Mark 15 Maintenance Manual
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.
THE FOLLOWING VIEWS ARE EXPRESSED ONLY AS MY OWN PERSONAL OPINION REGARDING HOW TO DEAL WITH DIFFERENT POSSIBLE FAILURES OF VARIOUS SYSTEMS FOUND IN THE MARK 15, 15.5, AND 16. THESE MAY OR MAY NOT BE IN ACCORDANCE WITH FORMAL TRAINING METHODS TAUGHT BY QUALIFIED INSTRUCTORS. I AM PRESENTING THESE TO YOU TO EXPOSE YOU TO MY OWN IDEAS. YOU WILL EVENTUALLY DEVELOP YOUR OWN, OR BE TAUGHT DIFFERENTLY BY YOUR INSTRUCTOR. IN EVERY CASE, YOU MUST FIRST RELY UPON THE TECHNIQUES THAT ARE GIVEN TO YOU THROUGH FORMAL INSTRUCTION.
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.
DO NOT BEGIN A DIVE WITH LESS THAN THREE VERIFIED WORKING SENSORS
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.
IN THAT CASE, YOU HAVE NO BACKUP WHATSOEVER. YOU SHOULD IMMEDIATELY GO TO SEMI-CLOSED MODE, AND ABORT THE DIVE.
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:
DO NOT FORGET THAT AS YOU ASCEND IN THE WATER, YOUR ppO2 WILL DROP. WITH A FAILURE OF THE SOLENOID, THE CONSTANT MONITORING OF YOUR ppO2 BECOMES CRITICAL. DO NOT ALLOW YOUR ppO2 TO DROP BELOW .21 ON ASCENT. TO DO SO RUNS THE RISK OF HYPOXIA, AND POSSIBLE DEATH.
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 Diving Home Page
Chapter One - The Mark 15
Chapter Two - Various Models of Rebreathers
Chapter Three - General Maintenance of Systems
Chapter Four - Operation
Chapter Five - Keeping it Working
Chapter Six - What to Look Out For (Warning Signs)
The Final Word...
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