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<font color='#0000FF'>Hello all,

Just have a curiousity on how a trimix dive can be planned using an Inspiration with trimix, to the already famous 70m depth. From where you start, what mix to use a diluent? Does a 3l bottom is enough? How much bail out gas to use?

Regards

Pierre
 

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Well, you can do the gas planning as per OC - calculate max depth, calculate ENDs etc and blend a trimix to suit the depth.  Or you can cheat and go for the standard gas (see, we're very DIR these days) and use 10/52 heliar as your dil.  This is obviously a hypoxic gas at the surface but cos the CCR adds O2 to maintain a setpoint whatever the depth, you'll be breathing 0.7 PO2 on the boat and down to the first set point change, then 1.3 PO2 from there on in.
Technically the 3l is ample, cos you'll realistically only use about 40bar during the course of the entire dive assuming you're using offboard suit inflation (which you will - you don't want 10/52 swilling round your drysuit).
BUT
Bailout is entirely another matter. To be 100% safe, you need to carry enough offboard gas to complete your ascent and deco if your RB goes utterly tits up at the end of the bottom phase.  Which means carrying the same size slings (if not larger - your RB's let you down, so you're not exactly going to breathing all smooth and relaxed) as the OC boys for a dive of equivalent time and depth.
But you can't use, say, 32% & 80% cos you'll be in all sorts of trouble breathing 32% at 70m. Your 3l of bottom gas isn't even nearly enough to get you from the bottom to 40m.  So you want to be thinking in terms of ,say, 21/30 trimix. Do the gas calculations in terms of breathing rate (stressed) for the time you spend on the deeper stops before you can change over to a high mix at 9 or 6m and you're really thinking in terms of a 10/12l sling of mix plus a 7l of O2/80%.

There is however more to all this than meets the eye and it all boils down to personal choice, preparedness, how much you trust the kit & the way you've put it together and the way you've carried out the dive in question. That's where the grey area starts and the grey matter has to be employed.
It's perfectly feasible to dive an Insp or Evo using 1l bottles to 70m for half an hour and back to the surface completing all deco safely with absolutely no slings.  I've seen it done. But if you flood your loop or lose all your O2 via a blown o-ring  at the end of the bottom phase and there's no-one around to give you extra gas, your options are
1 A very very bad bend which may or may not kill/paralyse you
2 Death

SO.  If you're taking the plunge and going over to the yellow side - take your time. Get plenty of hours in at shallow depths and work up gradually to the deeper stuff.  Give yourself time to get comfortable with the unit, think about how you want to approach mix diving, talk to other box divers, think long and hard about the pros and cons to the different approaches. There's no rush.  You'd be amazed the amount of fun you can have at 20-30m when you can stay there for bloody ages and the sealife doesn't bugger off when you turn up.

Make sure you find a good instructor - ask around, you may have to travel to get the best.
 

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<font color='#0000FF'>Thanks Rob,

When I will buy my breather, yes I will start from shallows and then progress deeper, but I couldn't figer out the above. Altought staying in shallow is hard as with warm water and good wrecks, maybe by then they will invent a depth limiter to the YBOD and if you don't pass eg 30hrs in 20m, if you go deeper it wouldn't work  
 Maybe I should do a trade mark for it  


regards

Pierre
 
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