Why Nitrogen?

First, I am not a Trimix diver nor am I planning on doing it (too much time, money and trouble for me) but I had a question about its use.
I understand that the idea is to reduce oxygen toxicity by reducing the % of O2 in the mix and to reduce nitrogen narcosis by reducing the % of N in the mix. (He) is then used as a filler (correct me on any of that if I am wrong).
So, and my numbers should not be taken literally as I have no idea what a valid mix is, say for example you have determined that you need 10% O2 and you put in 15% N so you would then add 75% (He).
Why would you not do 10% O2 and 90% (He) and just forget about the N as I donÃÕ see how it helps at all. I know there must be a valid reason for the Nitrogen but could someone explain it to me please?

Helium/Oxygen (HeliOx) mixtures are in fact commonly used in commercial and scientific diving applications.

For recreational use the main problem is that helium is outragously expensive so we try and keep the amount of helium in the mix to the lowest amount needed to get the narcotic properties of the mix to the desired value.

In addition, when doing partial pressure gas mixing it is much easier to top up to full pressure with the "free" HP air (which includes 80% N2) from the shop bank than it would be to try and pump up helium to 3000 or 4000 psi with a haskell.

There are also physiological/decompression considerations but I'm not really an expert in these areas.

For extreme depths you need to cut the oxygen with something, because it becomes toxic. Of course, Nitrogen becomes far to narcotic at extreme depth, so you also need to cut the nitrogen. So we use Helium as a filler because it is less narcotic. Helium is absorbed by our body at different rates and, dependent upon the depth, can add a great deal of time for the absorbed inert gas, the helium in this case, to leave our bodies; making us stay on our decompression stops longer. Meaning we would not want to much of it. Then, of course, there is the $ thing... However, I wouldn't argue this point for myself(not a good place to save a dollar)... For extreme depth there is something called HPNS. There is some debate over just how much nitrogen to add at these depths. However, I would posit that most divers rarely encounter these extremes in depth and they are not a huge factor to most deep divers.

There are also some gradient issues, but I'll let someone else hop on that...

Here is the wikipedia trimix article. It has references to parts you might want greater detail for. The general Breathing gas article also has some good information with references to further reading.

Hope these help!

Thanks guys, that helps.

OldNSalty:

Why would you not do 10% O2 and 90% (He) and just forget about the N as I donŽÃŽÕ see how it helps at all. I know there must be a valid reason for the Nitrogen but could someone explain it to me please?

Click to expand...

Extedned decompression and HPNS issues aside the main factor limiting the amount of Helium in a lot of divers mix is quite simply cost.

Helium is INCREDIBLY expensive and going up. For your example here if you had say 90% helium in a set of twin 12s thats 5011 litres of He needed. At Ž£0.02 per litre which is the going rate here that would cost roughly $180usd per fill.

There is a tradeoff between acceptable narcosis and cost per dive.

Its also worth noting that O2 partial pressure has to be above a certain level to maintain conciousness. 10% o2 would cause a blackout on the surface and in the shallows.

I am surprised to hear that helium cost so much as it looks to be in abundance at the party stores - of course, maybe that isn't pure He either....

No it is pure He that you breath. For the party balloons the cheap stuff can be used. The amount of He used in a party ballon is around one cubic foot. Now take a technical dive with a normoxic mix, such as 19/30 in a set of LP 108. That is approximately 60 cubic of He. You could fill 60 balloons with that.