Hi Don. Two things to remember during this short note: 1. We are still learning about physics in diving and 2. No one knows everything about those physics. Now, trimix contains oxygen, nitrogen and helium. Heliox contains no nitrogen. Nitrogen is the dangerous mistress in our tanks when we dive air deep causing nitrogen narcosis. Nitrogen is also the culprit when get bent because our bodies do not metabolize nitrogen, only oxygen. We always 'thought' that we did not need the nitrogen in our breathing atmosphere, since we do not process it. (More on that later!) Helium comes out of solution very rapidly. The molecules in He are so small that helium can evern penetrate glass! It comes our of solution in our bodies the same way - easy and fast, thus the necessitated SLOW ascent rates when on mix. We vary the amount of helium and oxygen that we add and the amount of nitrogen that we remove from our mixes according to the depth to which we are going. The deeper we go the less oxygen we need, just as the nitrogen. Deeper dives, (some commercial diving), can use Heliox as no nitrogen is needed and the cooling effect of He can be offset by hotwater suits. Deco is also (often) under much more controlled conditions. Also, the huge amount of VERY expensive He needed is easier to offset. Now for the kicker: Only in the last few years have we discovered that DEEP divers do better if a tiny amount of nitrogen is added back to the mix. Why? They're still working on that one! I hope this helps. Norm