O-Ring Failure

[Dayo Scuba]

It looks like I will be spending some time on the NC/SC border. Can you let me know of good places to dive?

 

[*Floater*]

I've been thinking about o-rings recently because of a tech class I took. It was an ANDI course and they push SafeAir, which is their trademark name for 22-50% nitrox that meets certain purity standards beyond what's required or often sold by non-ANDI facilities. The instructor said the gas quality was important because of how the gasses act at high pressures and how the effects of small amounts of impurities could be magnified at depth. He also said that the oxygen compatible o-rings are generally just more durable and give off less impurities as they degrade during use.

It made sense to me so I've recently replaced almost all my buna o-rings in my regs with vitons that I had in my o-ring kit, and I'm planning to go to EPDM o-rings next. They come in two types that I'm aware of; sulfur and peroxide cured. The latter ones are considered better so that's what I'd like to use.

Anyway, the obvious relevance of different o-rings to the topic at hand is that some are more durable than others, so it may also be a good idea to go with the high quality ones in order to minimize the chance of the kind of incident described by the original poster. So people should be aware that there are different kinds out there and some are better than others.

 

[DeepSeaDan]

I tried to signal my dive buddy but he was already too far ahead of me to know what was happening."

Glad you made it out of that squeaker o.k., but next time, ask your "buddy" to actually be a buddy to you, or plan / equip. / train yourself to go it solo, as that was what you ended up doing anyway...

Regards,
DSD

 

[montychandler]

The o-ring in the second stage end of the LP hose is technically a dynamic o-ring as it is between the hose and inside of inlet fitting as the second stage swivels on the end of the hose. In my opinion, it should be replaced annually, but some techs view it differently as the occassional movement as the second stage swivels on the end of the LP hose is a lot less than, for example, the movement of a piston in the first stage with every inhalation cycle. The problem is that this particular juncture also tends to accumulate salt, silt, etc, and it's often neccesary to dip the LP hose fitting in an ultrasound bath to ensure it's clean. If not, any contaminants will wear away on the o-ring as the second stage swivels, severely shortening the life of the o-ring. There is not much movement as dynamic o-rings go, but there is less lubrication and more abrasion than other dynamic o-rings experience.

One other potential cause of that particular failure is the LP hose fitting not being tight and screwing itself off the second stage inlet fitting. Long before it reaches the end of the threads and comes totally loose from the hose, the o-ring in the hose passes the end of the sealing portion of the inlet fitting where it then usually blows out. It's not uncommon for 1/4 to 1/3 of the o-ring's diameter to be blown out causing a very large and attention getting leak.

Thanks a lot for this explanation. I experienced this failure last week in Curacao on a night dive. I also dive with a Sherwood maximus and had just had my gear serviced. I guess I was lucky in that when the trouble started, there was not an explosion of air, but rather a steady stream. I used my alternate for a bit as i fiddled with the connection. I could see that it was not tight. I tightened it as much as I could, but still had a stream of bubbles. My buddy was close by so I just used her air supply as we aborted the dive - no emergency, just turned around. I had gone from 100 BAR at the time of the failure to around 50 bar when the dive was over - about 20 minutes of leakage..

Thinking it was just the loose connection, I tightened it with my little all purpose diver tool - it was pretty loose, 6-8 turns by then.. However when I turned the pressure back on I still had a bit of air hissing. That's when I took the 2nd stage off and looked into the hose fitting and saw that the o-ring had in fact blown out. I didn't even know it was there. Fits into a little groove around what I think you called an inlet fitting. Luckily the Habitat folks had a pick in their gear box (It was 10 at night and nobody was around) and I had my little dive saver box of O-rings. I replaced the o-ring and had no further trouble.

I've been curious about how this could have happened - your explanation is exactly what occured. Connection got loose, pressure affected the O-ring. My lesson learned is check the tightness of all fitting - especially after servicing! Like I said, I was 30 minutes into the dive before the problem became apparent..

No further trouble encountered after this little repair..

Thanks again..

Monty