DIY O2 or CMF "orifice"...

jaap

Hi

Thinking about the KISS orifice thread I had an idea while sitting in the lab tinkering with some fine high pressure tubing.

So after regular working hours I collected and cut a series of different lengths and inner diameters (ID) of tubing made from PEEK or Stainless Steel (SS).

Using swagelok fittings I tested if the fine tubings can act as Constant Mass Flow (CMF) devices. I ran them with a fixt inlet IP of 12 bar, varying the outlet pressure to simulate different diving depth while measuring the surface equivalent flow. The tubings can tolerate pressures up to >5000 psi so a much higher IP could be used if one wants to...

By varying the length of tubing I could replicate the CMF-function of the small orifices commonly used in CMF diving applications.

So one idea could be to use a tubing instead of a needle valve or precision orifice to achive the CMF-flow one wants. This would work either for CMF SCR or CCR ("KISS") operation.

Advantages of a tubing CMF-device:

• Very open to custom solutions, the right combination of ID and length can give almost any flow for a given IP
• A little larger ID than the corresponding orifice size, perhaps more resistant to clogging.
• The tubings are cheap so one could have a large number of them as spares.
• They are compatible with swagelok fittings, so very easy to fit and replace.
• PEEK-tubing is extremely inert, so no risk of corrosion.

Disadvantages:

• Unprotected PEEK-tubing is a rather stiff plastic and can get bent altering the flow.
• If PEEK-tubing is used with SS-ferrules and fittings it is possible to overthighten the fittings damaging the tubing and reducing the flow.
• The tubings could be rather long if one wants a hight IP, large ID and a low flow
• The tubing is cheap, but the fittings are not necessarily so...
• Homebuild solution, not properly evaluated for life support...

Som preliminary conclusions:

Using the correct ID-tubing one can get either KISS-style flows or higher (SCR-applications) flows using a tubing in the order of 1-5 cm long. Making it compact and easy to protect the CMF-tubing from damage.

The small ID SS-tubing is very stiff and hard to damage but more difficult to cut to a customized length. The PEEK-tubing is very easy to cut but is not as robust so it would need to be protected.

I used ordinary stiff PEEK and SS tubing but there is also coiled flexible PEEK tubing and SS tubing that is coiled.

AFAIK swagelok compatible 1/16" OD PEEK tubing can be bought with an ID of as little as 0.0025" (63,5 µm) and a pressure rating of 8000 psi so pretty much any CMF flow can be achived.

This is one site that sells PEEK tubing:
http://www.metachem.com/Products/MISC/Peektubing.htm

Or scrolling to the PEEK parts on this page:
http://www.alltechweb.com/literature/cat600/Pprod.asp

I have not used this sort of setup in a rebreather and I already have a number of more traditional CMF-devices such as needle valves, Hydrogom and various Russian and Dräger dosage devices, so I have no absolute need to build a diveable system.

But it would be fun to give it a try

Any comments? Or experience if someone has already made a similar system?

I did not mention any specific combinations of IP, lengths and ID. This was on purpose since I don't want that anyone that cannot setup and verify the flow (under a simulated dive) of such a system to rush off and buy some tubing and dive it... Especially so since I haven't spent more than an evening exploring if this is something that can work and if I have overseen some important thing.

whoizrob

But why reinvent the wheel? I understand that a kiss valve may be expensive to a homebuilder but a CMF orifice can be had for very cheap. If you do a search you can find manufacturers who sell the things for 30 cents. Yes CENTS. Granted they are not ruby but they work just as well, come in a large variety of sizes to fine tune your LPM flow, and are easily implemented with no specilized hardware. Run one of those inserts and a bypass line and you are golden. Sure it's not the piece of art a KISS valve is but if you want somthing to prototype with on the cheap, there you have it. Just make sure you keep an eye on your PPO2 at all times (which you should anyways!).

Rob

Gibbon

Can you please provide a link Rob?

Captain Randy

Thats part of the fun of homebuilding.

Randy

whoizrob

http://www.airtrolinc.com/components...n_orifices.htm

One of a few suppliers of said orfice. Just like the KISS valve, you must use a fine filter and as you can see this supplier also has filters (5 micron and cheap too!).

Plumb one line from your modified apex with the orifice and another to a bypass and you are good to go. It lacks the simplicity and design beauty of the KISS valve but is far cheaper alternative for home builders and effectively does the same thing.

HTH,

Rob

shell-back

yep, I have been playing with the same idea with a bit of a "twist" I will post when all the tests are complete.
there are some advantages to said system that should prove them selves in my tests. I would not hold your breath on it being available soon and would go with the Kiss MAV or Hydrogom valve any day , they are solid preformers and are relatively inexpensive.

__________________
The greatest obstacle to discovery is not ignorance -- it is the illusion of knowledge. ----Daniel Boorstin
Billy Beal
SDITDI IT #3507
RBW FIGJAM commitee member

Scuba_Vixen

I'm not sure I'd want to be trusting my life to a molded polycarbonate orifice rated for a max of 100 psi in a 100% O2 environment.

For something much safer, and still in the affordable range, ~$30, Look at Lenox Laser's laser drilled set screw orifices.


Darlene

whoizrob

Good find as usual Vixen! I'm going to have to take a closer look at those. Truth be told I did have some reservations using plastic but figured they'd do fine for compressive forces and orifice deformation. Reactivity? Not sure about the safety margin but I do know several people who've used them without trouble for a few years now. But I definately do like the metal orifices! You certainly are a mountain of useful info. I'd love to buy you some lunch and pick your brain one of these days, too bad you're bazillion miles away in paradise

Rob

denzel

yeah yeah, just her brain eh?

__________________

LukeN

Just no sense of adventure My one worked fine for a year and 50 hours untill I worked out the dolphin couldn't cit the mustard any more.

[quote=Scuba_Vixen]I'm not sure I'd want to be trusting my life to a molded polycarbonate orifice rated for a max of 100 psi in a 100% O2 environment.

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Luke Nelson

www.imagedive.com.au