Common mode failure

[pufferfish]

From the McMurdo data it is very apparent that certain reg designs outperform others and this was consistent from year to year. For example the Sherwood Maximus with the dry bleed system over four years and 1341 dives had a failure rate of 1.7%. Within one brand line though one can see large variations. The Poseidon Thor used over three years but only 182 dives had a failure rate of 1.1% The Cyklon on the otherhand with five years of data and 710 dives had a 5.6% failure rate. Scubapro was tried several years with various seconds on a Mark 10 first stage and the failure rates were very high. On about 60 dives the failure rate approached 33%. As most of the dives were within recreational depths many of the divers gravitated towards the Maximus as their working reg in this environment. When they pooled the Maximus dives against all the non-Maximus dives the failure rates were 1.7% vs 9.1%.

This supports the observation that certain brands do seem to fail more frequently than others.

 

[quimby]

If there is a common mode failure, then you need to look at a common causative factor. It could be regulator design, but it could also be the air being used

John I agree with you when I had a shop my air tested dewpoint was below
-60 as that was the best the lab could do and I saw plenty of ice dives and other than ice from surface interaction (read surface air) I never saw a freeze up and quite frankly don't buy into the current conjecture.
the link below might explain my point and the temps generated at the expansion of gas at the hp seat gets quite dramatic remembering the diag from some regs on "breathing machine", and a inhalation might be enough to just cause a crystal to form............Just a thought


http://us.lindegas.com/International/Web/LG/US/likelgus.nsf/DocByAlias/nav_news_table_dew

 

[quimby]

I had to chuckle on TDS when one of the divers from Norway said that after numerous cold water reg failures in the ocean environment subsequent air analysis revealed Norwegian 'trimix' should be labelled as containing mainly air, oil, and water.

Gotta love it, missed that one dam good one Pufferfish

 

[MikeFerrara]

Moisture in a reg can do it but my air tests show a dewpoint of below -90 F...no moisture there. I'd suspect most systems in good shape do as well.

A piston reg with some wear on it can let a small amount of water in past the piston but I suspect that the biggest culprit by far is the second stage and there isn't any way to keep it dry.

I think some do a lot better than others in cold water. Around here I think we see some shops selling carribean gear to divers who are going to dive the local 40 deg F quarries. One winter we ran into a bunch of guys with new (but very frozen) scubapro regs cussing them something fierce. I don't use them but from what Vance Harlow has posted here they have some good designs and some not so good ones.

 

[DA Aquamaster]

A frequent dive buddy of mine has frozen two composite cased second stages so far this year under the ice (one was an S600). I normally stick with my brass cased Scubapro Balanced Adjustables or a D400 for really cold water diving and have not had any problems.

In my opinion the all brass construction of the Balanced Adjustable allows much better heat transfer and prevents the second stage from cooling below freezing.

Older composite cased regs like the original G250 still used a brass air barrel while the newer generation of composite cased regs use a composite air barrel that I think severely limits the heat transfer that can occur making it easier for ice to form around the air barrel and on and around the lever.

The very different coaxial valve design used in the D300, D350 and D400 also works very well in cold water and I have never frozen one of them up either.

The Mk 10 first stage is quite freeze resistant if it has a properly packed SPEC kit, but if the silicone is not properly packed or if some has escaped over time they can freeze. The newer TIS system workes well on the relatively low performance MK2 Plus but does not allow much, if any, room for error on the high performance MK 20 or 25.

My personal Mk 20's are early production Mk 20's with the Mk 15 style cap that will accomodate a MK 15 SPEC boot. The resulting SPEC, TIS, and updated composite AF piston combination has worked well for me. Scubapro would probably not be impressed nor endorse it, but I have found it preferable to both the Mk 10 SPEC and Mk 25AF in very cold water.

Scubapro regards 50 degrees as cold water but I don't. Until they start doing testing in my neck of the woods instead of places like Costa Rica, they will continue to have problems in really cold water.

 

[Genesis]

Composite second stages (all composite) also give me HORRIBLE drymouth problems. I sold my S600 for this reason.

The G250s don't have this problem for me - they have brass air tubes. It makes enough of a difference....

An all-metal second would be even better, I suspect - good luck finding one nowdays.

 

[pufferfish]

Moisture in a reg can do it but my air tests show a dewpoint of below -90 F...no moisture there. I'd suspect most systems in good shape do as well.

That is real nice dry air there Mike but remember that is measured at 1 ATM. The actual dewpoint in your first stage is something much less depending on which pressure chamber you are dealing with. Our CSA air standard only requires a dewpoint of -63 F (-53 C) and I think NAUI says -40 F is good to go. I haven't been able to find a nice accurate atmospheric to pressure dewpoint conversion chart but have a look at the SCBA bulletin below and you can get an idea of why small amounts of moisture can become a problem. Let's assume if ice diving the internal temp of your first stage can get down to 14 F (-10 C) just to be on the safe side. When you first start the dive and the tank pressure is 3000 psi it would take an atmospheric dewpoint in the ballpark of -75 F to keep things from freezing up.

That may be the dewpoint when the filtration is first changed but three months down the road it is probably a lot higher (less negative). I have seen 25 F rises in dewpoint over three months of compressor use. A dewpoint of -75 F in September/03 may now be in the ballpark of -50 F if a filter change is due. What was good to go in the fall with warmer temps now becomes problematic with an ice dive. Care for some Canadian 'trimix' to test that reg. :biggrin:

http://www.survivair.com/techbulletins/TB88.pdf

 

[MikeFerrara]

That is real nice dry air there Mike but remember that is measured at 1 ATM. The actual dewpoint in your first stage is something much less depending on which pressure chamber you are dealing with. Our CSA air standard only requires a dewpoint of -63 F (-53 C) and I think NAUI says -40 F is good to go. I haven't been able to find a nice accurate atmospheric to pressure dewpoint conversion chart but have a look at the SCBA bulletin below and you can get an idea of why small amounts of moisture can become a problem. Let's assume if ice diving the internal temp of your first stage can get down to 14 F (-10 C) just to be on the safe side. When you first start the dive and the tank pressure is 3000 psi it would take an atmospheric dewpoint in the ballpark of -75 F to keep things from freezing up.

That may be the dewpoint when the filtration is first changed but three months down the road it is probably a lot higher (less negative). I have seen 25 F rises in dewpoint over three months of compressor use. A dewpoint of -75 F in September/03 may now be in the ballpark of -50 F if a filter change is due. What was good to go in the fall with warmer temps now becomes problematic with an ice dive. Care for some Canadian 'trimix' to test that reg. :biggrin:

http://www.survivair.com/techbulletins/TB88.pdf

Good points. I usually sample my air for testing right before I change filters though. That way it gives me worst case numbers.

 

[pufferfish]

Ok found a nice little program to convert pressure dewpoints.

If we use the McMurdo station water temp of - 2 C and first stage reg internal temps of around -5 C and take it down to -10 C for just to be safe it is possible to calculate the maximum acceptable dewpoint for diving in these conditions.

At 3000 psi and a -10 C pressure dewpoint the atmospheric dewpoint will be
-59 C or -74 F. This equivalent to 12 ppm or 9 mg/m3 moisture content.

This would be the maximum tank fill dewpoint one should accept for ice diving. If using high pressure tanks this would be even slightly lower (more negative).

So much for NAUI's -40F acceptable dew point. I wouldn't ice dive with any air exceeding -60 C or -75 F atmospheric dewpoint unless you want free flow skill practice :biggrin:

 

[Genesis]

Yeah and the fun part of it is that even a "sealed" first will freeze if the moisture is in the TANK air!