Apparatus for thermal transfer printing of an image from a thermal retransfer sheet onto an article, includes heating means adapted to supply heated gas at a variable flow rate. Such policy may be used to communicate to various mobile and other devices the \u201cmanners\u201d with which compliance is expected or required. The cross-linking agent is typically formaldehyde and the inactivant typically binary ethyleneimine. The heat exchanger has an inner wall especially a trough- or dish-shaped inner wall, contacting the inline measuring device externally at least partially flushly, as well as an outer wall fixed to the inner wall.