IDA-71 Nitrox unit Depth sensor

Gilbert

As we all know, If you put together an IDA-71 as it was meant to be, you have no oxygen supply below ~15m (45 ft) when nitrox unit is connected, cause most of us have no access to russian O3 lime.

I read through Daves pages hundred times, and at the end it hit me like strike of lightning:

What if, I connect a RG/UF-M CMF via an adapter to the HP side of the original O2 bottle and into the bag?

So; above switching point, I will have 0,9 l/min O2 and ADV for O2.

Below switching point, I will have 0,9 l/min O2 and ADV from nitrox unit.


Only one thing left: tell the depth sensor in the purging unit, it should work at a third of the depth it was designed to.
From the technical drawings I have, it looks like there is one bellows unit (like in an aircraft instrument) working against a spring pressure.

If I could manage to get three of them, in line, into a new made depth sensor case, it should work in theory.(Because in the same water depth you should have three times the compression/way to open the valve, as each bellow is compressed and the ways of each unit will add to each other) So you have the same purging cycle with three bellows, in a third of the depth it was originally invented with only one bellow.

Everybody should be confused by now, So I can ask you the real question

Does anyone of you guys can send me a photo from a disassembled depth sensor of a nitrox unit from IDA-71? (I want to find out if there is o loose bellow in this box or is it glued/welded in, as I have none to rip apart and see myself)


Thanks very much,any other suggestins welcome.
Sorry Dave for abuseing your photo,
Something is only impossible, if I failed for myself .

Attached Images

depth sensor.JPG (67.7 KB, 152 views)

Packhorse

Cant say I have really had a good look at the off board nitrox pnuematics in the '71.
I supose in theory you could mess with it and get it doing what you want but why bother?
Forget the RGUMF, just use a McMaster Carr or similar sourced CMF fitting. It works out cheaper and has standard type threads so is easy to work with.
Forget the stock nitrox pnuematics, just add a manual add valve instead.

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Skipbreather

Keep in mind the ’71 goes thru a CL purge cycle every time you cross thru the triggering depth. It uses a HUGE percentage of the available gas. Works for the original military application, but would be a deal breaker for any sport application.

Ken

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Gilbert

I wanted to do it like this, because i still have a RG/UF-M CMF regulator lying around. I also have no easy access to McMasters as I am from Austria. I could order the parts from overseas, or spend days searching local companies for parts. But I am lazy,I have the part anyway, and I am greedy and dont want to spend so much money.
Things tend to get expensiv by itself, as soon rebreathers come into play, so I'll start with the cheapest part available.
I also wanted to leave the Unit as much russian as possible and make only minor changes. As time goes on, maybe I'll get tired of the rig and rebuild it anyway.

I could fill the 2 caps that controll the amount of purging gas partially with epoxy, so the if the volume is smaller, the pressure at the nozzels will equal faster, and the purging cycle will be shorter. I was also thinking of changing the nitrox bottle for a 5 liter oxygen bottle as they have the same kleinkonisch thread and cost only 10$ on ebay.Maybe even extend the hoses and hook it onto the front "mine" bracket of the harness.

Anybody any photos or friends that work in a russian IDA-71 maintainance shop or storage...?

Dave Sutton

Well then....

(1): Fill the original 02 bottle with air (feeds the ADV and now you have diluent)

(2): Use a hose to take HP gas from any small cylinder and feed it to the RGU-F/M regulator

(3) Pass that 02 back into the loop.

(4): Add PP02 meter.


It's all a matter of how you slice and dice. Re-use the old offboard nitrox cylinder as the new 02 bottle, use the sleeve that holds it to still hold it, and put the new 02 flow thru the original offboard connector to the purge-line.

Just solve the 'hook old offboard bottle to RGU regulator" mystery and it's done. I can tell you how to do that too, but it's more fun to see if you can figure it out.

Exploration is the fun part!


Dave


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Gilbert

I've read Daves post, 10 min before going to bed.
So after 1 hour of lying around awake I decided to restart the computer and look through the pictures and sketches again. I have to say I would not have tried this approach, but I like it very much.

Only thing negativ about that is, I dont have an ADV for Oxygen anymore, as i would have with the purging unit below switch point.

Anybody tried to mess around with the purging unit/ modify it, yet?

Dave Sutton

Mixed gas rebreathers don't use ADV's for 02, they have them for diluent so it's exactly what you need. Putting diluent into the existing 02 bottle is the basic way to start making a CCR out of an 02 rig. Did the same thing on a LAR-V. All you need to solve is adding 02 to the system. That's easy.

I think you should abandon the depth switch idea, and work to a more normal system.

Dave

jaap

I can only agree with the others. It would most likely be easier, safer, more predictable and more compact if you go with the others suggestions (use ADV for dil only and add O2 manually). Since there are so many reports on IDA-71 conversions out there why not profit from them and do something similar yourself.

Anyway about the hydrostatic valve that does the switching. I had a mix of some russian colleagues and a russian dive buddy help me read parts of the IDA-71 manual. Also talked a little with a local guy who has use and rebuilds IDA-71's with sometimes minimal modification to the pneumatics for over a decade (Welcome to Divex.se! , Some of the stuff in his shop: Virtual Museum of Divex Sweden).

If I recall correctly it appears that the switch depth can be tuned (by varying the spring tension?) in a depth range of about 10-20 m. Modifying it for a shallower switch depth and eliminating the purge cycle is probably rather complicated.

If the intention is to build a rig for practical diving why bother? The hydrostatic valve assembly is really big and heavy, if you do some extensive modifications to it would you trust in it? Given its designed to freeflow serious amounts of gas inte the rig from pretty small cylinders I would not feel happy to have such a thing on a rebreather for regular diving. Feels like if your not 100% on top of it that thing is designed to bite you with runaway inflations, massive & rapid pO2 changes and to drain you tanks. Sounds like mr Murphys best tool to screw you day...

As I see it the only reason to keep the switch valve is if your interested in understanding how it works and experience some vintage style diving. But if thats the case why modify it at all?

Sidenot: Hmm, the switch valve could perhaps have some use transplanted into an SCR, say as an automatic switch from lean to rich feedgas set at say 20 m...

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Gilbert

First: Thanks to all for helping me out.

About the ADV: What I wanted to say: As the CMF provides the main O2 supply, it would have been a nice feature that the russian second stage delivers O2 when the Lung is fully collapsed during breathing in. This would make the breathing also more comfortable in O2 range. (Below setpoint, the russian second stage would act as an true ADV again)


Do it like Microsoft: Everybody calls it a BUG, we say its a FEATURE.

I think, I'll do what 99% of IDA-71 homebuilders did before

As the chinese say: "Copy is honour to the master" - So thanks Dave!