The dilution unit including the critical orifice is mounted directly at the probe body and therefore heated up with the probe to a stable temperature. The incorporated pre-heater heats up the dilution gas to the probe temperature. Both steps avoid that sample gas decreases under the dewpoint. Calibration gas enters the probe via an integrated connection. The probe can be heated up to 180 °C or 280 °C. As an option a heated manual operated 2 way ball valve is available. It is integrated in the sample gas inlet and shuts off the probe head from the process, for example during the change of the filter element. Additional the option ‘blow back in front of ball valve’ via the probe flange is available.