The introduction of sensor techniques has lead to a major change in the possibility to on-line control the plasma spraying process. By the use of such techniques reproducibility of the process can be enhanced by a more precise process control of flame and particle in-flight characteristics. In this paper a comparative study of four different on-line diagnostic systems and their sensitivity is presented. The systems evaluated are: the Particle Flux Imaging (PFI) from Zierhut Messtechnik GmbH, the PlumeSpector from Tecnar Automation Ltee., the Spray and Deposit Control (SDC) from the University of Limoges/Snecma Inc. and the DifRef M from Flumesys GmbH. Tests were performed using plasma spraying of a Nickel- Aluminium alloy. A statistical design of experiments (DOE) was defined using a ± 5% variation of the five process parameters current, argon flow rate, hydrogen flow rate, powder feed rate and carrier gas flow rate. System principles, sensitivity results are given and the systems suitability for use in industrial production is discussed.