The growth velocity determined by an one dimensional diffusion dominated transport model turns out to be proportional to the difference of the partial pressures at source and crystal surface ( = driving force) and reciprocal proportional to a diffusive transport resistance. For verification we have investigated the growth rate as a function of growth temperature, position of induction heating coil and carrier gas pressure for several growth runs. Contrary to the conventional procedure of performing single stationary experiments for each parameter set we have applied non stationary process conditions by varying one of these parameters with time. This allowed us to measure the growth rate for different parameter sets out of one crystal. The thermal data used were determined from pyrometrical measurements on the top and bottom of the crucible during the experiments and were combined with simulations of the 2-dimensional heat transfer in the assembly. The latter allowed us to obtain the temperatures (source, crystal) inside the reaction chamber which were not assessable by direct measurements.