next few pages. You should familiarize yourself with all parts of the unit, and refer to this document to help you better understand how each part works with each other.

I highly recommend as part of your training that you receive detailed information and "hands-on" experience in taking your unit apart, repairing what needs to be repaired, and putting it back together. This training should come from someone who is well acquainted with the operation and maintenance of the Mark 15 rebreather. Currently, I only know of about 3 or 4 people who fall into that category, outside the Navy - so be careful who you choose as an instructor.

With that said, I’ll add this pearl of wisdom:


By developing a "ritual" of maintenance of your unit, you will increase your knowledge of it, and continue to enjoy successful dives with it.


I can’t say it any simpler than that.


Section One : General Maintenance

Before each diving excursion, you should completely prep your rig for the upcoming dives. The following procedure is what I do each and every time I prep my unit:

• Completely remove hose/mouthpiece assembly and clean thoroughly.

• Completely remove center section and counterlung - clean both thoroughly.

• Remove, clean and re-lube all "o" rings in center section assembly.

• Apply thin coat of lube to counterlung surface.

• Inspect all "o" rings in pressure assemblies.

Since my dive trips usually last 2 to 3 weeks, with up to 4 dives per day, I usually go a bit farther than the above list, and include the following:

• Rebuild 1st stage regulators.

• Remove/lube or replace "o" rings in Bendix connectors.

• Remove/lube "o" ring located in counterlung overpressure relief valve.

• Rebuild mouthpiece with new "o" rings.

Basically, let me give you my philosophy regarding rebreather maintenance:

First of all, your rebreather is a "Life Support Device" - that means that it is responsible for keeping you alive underwater. As I have said before, these machines can save your life in about 5 different ways, but can kill you in a dozen different ways as compared to open circuit.

I keep that fact in mind when I maintain my rebreather. Before a trip, I will completely tear down the unit, and perform all the maintenance listed above, like a religion. But once ON the trip, I hardly touch my unit at all, except to dive it. The reason is very simple: If you have done all the maintenance and preparation of the unit properly before the trip, you shouldn’t have to touch it for quite a while, maintenance-wise. Naturally, you’ll be drying out your absorbent pads after dives, and cleaning out the accumulated gunk that seems to live in your hoses, but other than that, you shouldn’t have to do a single thing, except fill your spheres, and change your scrubber every so often.

Try not to become the kind of rebreather diver that constantly pores over his rig on a dive trip, attempting to catch up on maintenance that should have been done in the shop back home. First of all, working on a boat is a pain, and unless you carry a really large bag of spare parts and miscellaneous stuff, you’re probably not going to have the right part for the job anyway.

The Navy did a good job for us, designing a very simple, hearty rig. We’ve made improvements on their design to make the unit more rugged, so it should serve you many years and many dives.


Section Two : Routine Maintenance

(NOTE: Your instructor may add to this list, depending upon their personal experience - at all times, take the advice of your qualified instructor).

The following is a description of routine maintenance that you should undertake at the end of each dive day:

1) Remove hoses/DSV (Dive Surface Valve) from rig - wash out hoses with some kind of disinfectant.

2) Remove cover of Center Section - remove absorbent pads - rinse them in disinfectant.

3) Allow hoses/DSV and pads to air dry overnight.


The following should be done after every dive trip:

1) Wash down rig completely while still assembled (close the DSV)

2) Remove hoses/DSV from rig - soak them in disinfectant.

3) Remove absorbent pads from rig - soak them in disinfectant.

4) Visually inspect all Bendix Connectors and cable assemblies. Look for any water intrusion, or damaged/worn "o" rings. Spray some contact cleaner/lube in the end of each cable - replace/re-lube "o" rings, re-attach cables.

Once again, I’ll note that I go farther than this normally, including the removal of the Center Section, and Counterlung, with the Counterlung getting a thorough cleaning and re-lube as well.


If you are not sure which side is which, remember this: Put the DSV in your mouth and place your hands over the open ends of the hoses - remember "You have the RIGHT to breathe" - simply, that means the Intake should be on your Right side as you inhale, creating a vacuum against your hand.

Another "Juergensen-ism" is this: Whenever you are diving your rig, and can’t recall which side the Oxygen Manual Add valve is on, just say to yourself "Oxygen is the RIGHT gas to breathe"... Got it? Good.


There are many critical parts of your rebreather, but one stands out as a particularly vital part, the failure of which could go unnoticed by the diver during the dive, and potentially cause disastrous results. You should pay special attention to the following paragraphs.

Located inside your Center Section, you will find the "Cannister Seal" - this is essentially a big rubber band made of either an inner-tube tire, or natural rubber. This seals the Cannister from the rest of the Center Section, and prevents exhaust gasses from passing around the Cannister into the intake breathing loop.

The natural rubber bands that were original equpment on the Mark 15’s had a very bad tendency to snap after installation. Often, this could happen long after they were installed, and the case closed. It occurs when the band becomes old, or is subject to contaminants, such as air pollution and solvents.

Most Mark 15 divers have replaced this natural rubber with a piece of a truck tire inner-tube. While not very elegant, this solution works quite well. Inner-tube rubber is very tough, and can last longer in service than natural rubber.

However, this band is vital to keep exhaust gasses from mixing with "fresh" air in your counterlung. Should this band break during a dive, the diver may be subjected to a condition known as "Hypercapnia" where CO2 laden exhaust gas is mixed with scrubbed air, resulting in a cycle where the overall CO2 content of the breathing mix increases. Hypercapnia can result in confusion, loss of consciousness, and even death.

It is recommended that this Cannister seal be replaced by the diver during Routine Maintenance at least every 3 months. Further, when not in use, the Cannister Seal should be removed from the unit and stored in a plastic bag to prevent relaxing of the rubber compound, and deterioration from air pollutants and solvents. Failure to follow this recommendation can result in severe injury or death.

Make sure to check this band frequently - in my opinion, it is one of the weakest links in the Mark 15 system, and should be treated with care and attention.

Now lets move on to a discussion about how your rebreather works during a regular dive.


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 (which is included in this package).

Let me stress to you at this early stage the following:



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.


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:



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 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 5 : Keeping it Working


1) Preserving a sealed loop

2) Preserving Gas Transport Integrity

3) Storage and Handling of Sensors and Sensor Wires

4) Proper care of Displays

5) Proper care of Cables


Keeping your rig in top condition involves a lot of work, but thankfully, only in short bursts. If you adhere to a regular schedule of maintenance, you can look forward to may successful hours of diving your unit trouble free.

The following is my suggestions for maintaining your rebreather at its peak.


Section One: Preserving a Sealed Loop

Making sure your loop maintains its integrity is not as hard as you would imagine, but it is surprising how few people do it.

Lets start off with the closest part of the loop to you - the Dive Surface Valve (mouthpiece).

The DSV is actually the source of most leaks in the loop. It’s maintenance is good for a number of reasons, not the least of which is the fact that it spends most of its life in your mouth.

Since the DSV lives in your mouth, it, by its very nature, accumulates most of the goobers and gunk that you exude from your bodily orifice. This can have a detrimental effect on the "O" ring seals that live inside the DSV. As such, I completely disassemble and rebuild the DSV after every dive trip. I don’t mind saying that I am constantly amazed at what comes out of this thing...

You will go a long way toward preserving the integrity of your loop by taking the time to rebuild your DSV periodically.

The second part of the loop that needs attention is the hoses. These are rather simple devices, and do not represent a very complicated sealing issue. I generally will re-seal the hoses after a thorough cleaning by using only a tiny amount of lubricant over the inside diameter of the ends of the hoses. Too much, and you run the risk of the hoses squirting out from their connectors when pulled. If you use NO lubricant, they will seal fine, as well, but you then run the risk of "bonding" of the rubber to the mating surface of the connector, thus lowering the life span of your hoses themselves.

The last part of the loop is the Counterlung. The Counterlung is actually much more complex than what appears at first glance. It consists of several parts, including "O" rings that help seal it from the outside water.

The Counterlung is made of a particularly pliable rubber compound that easily binds to the stainless steel center sections used in the Mark 15 rebreathers. Because of this, and the sensitivity to other airborne compounds (such as hydrocarbons emitted by gas and diesel engines), I always coat my counterlung with a thin film of Silicone lubricant before re-installing on my Center Section. I personally don’t believe that the Silicone represents a fire-hazard in the Oxygen environment of the Center Section during diving, so I don’t waste my time with O2 clean lubricants for this task. However, you may wish to use Crystolube, or Halocarbon if you are an O2 clean purist.

Inside the center part of the counterlung is where the Overpressure Relief Valve lives. I make a point to always take this apart when cleaning the Counterlung, since that is the place where many expelled goobers make their way into the open sea. Lung-goobers seem to be the worst, and can cause sticking of that valve. All it takes is a few minutes to take it apart, clean the valve parts, and re-lube the "O" rings. This will ensure proper action throughout its life span, which can be many, many years.


Section Two: Preserving Gas Transport Integrity

The Gas Transport of the Mark 15 is comprised of several stainless steel tubes, hoses, and valves, each of which can experience problems over time. You can prevent these problems from occurring through good maintenance of the system as a whole.

One thing you will notice, if you are diving in salt water, is that many of the couplings will show signs of rust on occasion. I’m told that this really isn’t rust at all, but it is red, and looks like rust to me. It is caused by the "bi-metal" reaction that takes place in the presence of salt water. Even metals that are identical in composition (such as the 316 Stainless used in the Mark 15) can exhibit this reaction, due to minuscule differences in their metallic composition (such as would occur in different lots of the same grade Stainless coming from two different foundries).

Whatever its cause, I don’t like to see any rust on my machine. If necessary, I will disassemble the tubes from their fittings, and polish the "rust" off with my Dremel, using a stainless steel wire brush attachment.

After polishing, I’ll give the OUTSIDE of the fittings and tubes a spray of pure Silicone to help prevent this build-up in the future. So far, it seems to work great.

The next part of the Gas Transport that needs attention is the Manual Add Valves, located on either side of the unit. I don’t recommend you attempt rebuilding these unless you have a good working knowledge of valve rebuilds, and understand your unit thoroughly. Why? Because they are a pain in the ass to remove, repair, and align correctly once you are done.

However, I really believe that after 100 hours of dive-time, they should be removed and rebuilt, regardless of the difficulty.


Section 3: Storage and Handling of Sensors and Sensor Wires

Storage and handling of the sensors is a hot topic amongst rebreather divers. The maintenance of the sensor wires is a topic that I am very familiar with. I will give you the different opinions of the former, and my authoritative view of the latter.

Sensors: Oxygen sensors, more correctly known as Hyperbaric Sensors are essentially batteries that run on oxygen. The typical Oxygen sensor is comprised of an anode, a cathode, some reactive medium, and a gas permeable membrane for Oxygen to pass through. The Oxygen sensors in the Mark 15 are made by BioMarine Instruments. Inside the sensor, an anode of spun lead sits in a bath of Potassium Hydroxide (KOH). Across the KOH lies a cathode screen which is gold plated. The KOH soaked lead reacts in the presence of Oxygen, and releases electrons, which flow across the medium to the cathode. This flow results in direct current voltage flowing out from the sensor. The amount of electrons being released is (pretty much) linear with the amount of Oxygen present. Therefore, the higher the Oxygen partial pressure, the higher the voltage output as expressed in mV DC (millivolts, Direct Current).

Because this reaction slowly decays the lead and gold on the anode and cathode, the sensors have a limited life span, usually about one year. This has resulted in many different ideas as to how they should be stored.

Most all rebreather divers agree that the Oxygen sensors should be stored in separate containers when not in use. I personally store mine in small Tupperware containers that measure about 2" in diameter, and about 2 1/4" high.

Some divers believe that the containers should be flooded with Nitrogen, or Helium when the sensors are put away, to eliminate the presence of Oxygen as much as possible. My understanding of this practice is that they get about a year of service out of the sensors that way.

Other divers believe that the sensors should be stored in the refrigerator. I understand that using this particular method, they get about 12 months of service life out of the sensors.

I imagine that if you were to flush the sensor containers with Helium, and then store them in the refrigerator, you might expect to get as much as 365 days of service life out of them...

As you can see, you’re gonna get about a year of life out of the sensors before they die...

One important thing to remember is that the sensors should be marked as to their position in your rig, i.e. "#1, #2, and #3" you should also put the date of installation on each sensor so that you can keep track of how old a particular sensor is, and when it is likely to need changing.

It is important to keep the sensors numbered because mixing of the sensors can throw the calibration of your Electronics and Secondary Display way off.

I personally keep a small data tag in each sensor container where I write the output voltage of each sensor (measured in air) upon storage, and removal from the container. I have found that this gives me a good idea of the "health" of each sensor, and helps me to determine when its gonna die.

Whatever method you decide upon, the one thing you really want to do is somehow, somewhere keep track of the sensor readings as the one year life progresses. And don’t mix ‘em up - Sensor 1 is always in Position 1, etc.


The Sensor Wires

The sensor wires in the Mark 15, 15.5, and 16, are the single weakest link in the entire system, as far as I’m concerned. And I’ll tell you why:

Inside the Center Section, there is the Sensor Bridge (which, surprisingly, holds the Sensors). Outside of this bridge, you will see six wires, three red, and three black. These are the wires that carry the voltage output from the Oxygen sensors to the Electronics, via the "Horseshoe PC Board" and Bendix Connectors.

Now, if you take your Center Section out of the rebreather, and remove your Counterlung, you will see the area where the "Horseshoe" board lives. It is epoxy sealed. And herein lies the problem.

The Sensor Wires run through small holes opposite the Sensor Bridge, and pass directly into the Horseshoe PC board. These holes are epoxied as well. The wires are typical Mil-Spec wires, i.e. 22-25 awg wire, silver plated. Silver plating wires is a big thing with the Military. As many of you know, Silver is by far the best conductor of electricity - even better than Gold or Copper. For the millivoltage coming from the Sensors themselves, Silver makes a lot of sense. However, Silver is notorious for corroding.

The wires that lead from the Sensors to the Horseshoe Board are very sensitive to salt water and salt water spray. Once they have been exposed to salt water, their days are numbered. This is not fiction, but fact. Eventually, every Mark 15, 15.5, and 16 out there will have these wires go straight to hell. They get what I call "Creeping Necrosis." Essentially, the corrosion will begin near the end of the wire, and slowly creep down the length of it, under the insulation. This will go unnoticed by you. And one day, you will pay dearly for it... (sounds like a bit of a horror movie, eh?)

When the Sensor Wire finally gives out, it usually does so by breaking. You’ll notice that you don’t get a reading from a particular sensor. This usually happens when you are installing the sensors in preparation for a dive. You will examine the wires, and find that one of them has broken. The usual site for this break is right at the end of the wire, where it is epoxied into the Center Section.

I once watched a good friend of mine dig with a dental pick to try and expose 1/64th of an inch of wire right at the epoxy to try and get enough wire to solder on. It was a sad sight...

So - what can you do? Well, quite simply, you can try to keep these wires from coming in contact with salt water. I suggest that you use a good corrosion prevention spray, like "Caig Super Gold" or something equivalent on the ends of the wires. Also, try not to let too much water into your loop.

And what if your wires break? Basically, you’re screwed. Sorry. You will then have to have the old Horseshoe Board machined out, and a new one put in its place.

After seeing my fellow rebreather divers go through this horror, I took pro-active steps, and replaced my entire Horseshoe Board assembly and modified the way the Sensors hook up, to eliminate this problem from ever happening to me. It took me 2 weeks of work, but I got it done. If you want to know how to do it, you’ll have to pay me huge sums of money, and walk my dog for a week...


Section Four: Proper care of Displays

The Primary and Secondary displays are rather rugged devices, that shouldn’t require too much attention from you during their lifetime. However, some folks pay some additional attention to them, which you might want to follow.

The most fragile of the two displays, is clearly the Secondary. Not only does it have a delicate meter inside, but also a Bendix Connector on the end. I’ve already addressed how you should care for your Bendix Connectors, and the same applies to the Secondary.

Many divers like to keep the Secondary separate from the rest of the rig, usually kept in a padded case. This is a good idea. It’s not an idea that I adhere to, but I have all the parts necessary to rebuild one of these, should I trash my unit.

Remember one thing about the Secondary: There are no spare Secondary Displays out there. So, the likelihood of you ever getting a replacement, or a spare, is highly doubtful. So you should take care of it.

The Primary Display needs very little maintenance. The only thing that usually goes wrong with them is that the bulbs can burn out. Usually the "O" bulb. Replacing these bulbs is no walk in the park, either. If you aren’t good with electronics, and small part soldering, don’t try to replace it yourself.

You can also replace the light bulbs with LED’s (Light Emitting Diodes). I know one diver who did this, and is quite happy with it. However, he had to factor in resistors in line with the diodes - which is not impossible, but must be done.


Section Five : Proper Care of Cables

The Bendix Cables found in your rig are extremely valuable. You won’t find this out until you have one go out on you - then you will search fruitlessly for a spare, and eventually pay someone a fortune for a replacement. You can avoid this pain and suffering by maintaining your cables regularly. They are extremely robust cables, and don’t require any attention when in use, but between dive trips, you should take the time to keep them in top shape.

Essentially, the only maintenance that I ever give my Bendix Cables is a thorough cleaning of the connector attachment nuts (inside and outside), cleaning of the connector bushing (which is located on the inside of the connector attachment nut) and a good spray of Caig "Pro-Gold" contact cleaner/lubricator.

By far, the best tool for cleaning just about everything on a rebreather is a Dremel Moto-Tool. I happen to use the battery powered units for most jobs, and the AC powered unit for heavier stuff on the bench. You’ll need the Stainless Wire Wheel attachment, and the Straight Wire Brush attachment for the task.

First, I take the Stainless Wire Wheel, and clean the outside of the Connector Nut (the ring that you rotate to screw down the Bendix Connector). I then switch to the Straight Wire Brush attachment which fits nicely inside the gap between the connector itself, and the inside diameter of the Nut. I clean out any corrosion build-up (the green colored stuff that comes from the Brass Nut corroding). I then make sure to pass the brush around the Bushing (which is the smooth looking round surface just behind the connector itself).

The Bushing forms the seal with the captured "O" ring in the Bendix Bulkhead Connector, so you REALLY want to keep that clean and free of debris.

I then pass a Q-tip around the inside of the whole assembly to remove any dust or particles.

What I do next is a 2 step process : First, I spray the Caig Pro-Gold into the female connector plugs, so that it fills them up. Holding the connector perpendicular to the ground (so the Pro-Gold doesn’t run out), I spray the entire connector assembly with 100% pure Silicone spray. You don’t really want Silicone inside the connector plugs, so filling them with Pro-Gold first, keeps out any silicone spray that might get inside.

Once that is done, I verify that the "O" ring in the Bendix Bulkhead connectors are good, replace them if necessary, and re-install the cable. Try not to leave your newly cleaned cables lying around the garage. You also don’t want to leave your Bendix Connectors left exposed either.

True Story: A friend of mine once had another fellow install a new set of electronics in his rebreather. My friend (of course) didn’t bother to inspect the newly installed electronics (which weren’t new themselves, they were a "loaner" that had sat on the fix-it guy’s shelf for a while). During the first dive, he noticed his Primary display going wacky. He figured he had a flood in his battery compartment.

After exiting the water, he found that there was no flood. So he decided to disconnect the cables from the electronics pod to check the cables themselves. When he removed the Primary Cable from the Electronics Pod, he discovered a very large Cockroach, impaled on the 13 pins sticking out of the Primary Bendix Bulkhead connector. The roach had evidently ran in there to hide from the fix-it guy, and got impaled for his troubles. His bodily fluid managed to short out the cable connectors, and my friend lost a pin from the bulkhead side of the connector assembly, rendering the connector useless.

Chapter 6 : What to Look Out For (Warning Signs)


1) Leaks in the loop / Leaks in the gas transport

2) Failure of Sensors and Sensor Wires


Section One : Leaks in the Loop - Leaks in the Gas Transport

During your regular rebreather maintenance, you should ALWAYS perform a leak test of your complete system. Testing of the loop is done 2 ways:


Negative Pressure Test

The negative pressure test is essentially a vacuum test of the breathing loop. You turn off your Diluent sphere, and begin inhaling the gas from your loop via the DSV - exhaling each breath out of your nose. As the Counterlung collapses, it will eventually reach bottom, whereupon you inhale with all your might, to create a vacuum. You shut off the DSV, and let the hoses rest for a few seconds.

When you open the DSV after a minute, you should hear a "whoosh" of air being sucked into the loop, relieving the vacuum. If you do not hear that rush of air, you probably have a leak in your loop somewhere.

It is my personal experience that most leaks in the loop occur in the DSV. There are very few places for leaks to occur throughout the loop, and most are semi-permanently sealed (such as the Counterlung). You should first examine the DSV for possible leaks before tearing the Center Section out, and looking for holes in your Counterlung.

Note that you can lose vacuum in your loop during a negative pressure test through residual pressure in your Diluent pressure lines, or even through the Diluent 1st stage (if no sphere is installed). A slow leak of vacuum in the system is not really a sign of trouble - what you are checking for is a major leak somewhere in the system.


Positive Pressure Test

The Positive Pressure test is difficult to accomplish, and I feel is potentially harmful in a number of ways. In this test, you block the Overpressure Relief Valve located on the counterlung, and orally inflate the loop via the DSV. First, the blocking of this valve is not an easy task to accomplish, and second, exerting positive pressure in the loop could potentially damage the counterlung, or cause seals to blow outward. I do not personally perform this test on my own rig, and do not recommend its use.


Leaks in the Gas Transport

Testing for leaks in the gas transport can be accomplished in a number of ways, but by far the "dip-test" is the most effective.

Essentially, you will assemble your unit, complete with fully charged spheres, and turn the system on. You close the DSV, and then dip the entire unit into a pool, or bathtub so that it is completely submerged. You then look for any bubbles that might be coming from the gas lines or fittings. Any leaks in the lines or fittings must be repaired before diving, since even the smallest of leaks generally leads to larger leaks in the future.


Section Two : Failure of Sensors and Sensor Wires

The warning signs of failing sensors are very simple, and will reveal themselves at either very high voltage (over 21 mV) or very low voltage (under 16 mV) when the sensor has been resting in air.

Oxygen sensors are like a car battery - sometimes they will erode slowly and die in a constant, linear manner, and other times they will have "spikes" of voltage just prior to dying abruptly. I have been told that increased exposure to Oxygen at pressure can cause their death near the end of their life span, and I have experienced this myself first-hand. Essentially, the sensor will read fine when on the boat and installed into the rig, but once the dive has commenced, and the sensor is exposed to hyperbaric Oxygen, it will die.

What I have found to be the best guide for predicting when a sensor will die, is its actual life under use. Generally, using the BioMarine sensors, I can expect about one year of in-service life out of each sensor. How the sensors are stored, or how often they are used during this year does not seem to have much bearing on how long they live, either.

Nonetheless, I DO store my sensors in small Tupperware containers when they are not in use. I do not refrigerate them, like some guys I know, nor do I flood the sensor containers with Nitrogen. It seems like the bottom line is that once you remove the sensors from the factory sealing, and begin using them, you’ve got about a year before they die, period.

Bear in mind, that these sensors are manufactured by hand at BioMarine, so some variances in life can be expected. What I can say, however, is that the BioMarine sensors seem to have the most reliable performance of any sensor I have ever used in my rig, including the Teledyne R10’s.

(Yes, that is a blatant endorsement of the BioMarine sensor. It’s my guide that I’m writing, so you’re getting my opinions...)


The Sensor Wires

The sensor wires are probably the single weakest link in the entire Mark 15, 15.5, and 16 rebreather. They are weak for several reasons:

1) The wires themselves are not very sturdy - they do not take well to pulling, yanking, or bending much.

2) They run from the Sensor Bridge to the Horseshoe PC board (located on the inside of the Center Section in the area enclosed by the Counterlung), via two holes that are drilled in the Bridge, and the whole shebang is potted with epoxy.

3) The wires themselves are military grade silver-plated wires, which makes them prone to corrosion.

4) The small gray connectors used in the Mark 15 have gold-plated contacts that must be Zinc underneath, they corrode that fast. (That’s a joke. They’re probably some kind of steel base metal, but they sure as hell corrode in a heartbeat).

What typically happens with the sensor wires is this: Somehow, salt water gets into the area that houses the Sensors. This salt water begins to work on the silver plated wires, under the insulation, through capillary action. The sensor wires begin to corrode within their insulation. Eventually, the diver will pull on the wires, and they break, either internally, or externally. Once broken, the corrosion of the wire is so bad, that making an adequate solder-joint at the point of the break is almost impossible.

The small gray connectors also will have the gold-plated contact pins and receptacles corrode very quickly. Once they have begun to corrode, it becomes increasingly more difficult to get them to make a good electrical contact.

In the case of a wire break, there is almost nothing you can do, short of a complete replacement of the Horseshoe board and wires. That is not something I recommend anyone attempt who is not familiar with the electrical systems of these units.

In the case of a problem with the connectors, I have had moderate success in cleaning them with a small wire brush, and coating them with some kind of contact cleaner/lubricant.

In my own rig, I have completely eliminated this problem, by eliminating the wires themselves. Since this isn’t a "how-to" book, I won’t go into detail, except to say that I doubt most of you have any desire to go through the pain and suffering that I went through to fix this problem permanently.

Chapter 7 : Personal Philosophy of Handling Emergency Situations


1) Leak in the loop

2) Failure of a Sensor

3) Failure of a Display

4) Failure of the Electronics

5) Failure of the Solenoid

6) Failure of Gas Delivery System

7) Blown Lines


The following pages are going to be devoted to my own personal philosophy regarding how to handle different "emergency" situations. I want to point out that I have categorized all the possible failures in the operation of the Mark 15 as "emergencies." In reality, most of these should really not be given such an extreme term, especially when most can be dealt with rather simply, by using your head, and relying upon your formal training and knowledge of the Mark 15 system to keep you alive.




Section One : A Leak in the Loop

On occasion, you may go through your pre-dive checklist, and determine that all is well with your machine, only to find that when you are actually diving, you have water begin to accumulate in your loop. This is usually noticed when you hear a "gurgling" sound in your exhaust hose. This sound if formed when your exhaled air passes through the flapper valve in your DSV, and bubbles through the accumulated water on the other side.

 Sometimes (as gross as this may sound), this build-up could also be caused by excessive salivation by the diver. As we all know, some dive boats can serve some pretty nasty chow between dives, and this can cause a bit of "reflux" which leads to excessive salivation. If that is the case, you can simply blow the liquid back into your Center Section, and most of it will be absorbed by the absorbent pad. This will clear most of the gurgling sounds.

However, if you notice water (especially salt water) entering your mouth from the DSV, even in small quantities, this could be indicative of a leak in your loop.

There is NO WAY to determine the location of this leak during a dive. If this occurs, you should LEAVE THE WATER AS SOON AS POSSIBLE AND SAFE. The reason I stress this is the two times where I have witnessed a diver exiting the water after a long dive, only to have their intake hose fall off as they ascend the ladder to the boat.

As it turned out, the intake hose was not properly clamped to the coupling which connected the hose to the Center Section. Had the intake hose come off DURING the dive, the diver would have got a lung-full of water, causing a very bad situation to possibly get worse.

Since there is no real effective means of dealing with a leak in your loop during the dive, you are far better off aborting the dive, and checking your loop seals before entering the water again.

At this point, I’d also like to dispel a strange myth that I heard going around a while back:

As the myth went, you needed to buy Stainless Steel wire impregnated hoses to avoid a "tear" during diving (as in wrecks, or around sharp objects). The reason for this was that at the great pressures found in the sea, even a small pin-hole in your hoses would allow huge volumes of water to enter under pressure.

This, of course, is simply not true. Don’t forget that the pressure found inside your loop is AMBIENT. So it is the same pressure inside the hoses as it is outside in the water. Any hole that forms in your hoses will leak in proportion to its size.

Should you happen to have a major leak of your loop during the dive (and by "major" I mean you begin to take in water to the point where it floods your loop, and begins to enter your DSV), you MUST switch immediately to your Bailout System. The dive should be aborted immediately, and you should exit the water as soon as any decompression obligations safely permit.


Section Two : Failure of a Sensor

During the course of your Rebreather diving career, you WILL have sensors die on you in the middle of dives. It’s as simple as that. It has happened to all of us at one time or another, and there really isn’t any way to prevent it, save using brand new sensors for each and every dive.

Fortunately, the designers of the Mark 15 realized this, and incorporated 3 sensors into the system.

You actually need ONLY ONE sensor to dive your rig. That means, you can have up to 2 failures during your dive, and still make it through o.k. That is about as likely as having lightning strike you while sitting in the boats head, reading some past issue of AquaCorps.

If you have a sensor fail on you during your dive, you will notice it first on your Primary Display. The "Alarm" light will go on, along with the familiar and comforting "O" light.

When you see this, you should immediately poll your sensors using the Secondary Display, to see which one is out of whack. It will reveal itself by being out of range of the other two sensors. In other words, you will switch from Sensor 1 to 2 to 3, and one of them will be either higher than the others, or lower.

The dying sensor could go either way. I’ve seen them just crap out to nothing all of a sudden, and I’ve seen them climb way up there in the voltage scale before crashing down deader than Dolly’s Doornail.

My personal philosophy is this: If I have 2 sensors that are in accordance, and match the set-point that I established for my rig during the pre-dive set-up, then I continue my dive, and just keep an eye on the Secondary Display to make sure that the two "good" sensors remain in agreement with each other.

I also track the date of installation of each sensor in my system, and take "before" and "after" readings of them each time I install them into my rig. (i.e. when the sensors are removed from their individual cases for installation into my rebreather, I take a measurement of their mV and write it down. After I am done with diving, and they go back into their containers, I take a second reading, and note that as well).

Should you have 2 sensors which die on you during a dive, then you have much bigger troubles.

First, you need to determine if they are DEAD, or just reading low. Personally, I’d rather they register NOTHING on my Secondary Display, rather than giving me low readings, since low readings makes it hard to know which sensor to believe.

A quick way to determine which sensor is telling the truth in that case would be to flood the loop with your Diluent Gas, and calculate what ppO2 the Diluent Gas should be at that particular depth.

For example: You are on a dive where you are using plain old compressed air as Diluent. If you are at 154 fsw, the ppO2 of compressed air is 1.2 If you flood your loop with Diluent, your sensors should give you a reading of 1.2 Any sensors that DON’T, are dead or dying. Rely upon the one that is reading correctly, and begin to end your dive.

There is no reason to continue diving with two bad sensors. It is unsafe at that point, and you need to isolate the cause of the failures before diving again.




Section Three: Failure of a Display

It has been my experience that the displays do not fail very often, if at all. The only kind of display failure I’ve ever encountered personally was the green "O" light of my Primary Display burning out during a dive trip in Fiji. However, there are things you should consider if you have any kind of failure of the two display systems of the Mark 15.

If you have a total failure of your Primary Display, the cause is most likely in one of three areas:

1) Improperly connected/sealed Bendix cable.

2) Improperly connected/sealed Switch Cable.

3) Failure of the Electronics.

4) On/Off switch being turned off.

#1 and 2 above should have been checked while topside during your pre-dive check list. #3 above is highly unlikely, since it usually results in strange light patterns flashing in your Primary, rather than a total failure of any lights to glow. Believe it or not, #4 above is the most likely scenario for a failure of the Primary Display. I know it sounds pretty dumb, but dumb things happen...


Failure of the Secondary Display

The Secondary Display, in my opinion, represents the last line of defense in any rebreather emergency. As such, I do not like the idea of diving without it. The Secondary is established in the system as a fully autonomous part that reads directly off of the Oxygen sensors, and will continue to do so even during a total electronic failure. In this role, it serves an invaluable part in keeping you informed of your actual loop ppO2 during the entire dive, regardless of the status of your other display, or the condition of your electronics.

The following can cause a failure of the Secondary Display:

1) Improperly connected/sealed Bendix cable.

2) Flooded Secondary Display

With both scenario’s above, I would immediately abort the dive. The Secondary is that critical to your system monitoring that I would not proceed without it.

If for some reason, you experience a failure of BOTH displays during a dive, there is only one option - Immediately abort the dive.

However, you must remember that you now are flying the rebreather "blind" in that, you have no idea what your loop ppO2 may be at any given time.

For this reason, I would immediately begin a "semi-closed bailout" with my rig. This bailout scenario should be taught to you by your instructor.

Since the odds of this happening (on a well prepared rig) is higher than having all three sensors die on one dive, I would also visit a church after the dive, and ask God why he wanted to kill you so bad...


Section Four : Failure of the Electronics

Electronic failure in the Mark 15 can happen for a variety of reasons. The most common cause for failure of the electronic systems in the Mark 15, 15.5, and 16 is the small bleed valve on top of the Electronics Pod cap. Every now and again (and this has happened to just about every rebreather diver I know), the diver will forget to tighten this little screw after installing the battery.


As you descend, the salt water floods the battery compartment, and begins to short out the battery via the Molex connector. This will result in your Primary Display putting on a "Christmas Tree" show for you of various flashing lights. Essentially, the battery is frying itself to death.

The other common cause for Electronic Failure during a dive, is the 3 wires leading from the battery to the electronics themselves. These wires go through a lot of abuse, as batteries are packed in, and out of the Electronics Pod. Eventually, they will break. When any one of these wires breaks, the entire electronics will not function normally.

The electronics can also fail due to improper seating of the Bendix connectors. Remember, you have 4 Bendix connectors located in your Electronics Pod, 2 in your Center Section, and 1 on your Secondary Display. That is a total of 7 potential failure points that you must address during your pre-dive set-up.

The least common failure of the electronics is some kind of component failure. This is actually quite rare - contrary to what some people say, or what many non-electronic rebreather divers would like to believe. The electronics of the Mark 15 are extremely robust, and generally do not fail, provided they are kept dry, and that external shorting of power supplies do not occur.

One important thing to note here: If you happen to somehow damage your electronics by flooding them, or shorting them out somewhere else in the system, they are T-O-A-S-T. You can’t go down to Radio Shack, and have ‘em fixed. So exercise caution in how you handle all of your Bendix connections, as well as the sealing of your battery compartment.

Should you experience a failure of the electronics during a dive, however, the recovery is quite simple. You merely rely upon your Secondary Display to give you your current ppO2, and either continue the dive, or abort, depending upon your personal choice, and the demands of the dive itself.

There is one kind of failure that I have heard reports of that you should know about: It was reported by one diver that when they had flooded their battery compartment during a dive, that direct voltage was passed through the electronics to the Secondary Display, pegging the needle of the display to the far Right. This, of course, would render your Secondary Display useless to you.



There is always another day to dive, but not if you attempt to finish a dive with no electronics, and no backup system. The result of which could be your funeral.


Section Five : Failure of the Solenoid

Luckily for us, the Navy was smart in its design of the Mark 15. They chose a solenoid valve that is normally CLOSED. It requires voltage from the electronics (24-28 vdc, to be exact) to drive the solenoid open. What this means, is that any failure of the solenoid usually is in the Closed position. Most of us, after a while, get used to the "click" of the Solenoid every now and again, followed by the hissing sound of O2 being injected into our loop. When you don’t hear that for a while, and your ppO2 begins to drop, the first indicator will be the "1" light in front of the "O" light on your Primary Display will go on.

If you don’t hear your Solenoid firing then, you probably have a failure.

At that point, recovery is quite simple as well. You merely inject O2 manually as you need it to maintain your ppO2, based upon the readings you take from your Secondary Display. Depending upon your experience/comfort level, you may continue the dive or abort. If you are unsure what exactly the problem is, you should always elect to abort the dive.

But if you decide to abort, remember this point:




Section Six : Failure of the Gas Delivery System

Failure of the gas delivery system can take many forms. The most common is some kind of clog in your lines.

You can eliminate this problem by thorough maintenance of your system after each dive trip. You should always check for proper gas flow throughout your system, first by testing the Manual Add Valves, then by collapsing your Counterlung (by exhaling the stored gas from it) to test the action of the Automatic Add Valve. The AAV should add gas fast enough to the Counterlung to keep up with your normal inhaled breath. It probably won’t breathe like an open-circuit reg, but it should be close enough as to be comfortable.

However, if you find yourself in the unfortunate position of having a clog in your gas delivery system during a dive, you should take the following steps:

If the clog is on the Diluent side, immediately stop your descent, and begin to ascend. This will give you immediate relief if you are having problems breathing from the Counterlung due to lack of gas volume. Begin to monitor your ppO2 directly from the Secondary Display. You can then attempt to add Diluent by activating the Manual Add Valve. If gas still does not flow, or flows very slowly, you should abort the dive.

The above, of course, assumes that you have a full sphere (3000 psi) and it reads correctly on your HP gauge.

If the clog is on the Oxygen side, you should immediately stop your descent, begin monitoring your ppO2, and begin a controlled ascent to the surface. ANY problems with Oxygen delivery at the outset of the dive requires an abort of the dive. You should carefully attempt to add Oxygen via the Manual Add valve, while constantly monitoring your ppO2 via the Secondary Display. Do not, under any circumstances allow your ppO2 to climb above 1.4, or below .21

Once out of the water, you should do a thorough inspection of all gas delivery lines to find out where the clog is. Do not attempt to dive your unit again until you have fixed the source of the problem.


Section Seven : Blown Lines

I have never heard of this happening in a Mark 15, but like all scuba systems, I’m sure it can.

If you have a blown line, the most critical step you should immediately take is the shutting off of the sphere valve. There is a chance you may still be able to use some of the gas in the sphere by turning it on and off for short periods of time, but you need to shut the valve off quickly, to minimize gas loss.

Once the valve is shut, you can then begin your assessment of the problem, and solution. In the case of a blow in your Diluent line, aborting the dive can be done without ever turning on the Diluent Sphere valve until you are back on the boat. Should you blow an Oxygen line, however, you will need to begin planning a semi-closed bailout strategy. For this reason, I will often carry a dive computer that is designed for compressed air diving. Usually, if will give you an idea of how long any decompression requirement might be should you have to resort to the use of straight Diluent gas in Semi- closed mode for deco.

Obviously, if you have Oxygen left in your tank, you could periodically turn the valve back on, to flood the loop with O2 then turn it off again when you have reached your proper ppO2. However, make sure you are constantly monitoring your Secondary Display when adding Oxygen in this manner.

Once safely back on the boat, or preferably, in your shop, you’ll need to perform a complete leak-test of your system to determine where the blown line is, and the cause for the blow. In just about any case of a blown line, I would recommend complete replacement of the line itself, along with the connector fittings for that line.



The Last Word...

I’ve endeavored to put this manual together in order to help a lot of new guys who are just getting into rebreather diving, and buying a Mark 15. I also wrote it to give my own insight to guys who already have 15’s, 15.5’s and 16’s.

(Yes, I said "guys" twice up there - no offense to women, but so far, I don’t know of any women who dive these rigs - and even when I walk into a room full of females, I still say "Hi Guys"...)

If I can leave you all with one thing it is that the understanding and maintenance of these machines is vital to your continued diving career, and indeed, life.

As anyone who knows me well can attest, I keep probably the messiest office of anyone, and my shop is usually a total disaster. But the one thing in life that I am incredibly anal about is my rebreather.

Diving a rebreather is probably the greatest experience you can have in your diving career. It gives you damn-near limitless time underwater, you can breathe like a Hoover vacuum if you want, with no penalty for gas use, and the constant neutral-buoyancy you will experience is incredible. Also, the fact that you will all of a sudden notice that you "blend in" with the surrounding sea life is one of the joys of diving these things. You will be able to get closer to marine life than you ever could on open circuit.

There are a lot of things that you’ll have to learn all over again, when you dive a rebreather, but that is half the fun. I haven’t covered all the practical diving techniques required in this manual, because you should learn all of that from a qualified, certified rebreather instructor.

Also, you should beware of those that pontificate on how rebreathers work, with limited or no experience on your particular rig. The world if full of them, and they all want to tell you how these machines work. Try to avoid taking their advice on its face. Read this manual, and the Mark 15 manual that is enclosed. Talk to others who actually own and dive a Mark 15, 15.5 or 16. Never, ever take someone’s word on how to dive these machines unless they are certified instructors (who qualified on the Mark 15) or experienced Mark 15 rebreather divers.

Take care of your machine, and it will take care of you.

Safe diving to all.


Where to buy Sofnolime...

Copyright Northwood Designs, Inc. All rights reserved.
Revised: September 07, 2005.