Fig. 2.30 The sign of the Joule–Thomson
coefficient, μ, depends on the conditions.
Inside the boundary, the blue area, it is
positive and outside it is negative. The
temperature corresponding to the
boundary at a given pressure is the
‘inversion temperature’ of the gas at that
pressure. For a given pressure, the
temperature must be below a certain value
if cooling is required but, if it becomes too
low, the boundary is crossed again and
heating occurs. Reduction of pressure
under adiabatic conditions moves the
system along one of the isenthalps, or
curves of constant enthalpy. The inversion
temperature curve runs through the points
of the isenthalps where their slope changes
from negative to positive.