Anyone up to “no good” with design ideas?

AD_ward9

High performance nylons tend to have very low water absorption: well below that of Delrin. Nylon 12s are a good example. This reduces linearly with glass loading as well.
Cheers
Alex

onetime

When looking at building a rebreather you take it to a far different level than simply using one. While some may get that in depth when taking a course it's pretty doubtful.

I think our differing opinions come from where we see innovation coming from. The great leaps in innovation typically don't come from those using 3d flow simulation tools and the like but those who take established concepts and come at them from a new perspective.

Rather than throwing a pump in there why isn't the focus more on keeping the water out in the first place? Thinking of things in traditional ways (e.g., floods are going to happen, just pump the water out) only gets you so far. Profile wise I agree about the barns most homebuilders make but just because it's not out there yet doesn't mean it won't be at some point.

The end all be all isn't in the manufacturers' house. Modifying existing is one approach but certainly not the only.

AD_ward9

Safest and cheapest one. Allows the home builder to focus on the weakness they see and not have to worry about doing all of the rest.

I thought that such a cheeky post in the home camp of the home builders would inflame passionate debate. Let us vote on it!

<EDIT> Thanks, I have just put up the vote<ENDEDIT>

Remember this part of the thread is very tongue in cheek.

The discussion on Delrin, and on batteries was very good.
Cheers
Alex

caveseeker7

This is a three page thread that's already all over the place.
Why not start a new thread on the subject with a poll ...

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

divetheworld

Someone is obviously doing it wrong. Everything I made actually functions without failure.
And in case you missed it earlier, I said that not all ABS polymers give off volatiles, we tend to develop plastics with our clients with a little more technology that sticking our heads in a box of pipe parts.

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AD_ward9

I would love to meet you, as you have found a trick. Please could you post pictures of the things that never fail.
In the world I live in, everything fails, sooner or later. Second Law of Thermodynamics: most fundamental law of the universe. Just some cases the probability of failure is such one has to wait a long while.
Cheers,
Alex

onetime

I'll agree that it's the cheapest but I'm not convinced that it's the safest.

The downside of modifying what's out there is that you're pretty much relegated to back mounted designs engineered as a stand alone system using OC as bailout. How would designs be different if, from the start, the goal was to develop an Rebreather with RB bailout as part of the system?

AD_ward9

There are commercial companies working on this. We are doing a unit using two 5 inch canisters in one, similar to what I posted, where the unit can be configured by the user either as a single rebreather with very long scrubber capacity, or a rebreather with a bailout. Uses hoses that go over one shoulder, and a plug in bridge piece linking the two sides of the scrubber. I am aware we are not the only company working on this type of design for a client.

On another post, which complained all RBs are backmounted: Quite a few military RBs are front mounted. Then there are ten or so RBs which are the shape of an 80cuft cylinder, so it can be side slung, back mounted, dragged or whatever.

One of the issues, I am surprised no-one has brought up, is that it is only recently the volumes in the sports Rebreather market have justified any significant design teams. Until recently, one could say that all the companies in this area were just better funded home builds, but without the safety design infrastructure one would expect of a company designing professional products.
How many Rebreather companies have someone on their staff who has had formal training in the design of life critical systems? I know only a couple.
Cheers
Alex

divetheworld

Speaking of the 'other parts', I know you made a lot of parts (HP and LP) from titanium is there any chance of discussing this at any point? Maybe off the board if you prefer.

And in case you missed it earlier, I said that not all ABS polymers give off volatiles, a couple of my clients develop plastic components. I do them favours, they do favours for me. They tend to use a little more technology that sticking their head in a box of pipe parts and having a sniff. I dont profess to be an expert in plastics, but I do use the best in the business.

Someone is obviously doing it wrong. Everything I made actually functions without failure.
Homebuilders may not have state of the art design facilities but their kit actually ends up in the water and that is what gets diving done, which is what this is all about isnt it? I have to point out that your comment appears to be somewhat dismissive and I would guess even insulting to those who have spent many years building up practical experience. In most cases using low-tech but well equipped workshops, not bedrooms and building some pretty reliable and innovative stuff.

On the subject of homebuilders, would anyone care to recall the names of a few divers who have made successful rebreathers/products and then make reference to how many of them are/were homebuilders?
lets start this one off shall we;
1. D. Thompson
2. Bob Howell
3. Kevin Gurr
4. Kerry McKenzie
5. Uwe Lessmann
6. Uri Baran
7. Dave Sutton
8. blah..............

I might suggest that you have a look on Jan Willems site at the homebuild and modified rebreathers. Respect is due to all those who learned from practical experience and developed great underwater tools. Experience comes from getting down and dirty with a rebreather and actually using it.

How many rebreathers today started off as homebuild prototypes or development tools?
It would be easier to discuss the ones that were not such as the CIS. The most successful rebreathers in the world were all homebuilds. Unfortunately the best rebreather designs never made it into mainstream production, usually due to cost problems but others because they were talked about but never realised for whatever reason.

I've waited a very long while. Still waiting. I guess I must be lucky, or so they tell me.
Funny that. The more I practice the luckier I get.

Of course, there are those that know.

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AD_ward9

You make a few points, so deserve a full (sorry, long) answer.
Quite happy to discuss Ti on the forum. We are fans of Ti, but every material has its limits. There are some interesting things with titanium. O2 compatibility in particular, and the effect of adiabatic compression. The exact alloy one uses is very important, and the state it is in.

We have a reasonable experience of buying and machining titanium, of various grades. We do like titanium because it does not corrode in sea water, no environmental problems with chrome plating, or quality problems with plating, it is strong and light, and cheap ($4/kg sponge, and about 3 times that when we buy it in bar and thick sheet). Nitriding makes it gold or purple, or mixed.

The use of Ti with O2 has a long and mixed history: it is a very interesting material in that regard because it is highly reactive, but the oxide and nitride layers form so fast it has good performance. The problems are mostly caused by microwelding: it is important Ti does not slide over any other Ti surface, otherwise the oxide layer is removed, the material microwelds, and risks an O2 fire. Microwelding is so bad, one cannot run any threads where Ti is on Ti.

The microweld problem means if one does pick Ti for a valve body, the needle must not be Ti, and the design does need a fair bit of testing - computer simulations mostly, to determine what energy is impinging on the material. When it does light, Ti does burn very nicely in O2, but then so does SS. Some bronzes and Monel used for O2 lines will light off but requires a huge pressure. Design defects with Ti are unforgiving: there was a Ti valve I remember from Atomic that caught fire. Ti is one material that does not suit a home build.

The reactivity means that no thin section should be in Ti. If Ti is thin enough, a spark in 2 bar O2 is enough to set it off: if it is thick, this is not a problem. We have not tried this at 300 bar, but up to 30 bar.

Currently, our policy on Ti is to use it in pure O2 environments, but not in any area were metal slides on metal, no thin sections, no area where adiabatic impingement is likely, no continuous regions (e.g. a fitting on a hose is OK, but not running it all the way to the breathing loop). We only have plastic inside the breathing loop except for a few screws and the valve pins which are SS. We do not run any intermediate pressure gas lines inside the breathing loop: the solenoid or valve pierces the loop and connection is made outside.

Sintered Ti filter slugs are OK so long as they have a big enough cross section that the flow rate is low, but as sintered SS is much more available this is more theoretical than reality. Pressure sensor housings in Ti are OK fitting a normal chrome plated third party first stage, hose termination is OK, injectors are not OK except for the body and even then I would be nervous and take a lot of convincing: a ceramic or artificial saphire/ruby needle on a Ti body would probably be OK. Ti does not suit a first stage reg at all, except for the pressure sensor (contents gauge port fitting). Tubes to the injector are not OK (for different reasons), Ti O2 cylinders are fine and well tested. For all the studs, screws, clips etc, outside the loop Ti is ideal.

The real application of Ti is anything in contact with sea water, where a plastic is not suitable for reasons of ruggedness or strength. We do embed Ti in plastics when we want one property but not the other.

The liability issues are such that use of Ti for commercial products in O2 is rare. We would generally use SS 440C instead. However, our demonstrators use Ti as described above.
I am sure you are right, but all those we have used do give off volatile components. With Grilamid available, we just don´t see the need for ABS for rebreather parts. ABS has its place: instrument cases etc. We have designed around 20 parts using ABS: not a lot, but some were quite complex. Please could you quote exactly the ABS you refer to, with the link to their MSDS. I am genuinely interested: if there is a specialist ABS out there that is suitable for rebreather parts, it would be of use to know about it. We have looked at quite a few different plastics, used a few, and the state of our knowledge is that for CNC´d parts, Delrin is the material of choice, and Grilamid for moulded parts, preferably with a thermo plastic elastomer over-moulding so it feels good.

On homebuilders: Some on your list are still homebuilders.
Jan has done a good site and I have visited it a fair few times. My question was "why?" Exactly the problem with more than a few of them.
Come on: Military rebreathers are the most successful commercially, until their makers lose the contract, and very few modern military units are home builds. Of the sports home builds, I would suggest the industry has suffered as a result of their home build origins due to failure modes which would never have been there if they were designed professionally. No problem: just increase the population of them. Something that you have dived 400 hours on, does not prove something is safe for a billion hours: the baseline for a life critical design. The problem is companies have tested kit for hundreds, or even thousands of hours, release it to the market, and suddenly find their product has a 1% Club due to design defects.

It goes back to the points I raised: good design in life critical systems costs money and until the volumes started flowing, which is only very recently, there was just not enough return on the investment. Almost all rebreather developments were inadequately funded. Now funding is starting to get to the coal face, it is going to be interesting to look back in a decade at what people use today. The next decade is going to see a huge change in eCCRs, and I do not think those changes are going to come from home builds.
Cheers,
Alex

NB: Before anyone quotes it, yes we do know the NASA report NSS 1740.15
JANUARY 1996 on O2 compatibility of materials, which says:
"Titanium. Of the various titanium alloys tested ( b b a a , , - alloys) and
reported all showed very high sensitivity to mechanical impact in oxygen
(Key and Riehl 1964). Titanium must not be used with LOX at any
pressure or with GOX at oxygen pressures above 207 kPa (30 psia). Tests
have indicated that titanium, a -titanium, and 2 a -titanium alloys can be
ignited and sustain combustion at oxygen pressures as low as 7 kPa (1
psia). Frictional heating test conducted on titanium and titanium alloys
indicated that the Pv product for ignition is extremely low (see Table B-2,
Appendix B). Recent tests at WSTF indicate that titanium and its alloys
can also be ignited in air in frictional heating tests.
Titanium alloys must be avoided in storage or test facility systems since
titanium is impact-sensitive in oxygen. A reaction of titanium and LOX or
GOX may propagate and completely consume the metal (Kimzey 1970;
Laurendeau 1968; Pelouch 1974; McKinley 1971)."
The report has been superceded by another, and with
"NASA Glenn Research Center Glenn Safety Manual
Chapter 5 - Oxygen BMS Document GRC-M8300.001 Revision Date: 2/05"
which defers to the American Society of Testing Materials (ASTM) MNL 36, Safe Use of Oxygen System Design, Materials Selection, Operations, Storage, and Transportation of January 2000,
Quite a lot of Russian reports on this also, quite a few not saying the same thing as NASA due to use of different alloys
Ti is widely used with O2 in aviation, space, moutaineering.
Issues are friction with Ti in O2 (does not occur in anything I described above), adiabatic compression, particulate, oil in O2 in Ti. It is these issues that give rise to narrow application.