A key approximation in the analysis of upward thermal diffusion cloud chambers is that both the heat flux and mass flux (in the center part of the chamber) are plane parallel. Procedures for determining when this approximation is valid are described. The key experimental parameter is the amount of heat added to the chamber walls to prevent vapor from condensing on them. Underheating the walls causes the experimental rates to be as much as 10(5) smaller than they would be when the plane parallel flux approximation is valid. Overheating the wall causes convection in the chamber center, also invalidating the plane parallel flux approximation. However, there does exist a range of wall heat values between these under and overheating limits over which the nucleation rate is negligibly affected. The overheating limit depends on the type and amount of noncondensible carrier gas present, whereas the underheating limit does not. Decreasing the molecular weight of the noncondensible gases, or decreasing the noncondensible gas pressure, increases the overheating limit.