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Helium also has a higher heat capacity than nitrogen.  A diver will feel 
colder breathing heliox than when breathing mixes containing at least some 
nitrogen.  The choice of using trimix is a compromise between the loss of 
heat, the amount of narcosis accepted on a dive, and the cost of helium.

In a message dated 4/5/99 8:16:45 PM Eastern Daylight Time, 
ststev@un*.co* writes:

>  
>  Having nitrogen in the breathing media:
>  
>  1)  is cheaper than using heliox
>  
>  2)  is a slower diffusing gas than helium, and thus will decrease your
>  decompression obligation slightly as compared to heliox for some bounce
>  dives, and
>  
>  3) when present in small amounts, has been shown to retard the onset of
>  high pressure nervous syndrome (HPNS) in deep commercial exposures or
>  when compression to depth takes place very rapidly.
>  
>  Your father, however, is probably right, since helium is easier to
>  decompress from, having a much lower potential for physiological
>  damage.  It is also effectively non-narcotic, and in reducing the
>  density of the breathing media, reduces overall breathing effort.  For
>  technical divers, cost is the primary reason we are not all using
>  helioxes.
>  
>  -Sean
>  
>  
>  
>  On Mon, 5 Apr 1999 17:11:31 -0600, Scott &/or Julie Gudmundsen wrote:
>  
>  >I don't know a thing about trimix, but it interests me as I am pushing 
the 
> limits of deep air.  My father, a non-diver,  who has a Ph.D. in physics, 
> asked me the following question.
>  >
>  >It's my understanding that trimix is part helium, part nitrogen, and part 
> oxygen. Why does it contain nitrogen? Wouldn't it be better if it contained 
> no nitrogen at all?
>  >
>  >I told him, "I have no idea, but I can find out!"
>  >
>  >Please edumicate me.
>  >
>  >Thanks!
>  >
>  >Scott Gudmundsen
>  >Vernal, Utah
>  >


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