Tom Brosz made the following comment:

"There are probably other reasons that helium is not used besides the voice problem, which could be
computer-compensated nowadays for radio work. I understand it also conducts heat much better. People in helium
atmospheres are usually always cold."

Tom's comment further piqued my interest. I ran a thermodynamics code on a bunch of gas mixtures and found the following:

Air at 1.0 bar, 20 deg.C
Viscosity 0.18422 millipoise
Conductivity 0.2592 milliwatts/cm deg.K

Pure helium 1.0 bar, 20 deg.C
Viscosity 0.19616 millipoise
Conductivity 1.5328 milliwatts/cm deg.K

Pure argon at 1.0 bar, 20 deg.C
Viscosity 0.22295 millipoise
Conductivity 0.1745 milliwatts/cm deg.K

"Air" with argon replacing nitrogen at 1.0 bar, 20 deg.C
Viscosity 0.21932 millipoise
Conductivity 0.1927 milliwatts/cm deg.K

50/50 mixture of argon and oxygen at 0.42 bar, 20 deg.C
Viscosity 0.21394 millipoise
Conductivity 0.2178 milliwatts/cm deg.K

Notice that a pure helium atmosphere has a much higher thermal conductivity than air (this supports Tom's comment). Notice also that argon has a very low thermal conductivity. This is why argon is used to fill light bulbs and is between double paned glass windows. The 50/50 mixture of argon and oxygen at 0.42 bars has interesting properties. The partial pressure of oxygen is the same as sea level air. The viscosity is a bit greater than air while the conductivity is a bit less. The argon would replace nitrogen as a dilutant. I'm not sure of my facts but I suspect that a 50/50 mixture of argon and air would probably have adequate fire suppression properties. Because argon is a noble gas, astronauts would not have to pre-breath pure oxygen prior to an EVA to avoid getting "bent". Also the lower cabin pressure would require a smaller pressure vessel for the vehicle (less mass) and reduce the danger from explosive decompression.

Where is my logic wrong here? Why aren't we using an argon/oxygen gas mixture for the Space Shuttle?

Gary