Onboard or Offboard trimix?

JD White

Hi All,
What are the advantages and disadvantages of onboard or offboard Trimix?
My thinking is to keep air diluent in the onboard, and use a 10/50 offboard as diluent and bailout. This allows me to use the onboard as wing AND drysuit inflation, and use my diluent as bailout, and use air as SCR if required. It also gives me 2 diluents as a redundancy.
Also, do you plug your drysuit inflator into the manifold or 1st stage?
I am not doing really deep dives, just the 60-80m range, but using hypoxic bailout.
I do not have an ADV.
Appreciate your thoughts.
JD

Depth-junkie

10/50 is a shit mix for OC bailout, do the figures and you will see for yourself.

I personley use 10/52 onboard, then have 15/55 offboard as OC bottom bailout from 100m up (bit hot from 100m but i am only going one way if i am on this gas)

ATB
Gareth

nigelh

My personal take is bottom mix and O2 onboard with the wing on the dil and the suit on a seperate tank.

The bailout (also bottom mix and O2) is side slung and is never more than checked. It forms no part of the 'normal' dive plan so it can be handed off or dumped or what have you.

Also the bailout tanks virtually go from test to test on the same fill unless I'm away from home and need a quick topup.

__________________
nigelh

chunter

I am a bit confilicted on this issue. When diving my Inspiration, I always used onboard air and O2, and offboard 10/50. I also carried 50% nitrox to speed up deco on OC bailout. My feeling is that the decompression difference isn't really that significant (10 or 15 minutes) between 10/50 and 15/50 if you have a shallow mix as well.

On the Inspiration, I would plumb in an offboard connection into the manifold, and I would turn off the onboard tank for trimix dives until I wanted to flush my loop on the way up. The logic that I followed is this...always have the same tanks in your unit regardless of depth. This way, there is no chance of confusion about which gas is where. For shallow dives, use onboard...for deep ones, use an offboard connection. This configuration also has the benefit of switching over all of your connections at the same time (manual diluent button, ADV, BOV, etc). My trimix would be contained in an Alum 80, which provided plenty of bailout gas to get me to my next mix. The advantage of having a large tank for trimix diluent is that you don't have to blend the gas for every dive. I would use about 300 psi on a 300 foot dive.

On my Mark 15.5, there is another issue. Because the IP required for the ADV (Schrader Valve) is much higher, running offboard trimix has a major disadvantage. The feed is very slow. I could tune my 1st stage up a bit, but then I'd have to tune down my 2nd stage. Either way, there is no ideal solution. For this reason, I am considering keeping one of my diluent spheres exclusivly for trimix (probably 10/50). If I did this, I would raise the O2 in my bailout to 15/50. I would then use another offboard tank, mounted to the side of the unit, for bc and drysuit inflation. I would still prefer to leave my air diluent onboard.

Aloha,
Charlie

__________________
"Foolish consistency is the hobgoblin of little minds." - Emerson

"Hobgoblin is a cool word."
- Charlie

Dave Sutton

Here's the system we have developed, which is a "semi-alpinist" one, but works well even if carrying more OC gas.

Baseline rig is set up with air diluent, and is set to feed the drysuit from air and wings with 02 (and NO PLAN to inflate wings.... drysuit used for all normal bouyancy control). BOV run from diluent, with the same CEJN fitting on the BOV as on both drysuit and wings fitting. This means that a "naked" rig (IE: no offboard) can be used as a nitrox rebreather, with enough OC from the onboard gas (diluent and 02) for any "small" deco needed. The connection on the wings feed makes it easy to unsnap that hose and to plug it to the BOV in case of desire to use pure 02 on open circuit for deco.


For trimix, we simply snap on a stage of the desired gas, twist the rigs "diluent select" to "offboard" and snap the feed line to the shoulder. The offboard bottle has a second stage on it, and can be handed off. The rigs have a second valve on them that allows that offboard gas to also feed the BOV. Basically, the internal gas then simply feeds the drysuit and is held in reserve.

The nice thing about this is that we can just snap on the "gas type and volume dejour" to the rig to dive deep. No need to deal with the internal gas, it's always air, and the rig becomes a modular "plug and play" system.

The *only* change I make for sub-200 feet diving is to replace the air in the internal bottle with 50/50 nitrox. That, plus the pure 02, plus the offboard trimix is my open circuit supply for bailout and deco. We use normoxic trimix so we can SCC out if needed or use it as a reasonable deco gas OC, and stick with normoxic diluent until we are pressed into hypoxic diluent by depth considerations, using 1.5-1.6 PP02 as the baseline point to begin mixing dilute gas.

Just another way. We basically don't plan to hand-off gas (semi-alpinist), but "can" if pressed to do so. , IE each diver has his own gas and is expected to use it), but we have interchangable fittings, etc., and can buddy share if needed (and if diving with a buddy... another story altogether).



Dave

__________________
"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com

Dave Sutton

Hi Charlie,

Being a Mark-15 guy too, I just gave up on the idea of a "faster" ADV and came back to the original Mark-15 IP for the internal gas and leave it at 140 PSI. That's how the Navy ran them, and all you need to do is to "dwell" on the ADV to get gas on descent, or to simply ride the manual add valve.

I'm making up external manual add valves on the shoulders for the Mark-15 now, and as you know where the valves are, you'll realize the improvement of relocation. Just an idea. We can take it to the Mark-15 forum for more details if you like.


Dave

__________________
"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com

hoppyinca

I would like more details...

I have a good idea how/ why, but necessary?

hoppyinca

Dave Sutton

Start a thread there and we'll discuss. It's off topic for here.

Dave

__________________
"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com

sisco

Im new to rebreather deco so forgive me if this sounds a bit stupid

what connection do you use to connect your offboard into the manifold - is it now connected in a way that your offboard runs your adv etc..

also on the way up its said you switch back to your onboard - must you first disconnect the offboard/stut it off - both taks cant be connected in and swithced on together can they?

chunter

I use the GCS system from Silent Diving. I have the male end coming from the manifold, and the female connection on my offboard tanks. If my onboard tank is turned off (which it would always be if I had offboard gas plumbed in), then my offboard would be feeding the manifold, and everything running from it (ADV, BOV, manual add button, etc.).

When coming back up, I would disconnect the offboard connection, turn on my onboard air, and flush the loop. If you happened to have both tanks connected (on), there is the possiblity that you would have an unknown mix in the loop. It would be a combination of whatever trimix diluent you had and air (if that's what you were running for onboard). There is no chance that you would contaminate either tank because the manifold is only low pressure (150 psi), so if you realized that you had screwed up, you could turn off whatever tank you didn't want, flush your loop, and personally...I'd do some extra deco. Even between 10/50 and air as diluent (assuming the same PPO2), there wouldn't be an extreme difference. Besides, air and trimix sound very different coming into the ADV, so I think you'd have to be pretty "out of it" to make that mistake for long.

The GCS connections are VERY hard to connect while pressurized, so I have always found this to be a good method (as opposed to running onboard trimix and plumbing in offboard air later on).

Aloha,
Charlie

__________________
"Foolish consistency is the hobgoblin of little minds." - Emerson

"Hobgoblin is a cool word."
- Charlie