a method for operating a flow meter incorporating thermal loss sensors and an installation adapter to provide known flow conditions upstream of the meter . the method includes distributing a plurality of the thermal loss sensors about a passageway through which flow is to be monitored , wherein during normal operation a single sensor is energized to provide an output representative of thermal loss due to the flow adjacent that sensor ; and deriving an output representative of the flow rate from the output representative of thermal loss on the basis of a predetermined calibration relationship between flow rate and thermal loss from a single sensor ; establishing the calibration relationship on the basis of outputs derived from all of the sensors at calibration , a representation of the relative values of the outputs of the sensors at calibration being recorded , and during use the accuracy of the calibration being tested by energizing all of the sensors ; and comparing the relative values of the resultant sensor outputs with the recorded relative values , with changes in the relative values as between calibration and test being indicative of loss of calibration accuracy .