Musings about a homebuild BOB & pendulum rebreathers

Dave Sutton

Actually, it's a great platform for a second loop, without the bulk of OTS counterlungs, it accepts a chest-worn loop like the C-96 Pro with ease, and I share the gas supply between the two. Just bring quick connect hoses from each first stage in the rig (which are at the bottom sides of the Mark-15) under the arms and plug them into (2) drysuit valves in the C-96 chassis. instant bailout loop.... and I use it for my deco to keep it fresh and working.


Dave

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iain-hsm

Dave

Never looked at it that way all I see is kind of Swagelok, Honeywell Skinner and Bendix. I remember someone I know lost a "large" MK part last year when they left it on a hotel balcony. Describing the part over the phone to the hotel clerk as: “well it looks like the inside of a small washing machine”
I will remember to pass on your spatial comments. Iain Middlebrook

Marc T

Yes you shared this bakup way a long time ago and I know john Vollangten had used the same system with his special buddy for cave diving, but he dont use this system because not enough streamlined.
He actually use a sidemounted Joki as back up.
In fact sidemouted Rebreather are very dificult to USE in sea diving ( variable latéral buoyancy are very difficult to control ) and a chest mounted is safer, its exactly the contrary in cave diving where buoyancy problem are less important than being as slim, or at list modulable than possible.

I plan to build a double Rebreather with the unused counterlung(only and inspire one like MK15 ) collapsed by the used one(s) because being contained in the same non extensible box.

The plan for this Rebreather is now ready in all its details on my table, I "just" have to build it
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montyg

Please tell us more about the MK15 analog electronics?

I'm not sure which forum here is the best to start a thread about it, though.

rgds
monty

Dave Sutton

Mark-15 and Mark-16 use MILSPEC electronics developd by Biomarine, using much of the same technology and hardware used for any other man rated life support system used by the military. The system averages the three cells, and if one falls out of usable parameters the analog system assumes that the failed cell is at 70% of the output of the other two and averaged *that* value into the calculation. The system ALWAYS works, basically because of the robust code and VERY bulletproof hardware implimentation as used in the MILSPEC system is is incorporated into. Voting logic systems are better... but I have NEVER seen one a fully tested and as reliable as the old military electronics. They are simple and reliable and always work. Let's face it, NOBODY has the time and energy to test civilian developed systems like the military tested these systems. Good enough for space flight = good enough for me, digital or not.

Dave

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whoizrob

Why would the military chose 70% as the default failure value? I don't know much about how a failed cell reads, but is 70% really a reliable value (I'm guessing it is)? Anyone have any plots of Mv output in a cell as it dies? Interesting...

Rob

Dave Sutton

They are not worried about a cell "dying" as they change them out regularly... as in every 90 days perhaps. Who cares what the cost per dive is in that environment? The system looks for a "lost" cell, meaning "zero output". The 70% value keep the loop gas at a viable mix no matter what happens. As long as a cell is not "lost" the system simply *averages* the three cells, as opposed to "voting one out". If one cell is showing 0.7 and another 0.7 and another 0.3, the system is going to average them out... you do the math. It always results in a good mix (good = viable, safe for life support)

Let's not try to re-engineer what is obviously an obsolte pre digital-age system. All I am doing is pointing out that *simple things work best*, and that technical systems that worked perfectly 30 years ago will still work perfectly, no matter what "improvements" have been made since then.

Dave Sutton

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iain-hsm

Dave.

I would like the opportunity to refute some of the blanket claims made that “nobody like the military have the time and energy to test civilian developed systems like the military”. While I fully accept it being good enough for you and me for that matter, there are still more questions to what we accept as fit for purpose even with milspec equipment.

The analogy of the MK 16 series sounds suspect and this is with the marked improvement from the previous MK 15 units. When you take into account the repair budget for operational MK16 units even with the published 10 fold increase in effectiveness over the older MK15.

The MK 16 has a total of around 340 parts listed as reparable replacement parts. Out of this only around 250 odd are classed as user level replacement parts (level 0) From this about 180 again are classed as essential spare parts with about 80 classed as shore base repair items. I think considerations wont allow disclose the total cost of the MK16 but it is around three times that of a typical CCR sports production unit

However if you take the average MK16 diver over 3 years in peace time he will average only 1 to 2 diving days per week for training (EOD disclosure) If on the other hand you take Desert Storm 1991 a total of four (4) MK 16 per EOD detachment was diving around 5 times that amount. Six days on, one day off with some operations exceeding 10 straight days. (EOD disclosed out of 120 day deployment diving operations were between 90 to 100 of days diving). The parts breakdown history on the MK16 from peacetime training and exercises to the Gulf war was dramatic. Even the $50K spare parts allocation for the units was frankly inadequate. Whatever you call it its not reliable maintainable
I know we are miles away now from the original post about pendulum rebreathers but for $50K spares bill that would have paid for a shed load of RN Navy CDBA units!! lol. Iain Middlebrook (only kidding)

Dave Sutton

The *point* wasn't to compare maintenance issues between civil and military use of hardware, it was to state in essence that NOBODY has ever been harmed bec ause the VERY SIMPLE albeit VERY PRIMITIVE control system of the Mk-15/16 series is BULLETPROOF and ALWAYS WORKS and has been FULLY TESTED within an inch of it's natural life... ;-) and even though it ONLY holds PP02 and does so with analog "averaging" electronics rather than digital "voting" electronics, it ALWAYS works.

In military aviation my helmet and 02 mask and flightsuits are maintained by professionals and the annual budget for this is about $10K/year. When I maintain my own helmet and 02 mask and suits I budget $100/year. The difference is the "throw it out if it's not new" V/S the "inspect and use if reliable" metric of deciding when to replace parts. Ditto with the Mark-15/16 series, which I started using professionally since 1978 (!) and have used as a "self funded diver" as well. The difference between "throw it out" V/S "lube it and use it" is something I've seen from both ends.





Dave Sutton

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"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com

Mdemon

Got any pictures for us please Dave? The nearest I've come to a Mk15/Mk16 is reading your posts...

I love the confidence you have in your unit. I like mine two.

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