Mark 15 Maintenance Manual
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.
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 Seven - Personal Philosophy of Handling Emergency Situations
The Final Word...
Comments? Let us know! Send us some Mail!