REC TriMix

[JWU42]

Quick question about the mix suggested a few posts up...

30/12 for 120'. Why not use a standard mix like 30/30 (GUE standard that is)? Is there much evidence that He under 15% makes much of a difference?

 

[mempilot]

Quick question about the mix suggested a few posts up...

30/12 for 120'. Why not use a standard mix like 30/30 (GUE standard that is)? Is there much evidence that He under 15% makes much of a difference?

If you were getting banked trimix, then 30/30 would probably be just as cheap. But, if you were blending yourself, then the extra He is pretty much useless at 120 based on a Best Mix calculation. As for He under 15% not making a difference: Any offset of N2 is going to be useful down to a point where narcosis isn't much of a factor. Many shoot for END's of 60 to 100 fsw. Your calculation for Best Mix will vary depending on whether you believe O2 is narcotic or not.

 

[mempilot]

Added note:

Standard gasses are not 'Best Mix' blends. They do, however, allow for easy team gas planning. The standard gas blends make getting fills easier when fill stations bank their 18/45, 21/35, 30/30, ean50, ean36, and ean32. They can fill faster and spend less labor doing the fill.

If you like your PO2 and END to be spot on at MOD, then Blending will give you better results. It will be more expensive and take longer to fill though.

The other problem with standard gasses is what to do with the left over gas after the dive. You could own one set of tanks for each standard gas, but that can get very expensive. To receive banked gas you would have to dump the unused mix if your dive required a different standard gas. Or, you can blend down or up to get the desired mix for the next dive. If you are going to have to 'fix the mix', then why not go for the 'Best Mix' anyway (barring team planning where you don't have a telephone ). I dive with the same buddies all the time, so we concur on a mix prior to going to our relative fill stations. It may be standard, or it may be precise.

 

[JWU42]

Mempilot:

I am just going from what I have read (listened to) from GI3 et al regarding He (DIR 2 and DIR 3). His comments about a high He content were certainly geared towards deeper depths (190'+) in my opinion.

What would be the additional cost for a 30/30 Triox mix versus 30/12 in your opinion.

A link to look at:

http://www.baue.org/procedures/standardmixes.html

This would suggest a 21/35 mix for 120' to have a lesser END I presume. Then again, this is getting past rec. triox/trimix

 

[mempilot]

What would be the additional cost for a 30/30 Triox mix versus 30/12 in your opinion.

[snip]

This would suggest a 21/35 mix for 120' to have a lesser END I presume. Then again, this is getting past rec. triox/trimix

Part 1

Cost of 30/30 in a set of double AL 80's at 3000psi at $0.22 for O2 and $0.55 for He:

$17.00

Cost of 30/12 in the same doubles:

$31.00

Part 2

For the dive at 120 fsw:

30/12 , 1.4 PO2 , 80 END (target for Best Mix)
30/30 , 1.4 PO2 , 44 END
21/35 , 1.0 PO2 , 52 END

30/30 gives you the target PO2, but an arbitrary END. 21/35 gives you a really low PO2 and an arbitrary END.

30/12 and 30/30 will give you the longest bottom time or the shortest decompression obligation, while 21/35 will give you the shortest BT or longest deco obligation.

END is relative because we all experience it differently. While 44 and 52 are shallower than 80, most divers don't plan for narcosis shallower than 60 to 100 feet.

What would I do for this dive? If my tanks were empty and my fill station has banked trimix, I'd go with 30/30. If my tanks where half full of some other mix, I'd blend to 30/12 if I didn't have to dump gas.

Remember, the example given for 30/12 in the early post was to show how inexpensive trimix can be for recreational dives. You can dive 30/70 at 120 and no one would fault you. It's just a matter of $ and showing people that He isn't that expensive for recreational dives, especially in large single cylinders.

 

[mempilot]

Then again, this is getting past rec. triox/trimix

No. This is rec trimix.

The end results of calcuations for rec trimix are the same as for Nitrox:

PO2 and END

In rec trimix, you just have one additional gas to offset N2 without elevating your PO2 to dangerous levels.

 

[JWU42]

Well summarized!

 

[badgers]

Part 1

Cost of 30/30 in a set of double AL 80's at 3000psi at $0.22 for O2 and $0.55 for He:

$17.00

Cost of 30/12 in the same doubles:

$31.00

QUOTE]
I do not understand how less helium is going to cost more.
I read you post to be
30% o2 /30% He =17 bucks
30% o2 /12% He =31 bucks

do you have those backwards?

 

[mempilot]

Part 1

Cost of 30/30 in a set of double AL 80's at 3000psi at $0.22 for O2 and $0.55 for He:

$17.00

Cost of 30/12 in the same doubles:

$31.00

QUOTE]
I do not understand how less helium is going to cost more.
I read you post to be
30% o2 /30% He =17 bucks
30% o2 /12% He =31 bucks

do you have those backwards?

Why YES, I do!

Sorry bout that.

30/30 would cost $31 and 30/12 would cost $17.

Thanks.

 

[Spectre]

[blah blah blah... edited 'cause he responded before I finished typing]

So yea.. I think he has it backwards; but the point he was trying to make was simply that the cost really isn't that different [in the grand scheme of things].