In engineering, there are widely used relations for gas properties, e.g., the ideal gas law. Like most (every?) model, there are limits to the validity of those models. For gases, most engineering models will definitely become invalid when temperatures are high enough for the gas molecules to dissociate.
While the temperature-dependence of gas properties is well studied, and I have no problem finding the relevant models and their ranges of applicability; I have a much harder time finding the dependence on pressure.
Currently, I am asking myself whether common engineering models for gas properties are valid down to low pressures.
This specific question is motivated by the folling problem. I need to simulate heat-transfer in a vacuum-chamber of a metallurgical vessel. Thus, I have temperatures in the 0-2000 K temperature range (which is pretty well covered); however, right now I assume a low vacuum of 100 Pa (a thousandth of normal atmospheric pressure).
My question is now: are the ideal gas law and other common models (for viscosity, heat conductivity) still valid at 100 Pa? Or, in other words, at what pressure are gases too thin for common engineering models to become invalid?
As for what research have I done: if I search for "low pressure gas properties" I come across a lot of research in the field of atmospheric re-entry or hypersonic flows. These authors account for dissociation and reactions among other things, which aren't relevant in my case.