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Offboard-Trimix Conversion for the Franken-Rig
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In order to understand how the system will function using the offboard cylinder, you MUST understand how the offboard system worked in the original Russian system. Go study the offboard nitrox-cylinder system first, and then return here with the link provided on that page.
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Using the of the originally configured IDA-71 offboard bottle system allows Franken-Rig to be used as a Trimix rebreather. How do we manipulate this system for use on Franken-Rig? Easy: We fill it with Trimix or Heliox! When used this way, the internal air or Nitrox-based diluent mixture (carried in the original internal 02 bottle) is automatically purged out at 40 feet or so (adjustable as needed) and the rig switches to a Helium-based diluent. On ascent, it switches back automatically to an air or Nitrox-based diluent again! So, with NO modifications, we have a 3-gas CCR with automatic diluent-switch. I think this is pretty neat for an off-the-shelf system. I'm not sure I'd dive it for any really serious trimix stuff, but I'd think that 150-200 feet or so is not out of the question. The bottom line is that this system, with a little ingenuity, has gone from being a curious 02 rebreather to being a working trimix CCR. That's it. I'll dive it with only the internal gas for a Nitrox rig. Add the offboard bottle for use as a Trimix rig. Simple, isn't it? WARNING! Here's the big unknown: I have not yet measured the output pressure of the constant mass flow 02 regulator, so I'm not yet sure to what depth it will act as a constant mass flow 02 injection system. The depth of the system will in the end be controlled by this factor. I'm machining up an adapter now to allow me to connect a gauge to the system to measure this, and the pressure should be easy to adjust using the diaphragm screw adjustment on the MEDI regulator. DON'T try diving a modified rig to any depth until I determine the answer to this question. Of course, if you follow the golden rule to ALWAYS KNOW THINE PP02 you'd be manually adding 02 anyhow using the bypass button on the side of the rig, and it would be a moot point anyhow, right?
