Gas choice

[Mr.Dave]

I'm intending to go down the RB root this summer & have been watching this thread, It appears that a few of you guys use 10:52 for Dil & I was wondering why if gas cost is no longer an issue?

I suppose I'm really asking why not use something like 10:90 if your diving south of 60m or say 20:80 if your doing shallower stuff?

Also what do you RB instructors use for dives at say <60m, <90m & say +100 & why?

Just in case this is going to open a hole can of worms or I'm just missing the point, then sorry in advance,

Dave

[londonsean69]

10/50 is easy to mix. Half fill a cylinder with He, then top off with air, no need to add small bits of O2.

People won't generally use 10/90 because not a huge amount of places have booster pumps, which are required to create Heliox at high pressure.

This could of course all be rubbish as I'm not on a CCR, but I do mix gas for CCR divers and mixing Heliair is MUCH easier than trimix, especially in those silly 3l cylinders they like to use

[nigelH]

Originally Posted by Mr.Dave

I'm intending to go down the RB root this summer & have been watching this thread, It appears that a few of you guys use 10:52 for Dil & I was wondering why if gas cost is no longer an issue?

Lazy, one of the best motives. 8-)
No oxygen so it goes in standard, unclean (ding, ding) cylinders.
Plus, with heliair, you measure the oxygen and you can work out the helium.

[The Purist]

Theoretically you should match your dil to the dive you're doing, but if you pick one gas that covers the range of 'normal' diving then you're covered if one weekend's 65m dive is binned and the next one is a 20m bimble. And with all the time you save mixing gas you can strip, clean and fettle your rebreather.

10/50 also makes it dead easy to do the sums for a dil flush, but it does have some disadvantages, like not being good to breathe at the surface, and not being a viable part of a deco strategy...

[Mr.Dave]

Ok so besides making it easier to fill, no need for clean cylinders & obviously less deco than on say 10:90, it's really the lazy factor that makes it so common.

I know it's a bit more of a pain but why not 18:45 for the shallower stuff, at least you can breather it on the surface? Plus if you've got your own 02 you only need to add in 20b so just do it slowly then stick in ~100b of He...

[londonsean69]

Originally Posted by Mr.Dave

I know it's a bit more of a pain but why not 18:45 for the shallower stuff, at least you can breather it on the surface? Plus if you've got your own 02 you only need to add in 20b so just do it slowly then stick in ~100b of He...

I take it you mean breathe it on OC at the surface?? On the YBOD (Correct me if I'm wrong) at the surface you are breathing 70% O2, the rest being whatever, as the setpoint is 0.7bar.

[Mr.Dave]

Originally Posted by londonsean69

I take it you mean breathe it on OC at the surface?? On the YBOD (Correct me if I'm wrong) at the surface you are breathing 70% O2, the rest being whatever, as the setpoint is 0.7bar.

Yep - if you've gone SCR or OC

[jptaylor9]

Appendix 7 of the APD manual (available online) has suggested mixes, both HeliAir & Trimix.

Need to keep ppO2 no more than 1.3 bar at max depth (often 1.0 - 1.2).

APD suggest:

Depth Timix HeliAir END
50m 21/26 15/29 30m (or try 16/26)
60m 18/36 13/37 30m
70m 16/44 11/45 30m
80m 14/52 10/52 28m
90m 13/59 9/58 26m
100m 11/64 7/67 24m

The HeliAir mixes seem a good choice as you don't need to worry about O2 clean dil cylinders & been CCR the unit will add the O2 for you... then there's the bailout to consider

[Sofnolime]

I use 16/40 as my "standard" trimix. It is good down to 70m. and avoids any nasty moments on the surface breathing 10% Oxygen mixes. As is has slightly more Oxygen and less Helium than 10/50 it can also be included in decompression bail outs. I change over to EAN 50 at 21m.
My local shop has never had a problem blending it, but then he only has an Oxygen analyser to check the final mix........I really should invest in a Helium analyser!!