Trimix pros and cons...

[Cave Diver]

For deeper dives, it's not uncommon for us to spend $100+ on gas for a single dive by the time you factor in backgas, deco gasses, etc.

If you're following proper gas management techniques you can end up with quite a bit of gas unused, which makes the cost factor even more distasteful.

 

[wedivebc]

But the reasons for using helium are ..., and reduced weight of gas (affecting buoyancy characteristics of the tanks you are carrying)...

I can't think of a situation where I would choose trimix over another gas for it's bouyancy characteristtics. Can you explain that please?

 

[TSandM]

Dave, I didn't mean to imply that one would choose it for buoyancy characteristics. It's chosen for the other factors. The reduced weight in gas is just a nice side effect, and makes some deco bottles ride better (as I understand it, since I have never and will never do any diving which requires helium in a deco mix!)

The issue of unused gas is a pesky one, which often leads to people having one set of doubles for each bottom mix they tend to use . . . MORE expense related to technical diving!

 

[scubafanatic]

...aside from the high costs of HE, there's the additional factor of availability, especially if you travel far from the advanced 1st-world and all it's tech toys to some remote 3rd-world location....even the fairly widespread availability of NITROX is a pretty recent development and getting HE in many places is still science fiction......so maybe check out the type of diving and the locations you'd like to dive and just see if you can even get your hands on HE.

 

[PfcAJ]

I can't think of a situation where I would choose trimix over another gas for it's bouyancy characteristtics. Can you explain that please?

Adding a little Helium can help make a bunch of otherwise heavy stage bottles more managable for long, shallow cave dives (think Manatee). Really a non-issue until you get a bunch of stages, but its nice when you do get to that point.

Once again, for most dives, it doesn't really matter and isn't worth the $$.

 

[Rick Inman]

Adding a little Helium can help make a bunch of otherwise heavy stage bottles more managable for long, shallow cave dives (think Manatee). Really a non-issue until you get a bunch of stages, but its nice when you do get to that point.

Once again, for most dives, it doesn't really matter and isn't worth the $$.

I've never had to manage more than 3 80's (deco only), and I have noticed that my deep bottle with the HE in it rides a little higher in back. But is there really a management advantage with cave stages with HE in them? And why the HE if the dives are shallow?

I don't cave dive so I don't know. Thanks.

 

[Blackwood]

But is there really a management advantage with cave stages with HE in them? And why the HE if the dives are shallow?

You may have answered the first question by asking the second...

FWIW, here are masses for 80CF of various gases per information from Universal Industrial Gases, Inc. ... Physical Properties (English Units):

EAN32    6.531 lb[sub]m[/sub] 
EAN50    6.692 lb[sub]m[/sub] 
O[sub]2[/sub]       7.136 lb[sub]m[/sub] 
21/35    4.558 lb[sub]m[/sub] 
35/25    5.218 lb[sub]m[/sub]

Given an 80CF stage of 32% (assuming the shallow cave example), every 10% increase in Helium lightens the tank by about half a pound.

32/10    5.995 lb[sub]m[/sub] 
32/20    5.460 lb[sub]m[/sub] 
32/30    4.925 lb[sub]m[/sub] 
32/40    4.389 lb[sub]m[/sub]

and so on

 

[Blackwood]

Helium is a lighter gas. It saturates into the tissues faster, it comes out of saturation faster. Your deco stops on trimix need to be more precise, or the gas can come out of tissue solution too fast, causing DCS.

Note that some people are teaching that Helium is rather insoluble in our body tissues.

 

[PfcAJ]

Rick, it makes a difference in the longer runs. Stages with 32% are pretty heavy when full, and 3 (or more) full, heavy bottles is just uncomfortable. The two under your arm hang low, and the 3rd on your hip (leashed) is heavy on your leg.

Personally, if the cave is around 80-90ft and I'm doing anything more than a 1 stage dive, I greatly prefer 30/30 in my stages. There are so many advantages (weight, breathing resistance, narcosis reduction, deco) that I find it worth it. When I said it wasn't worth it, I was specifically referring to the bouyancy aspect. I'm a big fan of 30/30.

 

[wedivebc]

Note that some people are teaching that Helium is rather insoluble in our body tissues.

Where would they be getting that notion from?