Venturi nozzles or Venturi tubes are used in pipe systems, in which the rate of air flow must be continually measured and recorded. At the same time the components and the methods used for measuring must be suitable for the demanding industrial marginal conditions and the media with which they come in contact. The diameters used for the reductions, as well as, where the measurements are taken, conform to the diameters where the tubes connect in the system and with the amount of air that needs to be measured.