The Sonimix 2130 NOx converter tester has been designed to generate a stable NO2 concentration with a small reaction time. It is based on the sonic nozzles technology (ISO 6145-6 technology) and high stability pressure regulators. The NOx converter tester is often used together with a multi-steps gas divider also based on sonic nozzles and high precision pressure regulators. In order to validate NOx measurements, the efficiency of the NO2 converter must be higher than 90 %, it is therefore very important to check easily and precisely the efficiency of the NO2 converter. The SONIMIX 2130 converter tester is based on the process known under the name of Gaseous Phase Titration.