Expedition-level Mk 15.x MCCR in 90 seconds!

teksimple

Necessity is the mother of invention.

– Plato.

My necessity: an MCCR with no intrinsic depth limit.

The machinating started months ago when I was contemplating buying a backup rebreather. Diving a Hammerhead-equipped Mark 15.5, I already had an ECCR that was quite capable. But if I had to send in the electronics for an upgrade or repair, I would at the minimum miss a few weekends of diving, perhaps much more. I thought about the challenges in rigging up my unit with a KISS valve. That would require another regulator altogether, and still require manual control below the depth allowed by the orifice. Nothing simple about that at all.

What about a true mass flow controller that could take the place of a solenoid and electronics pod? As a science major in college, I was familiar with the electronic mass flow controllers used for gas chromatographs. What about a mechanical one? A Google search revealed a good candidate. In fact, as far as I knew, it was the only true mechanical mass flow controller available.

The Porter Instruments VCD1000.

Expensive, but way cheaper than a new Hammerhead. Sounded too good to be true, so I ordered one. It was assembled to my specifications a few weeks later. The sealing surfaces are Viton, with a stainless steel diaphragm for oxygen service, a Delrin knob, and an adjustable flow rate of 0 to 0.75 liters/minute. It required some modification: 1/8” to 3/16” Swagelok pipe reducers, plus replacing the standard steel assembly bolts and washers with stainless steel versions and forming a bit of 1/8” pipe.

My invention: the “Msense” for lack of a better word.

I first tested the Msense in February, with a 3+ hour solo shore dive. Since then, I have accumulated almost 50 hours on the Msense with a maximum depth of 110 meters at Lehua Rock. But the Msense by design has no depth limit. It does not require adjustment as would a needle valve, and I can literally replace my solenoid and electronics pod with the Msense in 90 seconds. The Msense weighs a total of 340 gram (12 ounces) and fits in the palm of my hand, and as such does not impact much on my tool kit portability. Frankly, I have decided I will never take my Mark 15.5 traveling without the Msense in my bag. I would think it would beat any form of analog electronics pod, especially since it does not require that wad of batteries.

As a thought experiment, lets replace the Mk 15.5 primary display with another Mk 15/Mk 15.5 analog sensor display. Conceivably one could dive indefinitely without ever having to change batteries, because there are none!

Since Mk 15’s are relatively rare, the question certainly will arise: can the Msense be fit on my rebreather? I am sure it could, but certainly not as cleanly or as readily as it does on the Mk 15.x series rebreathers. The Delrin knob is beefy, but I would want to protect it with a metal sheath before exposing it outside of a rebreather case.

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"I don't know the percentage of the Internet that's valid, do you? Jesus, it's scary." - Hunter S. Thompson

DepthCharge

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Later,
John

It's better to beg forgiveness than ask permission

jasondrake

I know I'm being a bit slow, but I'm not sure I understand what this valve is doing differently from a common cmf valve. How does it work?
thanks
jason

teksimple

FYI, the part number I ordered was VCDSVS400, which allows a maximum flow rate of .75 liters/minute of 100% oxygen. Flow element max rate depends on the gas. Very large divers may want a larger flow element than the 400. Larger elements would not be a problem for smaller divers either since one can still crank the largest flow elements down to zero, anyway.

Note: you will need to get 4 3/8" 8-32 stainless replacement screws and pry open and then squeeze a #6 stainless lock washer on those #8 screws.

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"I don't know the percentage of the Internet that's valid, do you? Jesus, it's scary." - Hunter S. Thompson

teksimple

I assume you mean a KISS-style CMF valve? The KISS valve has an absolute depth limit (which is inversely proportional to the size of the orifice) because it has a non-compensating 1st stage. Once the ambient water pressure reaches the intermediate pressure of the first stage, the oxygen flow stops.

The Msense uses a standard depth-compensated first stage and is a TRUE mass flow controller because the flow does NOT depend on the absolute regulator pressure, only the pressure differential. It will deliver the same mass flow whether it is 50 PSI or 250 PSI ambient, and thus has no depth limit.

For KISS rebreathers that do not have a separate oxygen-add valve (unlike a Mk 15.x rebreather), I would recommend the Gorilla Valve which also should have no depth limit, but has a way to add oxygen manually.

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"I don't know the percentage of the Internet that's valid, do you? Jesus, it's scary." - Hunter S. Thompson

Johnny Christensen

THIS is why I spend so much time reading rebreatherworld. Brilliant !!!
Have some green.

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Wreckphotography from the cold north

enkelkijken

The Porter Model VCD 1000 Flow Controller is precision-engineered to control low gas flows at constant mass flow rates regardless of changes in downstream pressure. The VCD 1000 maintains a preset pressure differential across a laminar flow element....


this is a copy of the porter website: seems that the unit is not CMF at all: the MASS flow will increase by increasing depth, as the upstream pressure will increase, and the unit just keeps the differential pressure over the orifice constant...

... or am I wrong ????

paul

jasondrake

What enkelkijken said. If the valve maintains the pressure differential across the orifice in order to maintain constant flow, then changes in upstream pressure will surely result in changes in actual downstream flow?
Or am I being thick?

Rebreatherlab

Great write up!
So just to make sure I get this right: You are saying that it can give the same CMF disregarding depth. No re-adjustment needed between 0 - 110 m. The web site says that it keeps the CMF disregarding down stream pressure. If it holds CMF disregarding depth it needs to compensate for variations also in upstream pressure. Are you sure you have not set it to match metabolism at 110 m and simply push the manual ad more frequently at shallow depth?

The Gorilla box does not claim to do this. Only at a constant depth. It merely claims to be 20% more accurate between calculated MF and tested
MF. This has of course very low significance as with any Rebreather application it means verify the MF with your PO2 monitor.

Cheers,

Andy

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Ben Field

"IF" (and its a big if....) thats what it is, it would be the holy grail of MCCR diving.

I was trying to think of a way of doing this with a compensating 1st stage but set to compensate at a different rate to an OC 1st stage, still only on paper as math at present, would much prefer an off-the-shelf solution!

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