Offboard 02 for the mCCR

[Joshua Smith]

So, I'm starting to do dives deep enough to overcome the Interstage pressure on my COPIS. Around 250', ambient pressure, and the IP of my first stage equalize, and it's no longer possible to add 02. There are various ways of overcoming this problem, but I think I've found a pretty slick one. Curt and Mel at Silent Scuba managed to find some really cute 3cf cylinders, about the size of a wine bottle, and I rigged one directly to my backplate:

One cool feature on the Megalodon is the extra gas port on the Left Counterlung- you can plumb anything you want int it, as long as you have a whip on it. Right now, I have the bottle on the right side, and the whip comes across to connect- as soon as I dig up a shorther whip, I'm going to switch sides. The bottle's so small, you don't even notice it there- I think I'll leave it on permanantly- even on shallower dives, having some extra 02 along couldn't hurt anything.

[Nwbrewer]

Who said there's not good use for a Spare Air?

[Dmitchell]

So, I'm starting to do dives deep enough to overcome the Interstage pressure on my COPIS. Around 250', ambient pressure, and the IP of my first stage equalize, and it's no longer possible to add 02. .

Seriously, your IP is set that low? I don't know much about the size of the COPIS orifice but I would have thought that your IP would have been high enough to get you to at least 300'. My IP should in theory be good to 330' But I plan to have offboard O2 before venturing below 300'. I've also got a smaller orifice that I could put in that would allow me to raise the IP and go deeper.

The nice thing about the Meg is if you don't mind all that stuff up front, you have the ability to plug in offboard direct. I can't do that on the KISS.

Dave

[Joshua Smith]

My IP is sort of low- I keyed it down twice during class. I don't know where it will stop working, actually. I've had it to 240' and it worked fine- but I'll be doing some 280-300' dives soon.(we tried to do a 300' wreck this weekend, but we couldn't find the silly thing.) And others have told me that their units stopped flowing right around 250'.

[ljjames]

I realize this is an old post but... you can very easily figure out the minimum IP you want to set it at for those dives.

Figure your ambient pressure at that depth... 300 + 33 / 33 = 10.09 and then make that into "gauge" pressure (multiply by 14.7, since our gauges read in psi) Voila, PSI for your IP.

Its right around 150psi.

This is old skool, and why you don't do deep dives with even OC regs with IP's tuned way down. This is also, obviously why with our inaccurate pressure gauges we made a practice of NEVER breathing our stages below 500psi. Cause that gave some fudge factor with regards to when that last breath was coming.

My IP is sort of low- I keyed it down twice during class. I don't know where it will stop working, actually. I've had it to 240' and it worked fine- but I'll be doing some 280-300' dives soon.(we tried to do a 300' wreck this weekend, but we couldn't find the silly thing.) And others have told me that their units stopped flowing right around 250'.

[Joshua Smith]

I realize this is an old post but... you can very easily figure out the minimum IP you want to set it at for those dives.

Figure your ambient pressure at that depth... 300 + 33 / 33 = 10.09 and then make that into "gauge" pressure (multiply by 14.7, since our gauges read in psi) Voila, PSI for your IP.

Its right around 150psi.

This is old skool, and why you don't do deep dives with even OC regs with IP's tuned way down. This is also, obviously why with our inaccurate pressure gauges we made a practice of NEVER breathing our stages below 500psi. Cause that gave some fudge factor with regards to when that last breath was coming.

My IP is sort of low- I keyed it down twice during class. I don't know where it will stop working, actually. I've had it to 240' and it worked fine- but I'll be doing some 280-300' dives soon.(we tried to do a 300' wreck this weekend, but we couldn't find the silly thing.) And others have told me that their units stopped flowing right around 250'.

Thanks- yeah, I know the math. Interesting you bumped this, though- I have a couple deep dives coming up soon.

The reason you want to avoid jacking the IP too high on a mCCR is because you don't want the constant flow to exceed your personal resting 02 metabolism. I need to get back in and measure my IP- just haven't done it since I set mine a year and a half ago.

One way to overcome THAT problem is to turn your 02 off if your P02 creeps up (as opposed to burning through your dil)- but I don't like doing that if I don't have to. Personal preference, but if I want 02 in the loop, I want it right now.

Another solution is to pick a dil that will yield a 1.2-1.3 P02 at your MOD. P02 drops at depth , you just breathe in a wee bit of dil through the ADV. Disadvantage here is that you lack some of the ability to lower a high P02, but not too bad- Normally I select a max P02 of 1.0-1.1 at MOD, so it's not a huge difference.

Yet another way to compensate is to just plumb in your hot mix. Bailout strategies are a whole different discussion, but I personally like to use 40% and a trimix blend, when I need 2 bottles. There are compelling arguments for 50% and 60% as well, but 40% is what I like. Now, I haven't used this method yet, because I was afraid of jamming a crapload of Nitrogen into my loop at depth- but looking closely at the problem with several CCR cogniscenti, I'm starting to believe that the amount, and effects, would be negligible. I still want to test this somewhere deep, but relatively benign, with a trusted buddy, before I try it on a real wreck dive in the Sound.

The solution presented above, with the offboard, is my attempt to have my cake and eat it too. I need more time to play with these options, to see which one works best for me. Unfortunately, I haven't been doing as much diving as I would have liked, since I posted this thread originally- but I'm getting the kinks ironed out for what I hope to be a banner year of dives- getting myself and my gear sorted out.

[ljjames]

have you talked to Leon at all about your issue?

[Joshua Smith]

have you talked to Leon at all about your issue?

No, I haven't talked to him directly. I guess I don't really see it as an issue, per se. Just a puzzle with several workable solutions. I have talked to people who use each of the methods I discussed.

[ljjames]

understood, just curious what the designer has to say about the issue.

[Joshua Smith]

understood, just curious what the designer has to say about the issue.

Me, too, actually. Probably "Buy an APECS!" Seriously, though- if you get a chance to ask him, I'd love to hear what the horses mouth has to say.

[Tangfish]

Josh, did you ever get my 3 cu ft bottle cleaned? I gotta swap you back primary O2 tanks too.

[Joshua Smith]

Josh, did you ever get my 3 cu ft bottle cleaned? I gotta swap you back primary O2 tanks too.

This isn't a PM, you know that, right, Calvin? No, it hasn't been cleaned yet- we're working on it. And yes, we need to swap 02 bottles back.

[Gill Envy]

...
One way to overcome THAT problem is to turn your 02 off if your P02 creeps up (as opposed to burning through your dil)- but I don't like doing that if I don't have to. Personal preference, but if I want 02 in the loop, I want it right now.
...

Josh, have you had an issue with water ingress when doing this? since the hose decompresses when the o2 is turned off (due to the MFO) and conceivably the "seat" on the O ring in both the swagelock and the DIN could be lost, i've been leary of this. Actually, with the first stage blocked, maybe ingress is possible without even turning the valve off if you go significantly below the IP.

Curious if you have considered adding a needle valve? I've got one on order and will be looking at it's value for deeper diving... I will post on the results.

cheers,
g

[Joshua Smith]

No, I have never done it, and someone else pointed out to me that same issue, so I can see that it's a bad idea. The needle valve concept is interesting, too- I'd like to hear more about that!

[Dmitchell]

...
One way to overcome THAT problem is to turn your 02 off if your P02 creeps up (as opposed to burning through your dil)- but I don't like doing that if I don't have to. Personal preference, but if I want 02 in the loop, I want it right now.
...

Josh, have you had an issue with water ingress when doing this? since the hose decompresses when the o2 is turned off (due to the MFO) and conceivably the "seat" on the O ring in both the swagelock and the DIN could be lost, i've been leary of this. Actually, with the first stage blocked, maybe ingress is possible without even turning the valve off if you go significantly below the IP.

Curious if you have considered adding a needle valve? I've got one on order and will be looking at it's value for deeper diving... I will post on the results.

cheers,
g

I've done this on the DIL side and not noticed any intrusion - can't say I was looking for it though.

I would think the o-ring would just have a tendency to reverse it's seal and still not leak. When pressurized the o-ring is sealing the HP gas in but when depressurized it would keep the HP water out as long as it's seated correctly.

No?

[Gill Envy]

I don't know, but I've been reminded many times that the seat of an "o" ring is maintained by positive pressure from the inside. the thought of what would happen if salt water entered the MFO has crossed my mind, something I wouldn't want to find out the hard way.

one of the reason's I'm pursuing the needle valve retro on the copis is that the IP would never be overcome by ambient pressure... but, as I've said, I just don't know enough about what consequences there might be.

I'm a ways off from really needing the needle valve, but figure becoming acquainted to it in shallowish depths is a good first step.

g

...
One way to overcome THAT problem is to turn your 02 off if your P02 creeps up (as opposed to burning through your dil)- but I don't like doing that if I don't have to. Personal preference, but if I want 02 in the loop, I want it right now.
...

Josh, have you had an issue with water ingress when doing this? since the hose decompresses when the o2 is turned off (due to the MFO) and conceivably the "seat" on the O ring in both the swagelock and the DIN could be lost, i've been leary of this. Actually, with the first stage blocked, maybe ingress is possible without even turning the valve off if you go significantly below the IP.

Curious if you have considered adding a needle valve? I've got one on order and will be looking at it's value for deeper diving... I will post on the results.

cheers,
g

I've done this on the DIL side and not noticed any intrusion - can't say I was looking for it though.

I would think the o-ring would just have a tendency to reverse it's seal and still not leak. When pressurized the o-ring is sealing the HP gas in but when depressurized it would keep the HP water out as long as it's seated correctly.

No?

[Joshua Smith]

I finally got a chance to test this little beauty yesterday. It performed flawlessly at 290 fsw. And I barely even notice it attached to my backplate. I'm very pleased with it- thanks again to Curt for hooking me up with the parts!

[Curt McNamee]

I finally got a chance to test this little beauty yesterday. It performed flawlessly at 290 fsw. And I barely even notice it attached to my backplate. I'm very pleased with it- thanks again to Curt for hooking me up with the parts!

Great, I'm glad it worked out for you, how was the dive ???

[Joshua Smith]

I finally got a chance to test this little beauty yesterday. It performed flawlessly at 290 fsw. And I barely even notice it attached to my backplate. I'm very pleased with it- thanks again to Curt for hooking me up with the parts!

Great, I'm glad it worked out for you, how was the dive ???

You know how great the viz can be some days out in the Sound? Well, it wasn't like that. We had awesome viz down to about 260, and from then on it was pea soup. We made it all the way down, tied off, secured our strobe, and the second Dan and Paul moved, they were gone. I couldn't see them or the line. I inched over to the strobe, found the reel line Paul was running, started to move along it, and Ran into Paul as he was doubling back- we exchanged "the thumb" and got the hell out of there- seriously, it was worse than the Lake. So, like, 7 minutes of descent and 5 minutes at depth cost us close to 70 minutes of deco....not to mention the money for the charter.....But I got to test my offboard! So, at least I was happy about that.

[dsteding]

You know how great the viz can be some days out in the Sound? Well, it wasn't like that. We had awesome viz down to about 260, and from then on it was pea soup. We made it all the way down, tied off, secured our strobe, and the second Dan and Paul moved, they were gone. I couldn't see them or the line. I inched over to the strobe, found the reel line Paul was running, started to move along it, and Ran into Paul as he was doubling back- we exchanged "the thumb" and got the hell out of there- seriously, it was worse than the Lake. So, like, 7 minutes of descent and 5 minutes at depth cost us close to 70 minutes of deco....not to mention the money for the charter.....But I got to test my offboard! So, at least I was happy about that.

Commencement Bay?

[Joshua Smith]

You know how great the viz can be some days out in the Sound? Well, it wasn't like that. We had awesome viz down to about 260, and from then on it was pea soup. We made it all the way down, tied off, secured our strobe, and the second Dan and Paul moved, they were gone. I couldn't see them or the line. I inched over to the strobe, found the reel line Paul was running, started to move along it, and Ran into Paul as he was doubling back- we exchanged "the thumb" and got the hell out of there- seriously, it was worse than the Lake. So, like, 7 minutes of descent and 5 minutes at depth cost us close to 70 minutes of deco....not to mention the money for the charter.....But I got to test my offboard! So, at least I was happy about that.

Commencement Bay?

No- Elliot bay. We were trying to dive the Multnomah. We were right on it- hell, we might have been inside it for all we know.

[dsteding]

No- Elliot bay. We were trying to dive the Multnomah. We were right on it- hell, we might have been inside it for all we know.

Seems like Elliott Bay and Commencement Bay are frequently crappy deep.

[CaptnJack]

No- Elliot bay. We were trying to dive the Multnomah. We were right on it- hell, we might have been inside it for all we know.

I imagine the Duwamish just rains silt down on that (and the Fuller). I'm guessing the right time of year for those is when the river is lowest and least turbid, but I have no clue if that's true.

70mins of deco? Jeesh for that kinda BT I would have done 20-25 (on OC), how on earth did you rack up that much deco in 12mins?

[bigsky]

70mins of deco? Jeesh for that kinda BT I would have done 20-25 (on OC), how on earth did you rack up that much deco in 12mins?

70 minutes was total run time including descent

[Joshua Smith]

70mins of deco? Jeesh for that kinda BT I would have done 20-25 (on OC), how on earth did you rack up that much deco in 12mins?

70 minutes was total run time including descent

D'oh. Yeah. I can't remember how much runtime we had when we started the ascent, but I think I'm close. And yes, I cleared at 70 minutes.