Open source rebreathers - an alternative approach

montyg

I have not found that, and I just had some HDPE pipe put in a lathe last week because it wasn't round inside, and now it's perfect and the o-ring seals like a dream!

The comments from the owner of ESO Engineering, Marc (who did the machining for me):

> Q:What's the HDPE like to machine in general, and what's it like to tap
> thread into it?
> A: it is beautiful to machine and takes threads well, on thin items like 2mm plate the threads can become a bit of a problem but on thicker pieces like 10mm and up they are quite strong.

rgds
monty

GKAM

Well, it depends how precise you want your machining.

For example, SS DPVs use HDPE for the scooter body. They process the pipes to make them perfectly round on the inside and straight/flat on the sealing face. These are large bore tubes.

However, you cannot machine a fine thread on HDPE. It is too soft compared to acetal and it also frayes (is this the right word?) if you try to cut fine pieces.

Bottom line, is that IMHO it is fine for making a scrubber body but a PITA for detailed work. It would be unsuitable for example to use for the scrubber head.

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montyg

GKAM wrote: "Well, it depends how precise you want your machining.
However, you cannot machine a fine thread on HDPE. It is too soft compared to acetal and it also frayes (is this the right word?) if you try to cut fine pieces."

OK I'll get them to to a trial run on an off-cut. Does anybody know if the fine thread on a female Drager P-connector is a standard thread? If so, which thread is it?

GKAM wrote: "Bottom line, is that IMHO it is fine for making a scrubber body but a PITA for detailed work. It would be unsuitable for example to use for the scrubber head."

OK so the question is then if it is good practise to fit a delrin "head" to an HDPE scrubber body? Will they expand and contract or swell at the same rate and stay water- and gas tight?

rgds
monty

Genesis

SS scooters use a UHMW machined plastic head on top of a HDPE pipe and have no problem. They use a face seal with a big O-ring - that should work even for items with slightly different expansion coefficients over the ranges being worked here.

That's my intention, by the way....

BTW the gas pipe you sent is great - but it turns out to be a bit short and narrow. Would work for the Extend-Airs but I'm slanting towards using a standard loose-pack scrubber and for ease of mounting on the backplate want something a bit longer... thus the search for a ready supply of pipe....

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rdmmdr

i have all the pipe you could ever need just done the street from me at the well/ irigation place and it all free. if you can not find locally send me size dia/lenght.
rick

Genesis

Update!

Just got a reply back from Lee Supply - Kim Wichterman (kwichterman@leesupply.com) is the customer service rep.

They have some sizes/types in stock (6" SDR11 in particular, which is roughly 1/2" wall thickness, and appears to be almost perfect for scrubber cans) and they will ship small quantities. $6.10/linear foot - not too awful.

So we've now got a source for US homebuilders for this stuff..... I'm having two 8' lengths sent for prototyping work; for roughly $50 for an 8' length its cheap.

http://www.leesupply.com

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"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
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Crazyduck

Karl,
What type of scrubber are you making?
I am guessing a Axial with 6” outside DM?
At suggested rating of 1.5 that’s what 4” deep?
Just curious buddy.

Has anyone (outside of Paul) made exotic scrubber (non-standard?)
I am considering a chest O2 style rebreather but there are some real serious drawbacks. Splitting the scrubber into two parts has some great benefits- but the wrong calculations could be Very Nasty...


Best Regards, Andrew

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Rob Davie April 2005-

Presently in a state of transition from Open Circuit to Closed Circuit.

"You will not be punished for your anger; you will be punished by it." - Buddha.

decoweenie

AFAIK, the 1.5 ratio goes the other way, Andrew...

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Crazyduck

9 Inches long
Yeah, that would make a little more sense.
Thanks mate...
Hummm, that humble pie sure tastes good.

__________________
Howdy Senor- What’s Happening!
Rob Davie April 2005-

Presently in a state of transition from Open Circuit to Closed Circuit.

"You will not be punished for your anger; you will be punished by it." - Buddha.

Genesis

6" DR 11 pipe has an ID of about 5.4". The actual OD is 6.625"; when you reference 6" pipe you're really talking about IPS (pipe size) and its not really what you spec'd, just like a dive tank with a 3/4" valve opening measures about 1" in diameter - not 3/4".

The plan is a coaxial scrubber at this point; the total length of the body will be roughly 18" to fit on a standard STA. The 6.625" OD will fit reasonably well on the STA flange. Active area should work out to be a bit under 12" long; I need enough room for the water trap on the bottom and sensors/solenoid (4" roughly) on the top. I am going to go with direct fill rather than an insert. as plastic pipe is a pretty darn good thermal insulator, especially with a wall thickness of about 1/2". That would give me a working ratio of somewhere around 1.75:1 or perhaps a bit longer (length : diameter)

Both bottom and top will be removable panels; the bottom will have the water trap attached to it, while the top has the electronics attached. The downtube will rest on the bottom plate and the bottom scrim will be fastened to it. Three stainless all-thread rods will provide the mounting for the top scrim, along with SS springs and hand nuts to hold pressure on the top scrim to help prevent channeling. The downtube will be mated via a double-O-ring seal to the inlet (which will be attached to the head); that's pretty simple to do with plastic pipe; you just cut the retaining groove for the O-rings on a lathe, so that when you put the head on the downtube seals to the inlet. Top and bottom panels will seal via face O-rings ala SS Scooter.

I am also going to see if I can get a length of 8", which has an OD of 8.625". That'll be a waterheater in the water though..... I suspect the 6" pipe is the one I'll want to actually use for drag/size/convenience reasons, although an 8" tube would make for an easier radial design. I suspect that a 6" radial might have to be too long to be workable and it requires much more effort in terms of machining, unless I come up with a ready source of perforated thin SS material (to make the outside of the can with) and want to start Tigging up an inside can.

I may also build up my original design using the ExtendAir carts off the other piece of pipe I already have, using that as a cross-shoulder mount. That creates other mounting issues for the rest of the kit tho (cylinders, etc)....

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket