Anyone up to “no good” with design ideas?

canuckdiver

and believe me Bruce, we GREATLY appreciate it!!

On a slightly related note, does anybody have any data on concerns about using ABS plastic for things like scrubber canisters?

Simon A

===Wish mode on====
Just a quick thought, This being the home builders section and all...
Might it be worth generating a list of materials that it's considered safe to use in the loop of a rebreather? both trade and generic names would be nice, +links to relavent COSH or MDS 's
----Wish Mode off--------

Simon A

AD_ward9

Those little boots over the Evo O2 DIN-Coax/SMB connector look like PVC. Would be useful for APD to confirm it is something else, such as nylon, which it could well be. Without embarrassing anyone, there is a scrubber canister here from a commercial rebreather, the plastic is not identified but looks like PVC and is definitely not nylon.

PVC is not a suitable material for RBs, but the US Navy line on Delrin is absurd. For rebreather parts, where Delrin is used it is not normally moulded: it is CNC machined. As a machined plastic, its properties are outstanding. If a part is CNC machined, I would prefer it out of Delrin than out of any other common machineable plastic. If it is moulded, then Grilamid is the material. It is not burnt in a rebreather, so a military regulation concerned with building safety, or ship safety, in the case of fire is not really relevant. Delrin, I suggest is eminently suitable for rebreathers.

It is a pity Grilamid is not available in thick slabs and rods to machine. It does machine nicely: we tested that too. As a Nylon 12 derivative, it is very stable.

We must keep risks in perspective. What is the risk of your Rebreather catching fire at 100m? What is the risk of a plastic fitting breaking at 100m? If a CNC´d part is replaced by another plastic that is a quarter of the strength or toughness of Delrin, then I suggest the latter risk is many orders of magnitude greater than the first risk. This means that by applying a regulation inappropriately, divers lives get put at risk.

I assume we all agree one can ignore the powder toxicity: at a LD50 of 11Kg/kg (you would have to eat a metric ton of it to have a 50% chance of death), Delrin is less toxic than carrots.

Anyhow, that is the US Navy market. It should have no bearing whatsoever on what materials are chosen for rebreathers for the sports market, unless someone is worried their ex is going to fire an Exocet at their dive boat. But it never ceases to amaze me how many people go with a certain dive operator who uses his oily compressor to pump O2, putting a nice mist on the inside of your O2 cylinder - perhaps it stops it going rusty?

On the suggestion to use ABS, ABS outgasses fairly badly. Just smell a box of some fresh ABS mouldings if you need confirmation. ABS is fine for cheap handset housings, but even there ABS would be an unusual choice from someone aware of the options. ABS also produces nasty chemicals on decomposing, which it does more easily than Delrin (e.g. UV if you leave your canister open on deck, and dive somewhere other than sunny Scotland).

Cheers,
Alex

caveseeker7

Cool, very convenient.
You wouldn't have to have SS5L1-AA-MAN D 10 handy, the NSSC's US Navy Diving and Manned Hyperbaric Safety Certifications Manual? That deals with the test requirements regarding offgassing and flammability.

I would hope not.
While the MIL HDBK-2036 may be for guidance only, the Navy seems to go by it.

__________________
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.'!"

AD_ward9

You have trailed your coat over something tantalising. Where is the report on batteries?

On primary cells, the contacts are a major failure point, the variation in cell characteristics a minefield for design validation (means, design are not completely validated), and if you try pressurising them to 300 bar they do more than wrinkle. I have seen a lot of problems with electronics used on RBs from particular types of batteries. I would be interested in the official report. Any chance of mailing it or posting it, as you refer to it? All our designs use rechargeable batteries, particularly Lithium Ion gel batteries. We have tested these quite a few times, including recently, to 300 bar, and they work quite nicely. Li Ion, as you know are rechargeable, with excellent power density - they are the batteries used in mobile phones. At 0 to 300 bar in 30 seconds, just the skin wrinkles. Not that we plan to use them to 300 bar, but we figure if they can take rapid pressurisation and depressurisation to 300 bar, then they will work at 20 bar. If they are ruptured then they can develop a nasty short, so they need to be in a hermetic chamber that can take the pressure and does not burn, just for that little risk that someone decides to fire a speargun through their Rebreather at 100m!

Cheers,
Alex

montyg

on another email list ( I forget which ) this was said:

Scrubber Material Choice

Below an extract from an email that explains the whole lot :

, Stuart M <divebimbo@l...> wrote:
> Hi Folks,
>
> I am looking for a watertight/airtight cannister, preferably
plastic, for a
> diving DIY project. Anyone have any suggestions?
>
> Something like an umbilical torch cannister (but cheaper!),
cylindrical
> with flat ends, something easy to drill and a bit more robust than
bog
> pipe. I've seen a few things that would work but are a little bit
too
> small, like the spaghetti jars posh kitchen shops sell. Ideally the
> cannister would be about... ooh... for the sake of argument... the
size of
> a Desperation scubber.

Some of us homebuilders here is the 3rd world have recently gone
through this learning curve, so please allow me to share our
experiences ...

Wrt the choice of materials methinks you need something that doesn't
crack or shatter easily, has good chemical resistance ( against salt
water or sodalime ), isn't porous so it doesn's absorb water, has a
wide range of operating temps, machines easily, is cheap and
avaialabile in correct diameter/ wall thickness, and doesn't break
down when exposed to UV.

We looked at some water filter housings which would have made ideal
scubbers, but the plastic can't handle 'sorb going at 40-50 degrees
celcius.

PVC ( bogpipe ) is hard to get in tube with adequite wall tickness
and solid rod is a waste of money and a lot of machining if you are
making a pipe.

Perspex/ Acrylic has tends to crack and shatter in low temp and
breaks down under UV.

Polycarbonate/ Lexan was bloody expensive and I can't remember waht
else was wrong with it.

HDPE ( used for piping gas in the UK I am told ) is obtainable here
in 131 ID and 160 OD, with awesome wall thickness, easy machining,
great properties ito temp, chemical resistance, etc and a 10 bar
rating. But the 10 bar rating applies to pressure in the pipe and I
am told 1 bar inside and 10 bar outside will have a very different
outcome. Did I mention it's cheap and you can get it in any length ?
I don't know about you but a 6 meter length doesn't fit in my car !!!

( also see
http://www.sump4.com/rebreather/p1ss_construction.html#scrubber )

So my scubber is being built from HDPE ...

For light cannisters we found a guy who does 115mm ID filament wound
glass fiber & resin pipe which is also cheap and can get in any
length, can handle the pressure diffs and is perfectly round out of
the factory.

Many people will tell you "it's perfectly round sir". When you tell
them that the o-ring must seal because it's life support equipment
and if it doesn't someone might die they change their tune ...

my 2c worth

rgds

monty

AD_ward9

It is a perturbing thought about how many people waste time reinventing the wheel. Today there are around 110 rebreather models on the market. On the second hand market there are several hundred models, a lot ex-military. What is it about rebreathers that cause people to want to spend time doing what someone else has done already with far greater resources?

We all know rebreathers made from plumbing bits and drainage tubes: one or two are successful commercially. Just this is not the way to do it properly, and trying to do it that way, kills people (not the drainage tube, but the gamut of things associated with that type of development).

As to the Third World, the only differences I see between the Third World and First World are:
1. In the Third World, the value of life is even closer to zero than in the West, but events in the West show we are catching up on this point quite fast.
2. The difference between rich and poor is greater, because instead of governments maxing the return on the population they govern, it is private individuals/criminals, or private individuals dressed up as governments. (Again, we are catching up on that in the West).

If someone in the Third World has enough resources to develop and use a rebreather, albeit home-build, but not enough to do it properly, they are just accentuating point 1 above. If they want a rebreather and resources are borderline, then why not buy a second hand unit? Some ex-military units are a few hundred bucks in the Third World. A unit developed by the military 20 years ago has, in most cases, less chance of killing someone than a home brew.

Linked with the above, is use of a home brew rebreather without any rebreather training. Does someone who makes a home brew out of plumbing bits in the Thirld World spend $1000 on a course on a commercial rebreather before using their own? Where is that flying pig ....?

In my book, it is one thing to modify a rebreather, another thing to build it from scratch.

I have lots of rules of thumb which have held out over the years. One or two may be of interest here. As a rough rule of thumb, development of a rebreather costs between 40 and 200 times the sale price of a unit today (40 for a minimalist development, 200 for a really good product). Buying an NRE package from a design house costs cuts the number by about a factor of 5, but they start with the 200x figure. With sales what they are, this means a lot of contenders never make money on rebreathers: companies nearer the 40 end of the spectrum very rarely break even on a complete rebreather.

Taking a concept from design to production, including proper testing and approvals, costs a year and $500K to $900K, or about half that if a retrofit to an existing system. Time is often multiplied by a "novice factor" which varies between e and pi depending on the novice. Once they have spent their $500K or $900K, then they should figure on spending 4 times that if it is going to be successful in the market. The short of it is we see a lot of folk who need to put their hobby to one side, go and earn some money, then buy a proper development and production package. I know raising VC cash for any life critical technology is incredibly difficult, so the industry is underfunded. You cannot get insurance for this type of work.

So all the homebuilders out there, why are you doing it from scratch? What is it you are trying to achieve? I will contact a moderator and find out how a vote is set up on this forum, so you can give your reasons and we understand your drive. It would be most illuminating.

My §0.02.
Alex

divetheworld

Sigh,

ABS polymers are available in many different derivatives. Did you know that ABS is used in kidney dialysis machines and artificial joints?
Avoiding high temperatures in the process and the product will reduce volatiles on certain ABS types such as the Inspiration, so I suggest that if you don’t like it, don’t leave it cooking.
We experimented with a number of different amorphous medical grade Acrylic polymers, and methods of eliminating volatiles in ABS polymers is widely available by incorporating other polycarbonates. Sometimes design and research comes from further than google.

.

My shit is tried and tested.

__________________

Crazyduck

I know the feeling; had the same thing happen to me.

THis should help- Most high end camping, climbing, paddling stores ask for the Gore-tex resealer. Here is some interesting info I "snipped" off a web bpage.
Later, Andrew

Gore-Tex® Care Products, Sources of Supply, and Info

Nikwax Tech-Wash - a non-detergent soap especially formulated to be used with Gore-Tex. Machine washing recommended if the garment instructions allow it. Tech-Wash should be available from local outdoor and paddling shops who carry Gore-Tex garments, or from REI.

Nikwax TX-Direct - a water repellancy restorer for Gore-Tex. Available in both a wash-in formula and a spray-on formula. This should also be available from your local outdoor and paddling shops, or from REI.

Gore® ReviveX® - another DWR restorer for Gore-Tex from the makers of Gore-Tex. A new product supposedly three times as durable as other available finish restorers. It's too new to know for sure if it will live up to Gore's claim. More expensive ($5-$10 per garment per application), but would definitely be worth the money if it will take an older, well used rain parka through a long wilderness trip in wet country without failing. Try your local Gore-Tex garment outlet, or REI.

Nikwax: A chemical company based in Everett, Washington:
http://nikwax-usa.com/ (Product use info)

W.L. Gore and Associates® in Elkton, Maryland:
http://gorefabrics.com/ (Some care info)

REI, a large retail outdoor chain store based in Seattle, Washington:
http://www.rei.com/ (Product and info)

Kokatat, a garment manufacturer based in Arcata, California:
http://www.kokatat.com/ (Some care info)

Ben Field

Best source for this is to ask the gas men when they are digging up the road to fix/fit more of the stuff, they always have offcuts.
I have a few meters in the shed that I "lifted" from them while they where making a mess of my road... if anyone wants a short bit from me I could spare a scrubbers worth, probably.

Only down side is its bright Yellow.

BEN

__________________

Beware Fridge Suck