Originally posted by watergal
Nothing special needed for helium. It is safer and much less corrosive than air.
Quite the understatement! Not only is He less corrosive than air, He is completely non-corrosive! He is an inert gas, and as such, doesnt react with anything (chemistry geeks, let it slide
)
This is why if you take a Nitrox mixing class, it would take all of 15 minutes more (if that) to cover mixing helium. Theyd talk about real gasses a little, give you a bunch of formulas and then, after all that, conclude just use a fudge factor and be done with it. The biggest part of any mixing class is O2 handling. Handling an inert gas after learning to handle O2 is a non-issue.
A separate mixing class for Trimix is one of the more transparent money making schemes proffered by the training agencies. This is why a number of us were very surprised when 1-800-PAY-PADI
offered one mixing class that covered both Nitrox and Trimix! (
http://www.scubaboard.com/t4727/s.html).
Scubabunny,
Let me try and draw all the terms that others defined above in one reply and see if it helps:
First to liken Trimix and Heliox to Nitrox:
Nitrox is ANY mix of Oxygen and Nitrogen only. Air is a subset of Nitrox, which is 21% Nitrox. So technically, if you walk into any shop and ask if they pump Nitrox, all should say yes though many would only offer EAN21
. Colloquially, Nitrox is any Oxygen/Nitrogen mix GREATER than 21% O2.
Heliox is ANY mixture of Oxygen and Helium only, just like Nitrox is any mixture of Oxygen and Nitrogen only. Helioxs big problem is getting fills. Because Helium comes in bulk cylinders typically at 2400 PSI, you have to boost or lift the Helium to get a complete fill (
http://www.haskel.com/). Commercial boosters are big bucks. The higher the PSI you need to lift the gas, the more difficult and the slower it is, which is one reason why LP cylinders are so popular in the technical arena. At depths over 500 feet HPNS becomes an issue with Heliox, but I dont think youre planning on going there yet.
Trimix is ANY mixture of Oxygen, Helium and Nitrogen only. If you add He to an empty cylinder and then blow air on top you get Heliair, which is a subset of Trimix, though Heliair is a range of mixes, all that have an O2 content less than 21%. Heliair is not used very often, because by the time you add enough He in order to keep your Nitrogen Narcosis to a reasonable level, your mix becomes hypoxic at the surface. That means you have to deal with travel mixes and gas switches and therefore task loading to get to a depth where you can breathe your back gas without passing out. Its hard to enjoy a dive when all your focus is on staying alive.
Because Heliair uses no pure O2, theres no need for your cylinder and valve to be O2 clean.
But Trimix in the colloquial sense refers to an O2/He/N2 mix other than Heliair, just like Nitrox usually refers to an O2/N2 mix other than air. So colloquial Trimix is the result of (well make it simple) adding O2 to an empty cylinder, adding He on top of the O2 (at this point you have some Heliox mixture) and then blowing air on top of that mix (where the Nitrogen and some more Oxygen comes from). This requires the same O2 procedures that are required for Nitrox.
Note that by saying the same, I mean exactly the same. If a shop partial pressure mixes Nitrox, theyll PP mix Trimix and your cylinder and valve should be reasonably O2 clean. If they have a membrane system, you may be able to build your mixes from membrane Nitrox without the need for O2 cleaning. For instance, filling a cylinder to 40% with He and then blowing 32% Nitrox on top results in a nice mix for 200 feet.
The above mix would be called 19/40/41 (O2/He/N2) or more typically just referred to as 19/40. This mix would have an O2 PP of 1.35 and an Equivalent Nitrogen Depth (END) of less than 100 feet. So your O2 exposure is fine and youre as clear headed as youd be at 100 feet.
Wow, Im way off subject. I hope this pulls together the relationship between Heliox, Trimix and Heliair.
Roak
