a corresponding x ray detector arranged such that a detection section of the pipe is placed within the optical path of the x rays between the x ray source and the x ray detector a calibration chamber is located parallel to the detection section within the optical path of the x rays application_number US 201214397181 A description a device 10 to determine a mass flow of a multiphase fluid within a pipe 12 is provided as shown in fig1 4 a volumetric flow is determined by a constriction 14 in the pipe 12 acting as a venturi device by measuring a difference in static pressure between a constricted part 14 and an unconstructed part of the pipe 12 flow speed may be determined in order to calculate the mass flow from the volumetric flow a density of the multiphase fluid is determined for known densities of the individual phases this may be achieved by measuring the phase composition of the fluid since in many applications e g for crude oil water natural gas u2014 mixtures the x ray absorption coefficients of the individual phases differ strongly x ray spectroscopy is a straightforward method to achieve this goal an x ray source 16 provides x rays at at least two different energies the x rays permeate a detection section 18 of the pipe 12 and are detected by a corresponding x ray detector 20 located opposite to the x ray source 16 to provide a constant and high accuracy of measurements the device 10.