Plugged in Offboard choices

nmanfield

An old chestnut prehaps but following lengthy conversations (over much 'refreshment' I would appreciate the opinions of some grown-ups! If one is doing deep dives e.g 20 minutes in the 100 metres range then there are a few options regarding bailout . There seem to be a few options:

1. Carry enough bailout with you to fully decompress - However the O/C gas required makes this a pretty much impractical solution.

2. Go Alpinist. Not my choice

3. Team bailout. Great if only you have a problem and the team is to hand.

4. Take 2-3 cylinders, eg bottom mix, intermediate trimix and deco mix.

5. Stage tanks - OK if you can get to the stages

6. Plumb in on-board gas which is matched to off-board. Semi close for recompression

7. Plumb in on-board gas which is not matched to off-board. Semi close for recompression. If so what do you use?

8. Decompress as far as possible, rely on surface support to bring you gas. Great if the surface support is available at the time (blown off line etc)

9. In-water compression - however conditions have to be right for this.

10. A mixture of the above

11. Other

I favour option 6 but there are compelling arguments for and against this and most other options. Should I rely on SCR to get me to the surface? If so am I making the right choice for my Offboard (it won't work very well for a pure open circuit deco), should I consider adding another cylinder, if so what? Should I be looking at staging another cylinder, if so what and at what depth (I'm thinking a 50% at 21 metres here)?

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Steve

Option 1.

Here are my thoughts on this matter.

http://www.rebreatherworld.com/64134-post123.html

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nmanfield

Maybe I should add that by matching onbaord and offboard gasses gives some other advantages. As I tend to run the unit manually (with a low set-point as an emergency back-up) this allows me to use the offboard as my primary gas i.e the onboard remains unused unless in an emergency - this means that on a liveaboard I don't have to be constantly mixing gas for each dive. This is very convenient, allows for a more accurate mix and also saves significant time and money as I'm not paying for and handing back a full 11 litre cylinder of mix at the end of the holiday. If diving a mixture of deep and shallow dives (typical liveaboard conditions) I could fill my onbord dil with Air which could also be used to dil flush on deep dives to remove helium and reduce deco. Any thoughts or am I on a one way trip to the chamber?

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nmanfield

I feel a poll coming on. Could someone add one please? Thanks, Nick

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nmanfield

Hi Steve

I think you make valid points. I have done the calculation for a 3 mix-dive based on 100m for 20 minutes BT at a (admittedly very low RMV of 17 (this is what my RMV is without any problems). I should temper this with the fact that I am, admittedly, relatively inexperienced CCR Trimix diver (though qualified). I would say that a full 3 mix OC bailout (at the end of the BT) dive with no buddy / surface support would require 4122 litres of gas. I would presume that this could be carried as a 11litre of bottom mix, an 11 of Air and a 5.5 litre of 80%. Is this how you would do it? Bearing in mind that one would probably have a very high RMV do you think it better to change the cylinder sizes / gas choice?


Dec to 60m (4) Diluent 10/55 0.70 SetPoint, 15m/min descent.
Dec to 100m (6) Diluent 10/55 0.70 SetPoint, 18m/min descent.
Level 100m 13:47 (20) Diluent 10/55 1.30 SetPoint, 38m ead, 41m end
Asc to 78m (22) Trimix 18/40 -9m/min ascent.
Stop at 78m 0:33 (23) Trimix 18/40 1.57 ppO2, 37m ead, 43m end
Stop at 75m 1:00 (24) Trimix 18/40 1.52 ppO2, 35m ead, 41m end
Stop at 72m 1:00 (25) Trimix 18/40 1.47 ppO2, 34m ead, 39m end
Stop at 69m 1:00 (26) Trimix 18/40 1.41 ppO2, 32m ead, 37m end
Stop at 66m 1:00 (27) Air 1.59 ppO2, 66m ead
Stop at 63m 1:00 (28) Air 1.52 ppO2, 63m ead
Stop at 60m 1:00 (29) Air 1.46 ppO2, 60m ead
Stop at 57m 1:00 (30) Air 1.40 ppO2, 57m ead
Stop at 54m 1:00 (31) Air 1.34 ppO2, 54m ead
Stop at 51m 1:00 (32) Air 1.27 ppO2, 51m ead
Stop at 48m 1:00 (33) Air 1.21 ppO2, 48m ead
Stop at 45m 1:00 (34) Air 1.15 ppO2, 45m ead
Stop at 42m 1:00 (35) Air 1.09 ppO2, 42m ead
Stop at 39m 1:00 (36) Air 1.02 ppO2, 39m ead
Stop at 36m 1:00 (37) Air 0.96 ppO2, 36m ead
Stop at 33m 1:00 (38) Air 0.90 ppO2, 33m ead
Stop at 30m 1:00 (39) Air 0.84 ppO2, 30m ead
Stop at 27m 1:00 (40) Air 0.77 ppO2, 27m ead
Stop at 24m 3:00 (43) Air 0.71 ppO2, 24m ead
Stop at 21m 4:00 (47) Air 0.65 ppO2, 21m ead
Stop at 18m 4:00 (51) Air 0.59 ppO2, 18m ead
Stop at 15m 5:00 (56) Air 0.52 ppO2, 15m ead
Stop at 12m 7:00 (63) Air 0.46 ppO2, 12m ead
Stop at 9m 6:00 (69) Nitrox 80 1.51 ppO2, 0m ead
Stop at 6m 8:00 (77) Nitrox 80 1.28 ppO2, 0m ead
Stop at 3m 12:00 (89) Nitrox 80 1.04 ppO2, 0m ead
Surface (89) Nitrox 80 -9m/min ascent.
Off gassing starts at 71.3m
OTU's this dive: 92
CNS Total: 107.7%
1033.5 ltr Trimix 18/40
2363.9 ltr Air
672.3 ltr Nitrox 80
4069.6 ltr OC TOTAL

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EBT

I didnt think ANYONE was still thinking of doing this?

Nigel, this is commonly called the bendy flush.... Im sure you can hazard a guess as to why.

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nmanfield

Hi Steve, of course, you're right - I think it's me being dozy! The more I look at it the more I think that a mixture of pretty much most of the options is ideal. The carrying of enough gas as a team, with enough gas to get out of most situations when on one's own, plus surface support, plus stages gas,plus the ability to semi-close seems to be a sensible solution. Always remembering that one's RMV can be very high if the do-do hits the fan. Again i'm no expert.

Enjoy the course with Dave - he is excellent (did my Meg cross-over with him). I am now of the opinion that I need an expedition trimix course to fill my gaps in knowledge.

Regards,

Nick


EBT - Of course you're right re the bendy flush. My knowledge of Isobaric counter-diffusion is weak. I've not done this yet due to a lack of knowledge in this area. I know that a loop flush on ascent can be useful to reduce the amount of helium. But where and when (and with how much helium in the flush) should it be done - if at all? I am happy showing my ignorance and am ready to be seen as fool but I am very keen to learn and ensure that there are no gaps in my knowledge. I defer to experience!

Thanks, Nick

I should point out that i was never taught to carry out a flush (unless at 6m) therefore have never done it- rather I have seen other people switching to a 50% at 21 metres. I wouldn't do something that i was not sure the reason for doing and /or not taught. Hence the question

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