into the opening in the nozzle this opening communicates with the sample nozzle collection manifold 16 the number of probes 12 and sampling nozzles 14 is determined by extent of the non uniform properties of the velocity temperature and fluid concentration profile in the conduit preferably the plurality of probes and nozzles are arranged on a cross sectional area of the conduit thus there is a least one sampling nozzle 14 placed at equal areas within the conduit facing the flow each of which has a mass velocity that is linearly proportional to the local mass velocity of the fluid in the conduit such that the resulting flow rate from all the nozzles represents a truly representative sample of the aerosol and gases in the conduit this sample flows through the particulate analyzer 38 if used the particulate filter 18 if used the fluid composition measurement chamber 20 the in line mass flow meter 22 through the vacuum pump 24 or other suitable device and is exhausted back to the conduit 10 thus completing the sample loop the mass rate of the sample loop is proportional to the mass flow rate for the area of the conduit being measured one or more such multi point sampling probe assemblies may be used to obtain the average fluid concentration.