referring to fig1 and 2 , the illustrated assembly comprises a flow meter body 1 defining flanges 2 and 3 sandwiched between a flange 4 mounted on one end of a pipeline 5 and a flange 6 mounted on one end of a pipeline 7 . in use , a flow meter sensor ( not shown ) is supported in the flow meter body 1 so as to be exposed to fluid flowing in the direction of arrow 8 through the assembly . a meter installation adapter 9 extends into the upstream pipe 5 from the body 1 . the adapter is sealed to the meter flange 2 by an o - ring 10 that seats against a flange 11 on the downstream end of the adapter 9 . the outside diameter of the adapter 9 is less than the inside diameter of the pipe section 5 such that an annular space 12 is defined outside the adapter 9 . the inside diameter of the adapter 9 is the same as the inside diameter of the meter housing 1 and the flanges 2 and 11 are dimensioned such that the axes of the body 1 and adapter 9 are in alignment . thus there is no surface discontinuity at the interface between the adapter 9 and the body 1 represented by line 13 . as best seen in fig2 , the flange 2 supports a lip 14 which projects over the edge of the flange 11 from which the adapter 9 extends . the flange 2 remote from the lip 14 also defines a recess to receive a projecting portion 15 extending in the axial direction from the flange 11 . this ensures that the installer can ensure that the extension 9 is correctly aligned with the body 1 . in the illustrated case , the axial length of the extension 9 is greater than the spacing between the flanges 4 and 6 . such an arrangement could only be installed if at the time of installation the spacing between the flanges 4 and 6 can be increased to at least the axial length of the adapter 9 . without any movement of the flanges 4 and 6 however the flow meter body 1 can be slipped out from the gap between the flanges 4 and 6 without having to displace the adapter 9 . although clearly it is desirable for the adapter extension 9 to be as long as possible , even the use of a very short adapter extension enhances the predictability of conditions within the flow meter . for example , an extension only a few millimetres long would be sufficient to remove unpredictable conditions immediately adjacent the interface between the pipeline flange and the flow meter flange . for example such a short extension would prevent a sealing gasket protruding across the inlet end of the flow meter . fig3 and 4 show the assembly of fig1 and 2 after a flow meter has been mounted on a side of the flow meter body 1 not shown in fig1 and 2 . the flow meter comprises a housing 14 extending upwards from a base plate 15 . as best shown in fig5 and 6 , the base plate 15 supports a projection 16 defining an arcuate inner surface 17 which is a continuation of the inner surface of the body 1 . a ring sensor 18 is supported on a radial rod 19 extending out of the surface 17 . the body 1 defines a groove 20 which in use receives a sealing ring ( not shown ), the plate 15 being secured onto the body by four bolts ( not shown ) extending through the apertures shown in fig5 and 6 . the ring 18 supports four sensors ( not shown ) spaced at 90 ° intervals around the ring . the rod 19 also supports a sensor ( not shown ) adjacent its end . in use , the sensor supported by the rod 19 is located on the axis of the passageway defined through the body 1 and each of the sensors supported by the ring 18 is located at a distance from the passageway axis equal to three quarters of the radius of the passageway . referring to fig3 and 4 , given that there is no discontinuity between the inner surface of the adapter 9 and the inner surface of the body 1 , flow conditions upstream of the sensor ring 18 are determined largely by the characteristics of the adapter rather than the characteristics of the pipe 5 into which the adapter 9 has been inserted . therefore the meter can be calibrated in the factory on the basis that conditions upstream of the meter are known . if the adapter 9 was not present , there would be a discontinuity as between the opening defined by the meter body 1 and the inner diameter of the pipe 5 . furthermore , if a gasket was used to form a seal between the meter body 1 and the pipe flange 4 , the gasket might well project radially inwards on at least one side so as to partially obstruct the inlet end of the meter body 1 . the use of the adapter in accordance with the present invention therefore ensures that conditions upstream of the meter are known with certainty as far as the upstream end of the adapter . the sensor support structure shown in fig3 to 6 provides a robust structure which is easy to mount within the flow meter , does not present an unduly large obstruction to flow through the meter , and yet makes it possible to monitor conditions at four positions spaced around the axis and on the axis itself . as a result a very accurate representation of flow conditions within the flow meter can be obtained . the dimensions of the sensor support structure are such however that a relatively large opening is required as shown in fig6 to enable the insertion and removal of the structure into the flow meter body . an alternative arrangement which does not require such a large opening as illustrated in fig7 to 10 . referring to fig7 , a support structure 21 is mounted in a flow meter body 22 sandwiched between flanges 23 and 24 of a pipeline . a meter installation adapter 25 is fitted upstream of the body 22 . as in the case of the embodiment of fig1 to 6 , the adapter is accurately located relative to the body 22 by virtue of the cooperation of formations provided on the body and adapter flanges . in particular , fig7 shows a portion 26 projecting from the adapter flange as it is engaged in a radial slot in the periphery of the body flange . the support structure 21 differs from that shown in fig3 to 6 however in that rather than being circular it is only part circular so as to define what may be loosely described as a j - shape . such an arrangement can be manoeuvred into position through a much smaller opening in the body 22 than is required to receive the full ring sensor structure as shown in for example fig5 . fig8 , 9 and 10 illustrate in greater detail the form of the sensor support structure 21 of fig7 . the structure comprises a radially extending hollow rod 27 and a part - annular portion 28 which is also hollow . a sensor 29 is located in the closed end of the rod 27 and four sensors 30 are supported inside the part - annular portion 28 . the sensors 30 are received in an inwardly facing slot formed in the part - annular portion 28 and a metal shim 31 is used to close the slot and protect the sensors 30 . the overall assembly is lightweight and the sensors are in good thermal contact through the structure with the surrounding fluid flow . given the disposition of the total of five sensors , outputs from those five sensors can be used to derive an accurate representation of flow conditions around the structure . conventional techniques can be used to relate heat loss from the sensors 29 and 30 to local flow conditions . such conventional techniques may be as described in for example european patent no . 460044 entitled “ thermal mass flow meter ”, proprietor endress + hauser limited . the arrangement of fig7 to 10 includes five sensors . a three sensor array could be supported on a structure which could be threaded through a relatively small aperture and yet would still be able to sense conditions on the meter axis and at points offset by 90 ° about the axis . one such structure would have a radially inwardly extending position , an arcuate portion extending around 90 ° of the passageway , and a further radially inwards portion extending to the passageway axes , sensors being located on the axis and at each end of the arcuate portion . with an arrangement such as that shown in fig7 to 10 , if each of the thermal sensors 29 and 30 is continuously energised so as to continuously monitor thermal losses to the adjacent flow , the energy consumed will be five times that associated with a conventional single sensor thermal flow meter array . such losses could be significant in certain applications , particularly where many flow measurements must be taken . the present invention makes it possible to minimise such thermal losses by using all five sensors to calibrate the meter and at intervals to test the accuracy of calibration but relying upon only a single sensor for routine flow measurement purposes . for example , when the flow meter is initially calibrated measurements will be taken from all five sensors for each of a representative set of calibration flow rates such that the output of each sensor at a particular flow rate is determined . it may be that the outputs from different sensors differ from each other as a result of for example an asymmetry in the flow around the sensor structure . a relationship can nevertheless be derived as between the output of one of the sensors , for example the sensor 30 closest to the free end of the part - annular portion of the structure , and the total mass flow . the relationship between the output of that sensor and the other sensors can be recorded however so that if at some time in the future flow conditions change that change can be detected by a review of the relative values of the outputs of all the sensors . for example , if the flow meter is calibrated on the basis of a fully developed flow such that the outputs of all four of the sensors 30 are identical , the output from any one of those four sensors could be taken to provide the basic measurement value . periodically the outputs from the four sensors 30 could be compared and , if those outputs were no longer the same this would indicate a need to re - calibrate as either one of the sensors is malfunctioning or flow conditions have changed from those used at the time of calibration from a fully developed flow to a flow in which conditions adjacent different sensors 30 are no longer the same . initial calibration could be conducted in the factory . the accuracy of the calibration could then be checked on installation of the meter and its intended site of use , differences between the sensor outputs being taken as an indication that the calibration had to be checked . similarly , after meter installation the accuracy of the calibration could be checked at periodic intervals so as to detect longer term drifts or loss of calibration accuracy as the result of for example changes in the flow conditions immediately upstream of the meter . this facility can be achieved at minimal extra cost in terms of energy given that except for relatively short test periods only one of the sensors is energised . fig1 is a perspective sectional view through a second alternative embodiment of the present invention . according to this preferred embodiment of the inventive sensor the sensor support has a star - like structure . different views of the sensor support structure of fig1 are shown in fig1 and fig1 . the star configuration provides a ‘ balanced ’ mounting arrangement for each of the flow sensing elements . this arrangement allows a more balanced distribution of the overall electrical power to the element network with similar performance from each sensor element . that means the performance differences between each individual sensor element ( time response , stray body heat losses , power required to each element ) are less than with regard to the non symmetrical arrangement of the sensor elements in the before described circular design of the sensor support . according to a preferred embodiment the sensor elements 30 are arranged to the sensor support as shown in fig1 and 13 . it is also possible to attach the sensor elements 30 to the sensor support in a different arrangement . it is a further advantage of the star - like structure of the sensor support means that the sensor can be manufactured potentially cheaper and easier . the new design allows a set of modular components to be created . therefore it is quite simple to manufacture an extended range of star sensor assemblies to accommodate a wide range of pipe line sizes .