vjongene:
"...His answer was that while this would certainly reduce the risks associated with nitrogen narcosis, the greater solubility of He in human tissues (and hence faster on- and off-gassing rates) would increase the risk of DCS associated with uncontrolled ascents. Since a significant part of the curriculum consists of performing emergency ascents (unconscious diver, buddy breathing, etc), he argued that the risks may outweigh the benefits.
So the question is, is there really an increased risk of DCS after an uncontrolled ascent while diving Trimix? And is the onset of symptoms more rapid than after a similar profile dived on air?" (emphasis added)
Victor,
Based on what I have read and understand regarding the use of helium mixes, your instructor's response was true as far as it goes. One of the primary tenets of my trimix training was that helium is an unforgiving gas with respect to ascents. If you cannot control the rate of your ascent (for whatever reason) there is an increased risk of DCS due to the helium in the mix (for the reasons your instructor provided). (As an aside, if potential students failed to demonstrate an adequate ability to ascend slowly and under control, my instructor refused to accept them as students.)
Given that your curriculum appears to include several fast ascents (I am not sure how 'rates of ascent' translate to the various adjectives: "emergency", "rapid", etc.), it would seem that these ascents may indeed be safer using non-helium mixes.
With respect to your second question above, a "similar profile dived on air" would not have helium in the mix, hence, no helium could come out of solution due to a fast rate of ascent. Ergo, there would be no symptoms of DCS based on helium bubbles if you were diving the same profiles on air. (Perhaps I misunderstood your question.)
If you're simply referring to how rapidly you would
perceive symptoms of DCS, caused by helium versus nitrogen, while I'm unaware of any empirical data to base an opinion on (and I may be reading it wrong), onset of DCS will be perceived when bubbles exist to a degree that they cause symptoms that you recognize. DCS is DCS - even though in one case the bubbles may exist due to helium rather than nitrogen, I doubt you would perceive the symptoms any sooner. But I'm by no means an expert - others on the board may be able to offer a medically-based response.
[One organization funding research in this area is DAN, and I advise you to call them with your concerns.]
Doc