Change of diluent?

rob.laurie

Hi all,

I don't mean to hijack this thread, being my first post and all, but seeing how most of you do not change diluent I was wondering if any Meg divers actually use the MGP ( mixed gas bypass ) on their rigs? I am looking at going to a meg and can't see why this option would be needed-SCR bail out on offboard maybe ? As I have not yet been certed on this unit so maybe I'm missing something.Does anyone use the MGB in their dive planing? Any input would be apreciated.

Thanks, Rob

bubnotbub

Hi Rob,
the MGP of the Meg is a very useful accessory, because it allows you to be able to easily insert in the circuit a diluent offboard.
As it regards the change of diluent (the flusch gas to 6 meters with the oxygen is not a true change diluent) I sometimes perform it according to the type of dive.
I change diluent to 30 meters, I make a partial flush with only 1 or 2 expirations, for then to make a total flush to 18 meters, in practice use a travel mix and pass to a 50% to 18 meters.
My personal impression is to go out less tired from the long dives where I have had to effect some prolonged efforts.
I know that may be this can provoke the criticisms of many, but on me it works well, and a sistem that works well it doesn't change.
I have manufactured one switch , to be able to easily effect the change.
In my Trimix course, I teach the two systems, 1 only diluent, or with two diluent, also to practise to the student the exercise of the diluent off board in case of malfunction of the principal diluent.

Best regards

Aldo

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gabrielesub

Real interesting post;
In the past I used to switch between 2 different diluents during deep dives (deeper than 70mt)...for example between trimix and normal air or... for 100mt and deeper dives between ipoxic trimix (i.e. 8/70) to normoxic (i.e. 18/35); sometimes I did a switch on the Pp at the same time (from 1.2 to 1.3 or 1.4).
To be honest I performed also a triple diluent switch during a dive (6/75 to 18/35 to the "trimix" from my argon tank Ar/O2/N2)...
At the moment (since last summer) I am not doing the dil switch anymore. I start the dive and maybe I switch the Set Point at a certain point to faster the off gassing... but no change in diluent (like Tom Mount suggest me me to do).
Well... to be honest... nothing changed.
I still follow my VR3 deco profile (or the Vplanner deco plan) ... I cannot feel any difference.
I think it is pretty difficult to be able to appreciate diffenence just on a "feeling" way... no data to support our opinion.
Like Aldo said: if something work I won't change it!
Actually I find the "no dil switch system" better than the old one... it is just a matter of semplicity during the dive (and the dive planning).
You need just to breath from surface to bottom.... to surface.... same gas.
Also... I try to use a 10%oxygen trimix everytime I can... just becasue is very easy to do the math at every depth to check sensors!

I have read a lot about DCI Counterdiffusion: at the moment seems to be a "dark hole" in the science... I ll wait to have some important and official answer from doctors before changing my diving procedures... (I face the DCI Counterdiffusion in the Navy too... in a complete different way! ).

Safe diving,
Gabriele

J. V.

I do that on my first rebreather too, and I found the system very clever, but a day I feel me uncorfortable deep in a cave, I continue the descent and feel more and more unconfortable, in a flash I realize I was on air and not on trimix like I though. After that I've stoped to use multiple diluant. The system is clever, but I'm not enought clever to use it

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Gilles

A very strong mandate for single dil diving. I am very curious about this and regret not questioning my instructor more about this topic. So here goes.

Is it all because of simplicity? There seems to be substantial deco advantages with dil switching (if safe CD and brainfart risk mitigation measures are exercised).

Surely if one can plan and execute a bail-out, one could likewise switch dil using accompanying bail-out gas. Please educate. Is it just much easier said than done?????

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Curt Bowen

I use 10/50 for the whole dive

Why 10/50, because it is easy to mix.

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Captain Nemo

I change to air ...... on the surface......did you not listen in greece....

jmurba

HI aldo,
I keep my dil all dive long,
back on the zodiac I switch straight on Beaujolais (((:-
to be serious I would never go from trimix to air dil on the acscent, don't know if it is true but i fear isobaric counter diffusion,

and U ?

regards

jean mi

Gilles

Of course no swiching to air, but could one use your bail out gases. Eg, for the sake of simplicity, flushing your loop with off board EAN 50 at the end of your 20m stop? Maybe switching to a less He rich Tmx (used as part of bail-out) on or near bottom?

Other switches (not using air but less rich He) seem to create more significant deco advantages (according to VPM).

What I'd like to get to, are applied ICD risk mitigation measures. VPM has a default 0.5 ATA dPN2 setting. Someone here posted a rule of fifths. Haven't found a lot of discussion about this yet.

The principles, if any, could provide a range of interesting and effective bail out and on-loop deco planning. This of course, on how difficult it is to correctly execute in reality.

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Mark Chase

Single dill all the way out but I wont say that ill never switch diluent. We were looking at some profiles for diving the Victoria at 145m and significant decompression could be saved by diluent switching on ascent.

I was surprised at the dive show talking to Kevin Gurr. I was suggesting the dill tank on the Boris was a bit small for deep dives as there was too little gas to do diluent flushes with. He said he ran the unit off of off board trimix at depth in large cylinders and ran air inboard which he switched to on ascent to speed up deco??? I thought such ideas were ancient history, but apparently not.

ATB

Mark Chase

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