DIY rebreather under construction

marekm

Hi Ekke,

I've just done an experiment where I had a small sheet of PC in contact with a worst case scenario caustic cocktail and crushed scrubber lime dust for a period of four days non-stop. After this time there has been no change to the PC plastic from either the solution or dust. Compared to my eariler tests with concentrated (several molar) NaOH where PC started to degrade slightly after 24 hours, a caustic cocktail from rebreather lime is too dilute (pH ~13.5) to degrade PC at a significant rate. Thus, there should be no problem with building scrubber parts from PC. This will also avoid the crazing problems associated with PMMA, and give better structural integrity (which is very important in rebreather design).

Cheers.
-Marek

Ekke

Thanks Marek!

I'm not sure if there are any good reason to make the lid from PC, but that's nice to know that it can handle even caustic cocktail. Well, PC is cheaper than POM but there are so much "stuff" in the lid that you can't see much.. And I'm not sure if there anything to see at all..

Anyway, there hasn't been so much progress with our project. Today I made connectors to Ray hoses and perforated plate to scrubber, but we are waiting some parts to arrive. I didn't have access to milling maching today so couldn't do a lot.. Maybe we can start to make counterlung in weekend.. We will see.

-Mikko

montyg

May I ask if anyone knows anything about the suitability of HDPE for the same purposes as discussed with PC and such please?

rgds
monty

Duncan Price

I work for a company that makes wet and dry gas scrubbing units for the semi-conductor industry. We have to deal with all sorts of exotic nasties. Polyolefins, PTFE, cPVC etc are the materials of choice. Polypropylene is the workhorse material. PTFE tends to creep (which can be good or bad depending on its use). cPVC is brittle.

I would not use polyamides (nylon) in any wet environment as they tend to absorb moisture. PC and PMMA tend to suffer from envronmental stress cracking/embrittlement.

HDPE or PP are good choices and can be hot welded. Gluing is more problematic. I'd prefer PP over HDPE for dimensional stability.

We also use more exotic materials such as inconel and hastalloy for certain applications.

jmhardingjr

Can anyone provide information regarding the use of ABS for the above applications?

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". . . the sea's in my veins, my tradition remains, I'm just glad I don't live in a trailer . . ."

Duncan Price

Poly(acrylonitrile-butadiene-styrene) terpolymer should be OK for service with low pH substances like sodalime.

Basically I'd avoid anything in contact with a caustic compound that is a condensation polymer of some sort. This includes, polyamides, polyesters and polyacetals. Delrin (polyoxymethlyene) is readily machined and dimensionally stable but I wouldn't use it in a scrubber (though I wonder what Draeger use - some bits I have from a Ray are marked POM.

I'm probably being overcautious. Mind you we scrub HF, Cl2 etc. here.

jmhardingjr

I've heard that there can be some dimensional stability issues with ABS as temperature changes. Is this insignificant under normal diving cirucumstances or perhaps not an issue at all? The reason that I ask is that most designers who use engineering grade ABS sing its praises in terms of machinabilty, tensile strength, etc. Seems like it might be an economic alternative to POM or other more exotic materials.

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". . . the sea's in my veins, my tradition remains, I'm just glad I don't live in a trailer . . ."

Duncan Price

Not a problem under normal use. A lot of common plastics don't like temperatures approaching the boiling point of water. Embrittlement could be an issue with some polymers, if for example you went ice diving. ABS has good impact properties if you are going to be harsh with your rebreather. PC and PMMA aren't very good especially after long term use in salt water. If you want better dimesional stabilty then glass filled polymers are the way to go.

You have to remember that polymers have several orders of magnitude increase in thermal expansion over metals. This might lead to issues with tolerances if you are going from extremes of temperature. The low thermal conductivity is useful in scrubber bodies because the stack will keep warm.

Tom Rose

Some folks have real issues with ABS....something about the ABS breaking down into toxic materials when heated I think.

Having said that, I have used ABS for scrubbers, backpacks, and other items in my home built units a number of times with no problems. It does machine well with sharp tools, is already black and polishes up really well. If you are careful not to overheat it, it thermoforms quite well and it glues with all purpose glue in the plumbing department at Lowes.

One thing about my endorsement of the material....my diving is the slow easy type not generating a steam plume from the scrubber....your mileage may vary as EPA says.

The stuff is strong.

On the down side, there are less fittings available for ABS than PVC...

Tom

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Duncan Price

I think that if your rebreather is on fire then you have bigger problems than inhaling the decomposition products of the materials of construction of your life support apparatus.

ABS will decompose to give off hydrogen cyanide and styrene amongst other nasties. PVC readily decomposes to liberate hydrogen chloride and some benzene, teflon produces hydrogen fluoride. Perspex generates lots of monomer to the extent that you can smell it when you machine it. In fact seeing how a plastic burns is an undergraduate experiment that is used to identify them. Unless the polymer is highly plasticised then I wouldn't be to concerned about breathing in anything bad for you under normal operating conditions.