battle royale: scubapro hp piston stem oring material

[Candiru]

I am intrested in seeing what people are using in their hp piston applications. Please vote and add comments if so motivated. Thanks!

 

[Eric Sedletzky]

I want to learn from this thread.
Please vote and then explain.
Thank you.

 

[ams511]

Buta N? I think you mean Buna-N.

 

[Candiru]

Ah yes, spelling error is correct. I'm not sure how to correct this.

 

[redacted]

I guess that one vote for Viton is me. I have not been able to find EPDM in duro 90 so that was not an option, I did buy 100 Urethane 010 in duro 90 a few years back. Before that I was using Viton in duro 90 but had some problems with them on my Mk10. I was experiencing a 10 to 15 psi swing in IP as tank pressure fell from 3000 psi to under 500 psi. That is much more swing than should occur even with an older BP. I suspected this was the result of some extrusion and the resultant increased friction. Of course, it may also have been related to the tolerances of just a few Mk10s. In any case, I went to the urethane because of its superior dynamic qualities including resistance to extrusion. Urethane worked well but aged badly in storage in only a couple years to the point where they started to break when folded during installation. I was using them in My Mk5/7/10 but did not use them in my Mk20 which are probably produced to much closer tolerances. So it is back to viton in my Mk5 & 7s where the larger piston reduces the effects of piston o-ring friction. I don't dive my Mk10s any more.

I also have an older bag of nitrile duro 90 which were easier to find 10 years ago but I stopped using them when I found more suitable materials available in duro 90 for use with nitrox.

 

[Zung]

So far, for my main MK20/25 fleet, I've been using EPDM Duro 85 o-rings from the original SP kits. I've recently fitted a PU 90 in my main MK20 just to try, and didn't notice any change. For the next round of service, maybe in a year or 2, I'll try to re-use the bushings and the filters, and replace only the seats and the HP o-rings, and that'll be Viton's.

For my MK5's and MK10's, I've used Nitrile, Viton, and PU, and I don't have anything special to report.

Nevertheless, my vote goes to Viton because EPDM is nearly impossible to source, PU has at least 1 case of a whole batch becoming brittle with age, and Nitrile is too "common" to be worth writing about . Furthermore, Viton has become very affordable these days, either from McMaster or eBay.

 

[Couv]

FKM (Viton)

I too had issues with some PU o-rings (what do you expect considering the initials?) However, I suspect a few of us were using o-rings from the same batch. Below is a cut an paste from a previous post I made a while ago. The only change I would make would be to say, I tried to keep things too simple by ordering only duro 90 seals. I now use duro 90 for high pressure differential application and duro 75 for low pressure applications.

.................................................................................................................................

My fellow suspects,

(for the short version, skip to the bottom)

We went through a similar debate on selecting o-rings a few times before, but it is always good to see fresh ideas, points of view, and anecdotes.


I did a little homework after taking apart a few regulators with all sorts/unknown types of o-rings, some looked like miniature toasted bagels and crumbled upon removal. Conversely, some had no apparent problems at all so I have tried to add a bit of logic in selecting o-rings for my equipment.

An engineer I corresponded with who's job is failure analysis recommended FKM (Viton). However, his bottom line for scuba regulators was not to worry too much over it as any quality o-rings do a good job, are relatively cheap, and are easy to replace. He recommended FKM over EPDM because (in his view) the final product is more consistent in hardness and elasticity. Now, there are few post here made by people for whom I have a lot of respect (Pesky, DA, Oxyhacker) recommending against FKM, but on the other hand Global (for tank valves), Dive Rite (regulators and tank valves), Thermo (valves) and probably a few other companies in the industry use and recommend FKM even with 100% O2. I have been used FKM o-rings for a several years now and have never had one fail.

Well, where does that leave us? The o-ring materials most often mentioned here, FKM, EPDM, Polyurethane (Polyester or Polyether) will all work well. Even Silicone, Butyl Rubber, Nitrile etc will work in almost all of the applications, but lets not get too carried away because we might be here forever trying to come up with a candidate.

The three most popular materials, FKM, EPDM, Polyurethane are all safe for 100% O2 use so why not use one of these just in case high O2 ever becomes an issue.

So here we go with my selection method:
Cost: If you buy in bulk, hardly ever an issue, I like the idea of a group purchase so count me in, but if you stock too many....

Shelf life An issue I do not see mentioned often. According to O-rings, inc. http://www.oringsusa.com/html/shelf_life.html FKM and EPDM have unlimited shelf life, PU shelf life is only 5 years....so there goes PU.

Quality: Working in the aviation/aerospace industry for over 30 years has made me anal about quality. For this reason, I only buy aviation grade o-rings. Hardware store o-rings will probably work just fine, but it is nice to know there has been some quality control and sample testing in the manufacturing process.

Hardness: As pointed out before, Shore A 70 or higher duro for most application, 90 for high pressure differentials, like piston stems and tank valves. In a post long ago and far away, I mentioned only buying duro 90 for the -010 o-rings, but I have had problems in one application. The second stage orifice gets difficult to install with a high duro o-ring, so I try to use something a bit softer there. (do any of you have any 70/75 to swap?)

Material: According to the engineer mentioned above, FKM is more consistent than EPDM and less subject to compression set, but I would not have any problem using either. Both have a wide temperature range. EPDM -40deg F to 25deg F having the edge over Viton's -10deg F to 400 deg F, but I usually avoid diving in those conditions if I can.

So there you have it- to keep things simple, I use FKM duro 90 for pretty much everything except my orifice. ;-0 YMMV

couv

A few helpful links:
http://www.efunda.com/designstandards/oring/oring_chemical.cfm?SM=none&SC=Oxygen, Cold#mat
http://www.oringsusa.com/html/scuba.html
http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf
http://www.marcorubber.com/material_chart.htm

 

[HenrikBP]

My vote is for Viton; simple unscientific reasons:

- price is not bad at all, if you're buying o-rings in batch anyway

- I use my MK10 as my deco reg - so far only with 50%, but will at some point to to 100%

- other DIY-ers, much more experienced than I said it was ok

Henrik

 

[halocline]

I'm voting for polyurethane just to be different. The good ones I had did work better in my MK10s, but I had a lot of bad ones, same batch as awap and couv. The short shelf like is a bummer. And to be honest I don't know nearly enough about the properties of each compound to make any meaningful comparison. I've read that viton has crummy abrasion characteristics, then I read in a different place that viton is good with abrasion. I've been told that polyurethane absorbs water, so that's probably not good, but maybe someone else will tell me tomorrow that it does not. Who knows...clearly if there were a compound that was substantially better than the rest, or if nitrile o-rings were not suitable for scuba use, we would not see the kind of varied o-ring composition we do in scuba use.

So, my real vote would be.....it doesn't matter, just use a high enough duro rating for the pressure differential.

 

[Candiru]

So lets hear from from the OEM people. EPDM anyone, I see a vote, so someone thinks scubapro knows what they're doing.