DIN over 3000 PSI?

[bradshsi]

There are a lot of O-ring parameters which are not easily reduced to numbers, like mechanical strength. Urethane is probably the "toughest" O-ring material made, which makes it ideal when extrusion is a problem.

I guess I'd have to disagree with that. Engineers have spent a long time comming up with objective tests for material properties. I don't believe that O Rings are an exception to that.

Urethane has very good tensile strength, short term resilience and wear resistance which is probably the "toughness" you refer to.

[Just going to a higher duro nitrile or EPDM isn't a perfect solution because. While the higher duro improves resistance to extrusion, it is often at the expense of sealing qualities. And it doesn't necessarily make the O-ring any stronger - just harder - so while it will resist extrusion better in the short run, it will often break down sooner than a urethane O-ring would in the same application.

This is true, but just to point out that if you are already using a 90 hardness urethane O ring, changing to the same hardness of EPDM is not going to affect the sealing qualities either way.


I'm interested in your use of urethane given it's tendency to have a large compression set when exposed to temperatures >50 degC. One scenario I could think of is when the yoke valve/tank assembly is on a dive boat exposed to the hot sun. Under this scenario the urethane could take quite a large set which would probably shorten it's life. Have you encountered any similar problems in your use of them ?

 

[DivePartner1]

Yeah, the "tech world" uses DIN. So what? They also favor low pressure tanks, duh. The "rest of the world" does not use DIN, just some Euro countries and former colonies which get their valves from Europe.

The INTERNATIONAL yoke connection is equally as reliable up to 3500 psi. There is no difference except that the yoke screw is easier to snag on fishing lines or catch on an overhang, but accidentally knocking the reg/tank joint ajar is very unlikely. The yoke valve is easy to keep clean with a simple piece of tape, and can be blown dry easily, but suffers in rental situations where the operators or customers use an ice pick to change out the O ring. However, the claims that the yoke clamp is not "safe" is a bad rap. The DIN is best suited for use on VHP (very high pressure) tanks up to 4500 psi. It is optional on other sets, no more, no less. It is superior for complicated cave rigs, the kind with multiple tanks and regulators and a lot of other crap. The DIN is simply a cleaner installation. I wouldn't use it at higher pressures (over 4500) due to the thin walls of the DIN valve. The socket in the DIN valve will collect moisture and grit. Compressor operators have to be aware of this and clean the valve thoroughly. When not in use, a large plug is necessary to protect the socket. Also, the regulator threads are exposed, not protected by a yoke clamp. The threads and the inlet have to be shielded with a boot.

About "bar". A bar is not one atmosphere, it is just an approximation, a happenstance which makes it handy to calculate (roughly) volumes. Please, no more "physics for dummies" lectures.

The ISO and CGA "standards" are not specifications. The specifications for the USD and SP yokes far exceed the stuff published by these bodies.

Thanks to all for an informative thread.

To take this to its pedantic conclusion, do you know if ISO developed the standard specific to scuba cylinders orwas it more of a 'one-size fits all' standard for dissimilar applications? Do they publish there deliberations?

 

[oxyhacker]

You can test for specific material properties, but since an O-ring's suitability for a job depends on all those properties working together, and selecting one always involves tradeoffs, just the numbers on the chart are not always going to lead you to the right material - it requires a good feeling for how the different materials perform in different situations. So I tend to trust the judgement of people who work with this stuff all the time, rather than trying to second guess them.

Also remember those numbers are usually based on standard compositions, and can be customized for the quantity buyer.

I guess I'd have to disagree with that. Engineers have spent a long time comming up with objective tests for material properties. I don't believe that O Rings are an exception to that.

 

[bradshsi]

You can test for specific material properties, but since an O-ring's suitability for a job depends on all those properties working together, and selecting one always involves tradeoffs, just the numbers on the chart are not always going to lead you to the right material - it requires a good feeling for how the different materials perform in different situations. So I tend to trust the judgement of people who work with this stuff all the time, rather than trying to second guess them.

Also remember those numbers are usually based on standard compositions, and can be customized for the quantity buyer.

I'm in full agreement with that statement. The point I was making is that the material properties are not mysterious subjective things. The knowledge of what works in what application (due to tradeoffs), is a bit more subjective (as you point out in your post).

I also rely heavily on the seal manufacturer's advice when designing reliable joints in equipment depending on what flamable and/or toxic fluid needs to be contained.

I realize that compositions can be optimized for a specific customer. The reason I prefer to highlight the strengths/weaknesses of standard compositions is that most people don't have the buying power or access to specifications to determine whether composition A is better than B. They go to a store and say "I'd like an EPDM O ring please".

Thanks for a really interesting thread. I've learned a lot about the relative merits of urethane O rings for scuba use.