a technique for calibrating an ionization detector by simulating the presence of particles of combustion . in one embodiment a test probe is employed extending into the chamber . the probe may be partially conductive and partially non - conductive and be either rotatable or able to be withdrawn or partially withdrawn to vary ionization current . in another embodiment the source may be moveable to alter the ionization current . in still another embodiment there is provided the temporary distortion of one electrode to change the geometry of the chamber to thereby effect ionization .