a hollow polished stainless steel tube and each contain a heating element 38 42 and a temperature sensing element 40 44 respectively a heating current derived from the controller 24 is provided to the heating element 38 and 42 via a suitable electrical conductor 46 and temperature measurement signals are returned from the temperature sensing elements to the controller via a pair of conductors 48 the conductor 46 and 48 are attached to a connector 49 which may be attached to cable 22 the sensor operates on a thermal dispersion principle based on newton 39 s law of cooling one of the probes is selected and its heating element is supplied with a constant energy which radiates out as heat we generally refer to this probe as the energized probe its counterpart probe or unheated probe is generally called the ambient probe both the probes provide a temperature signal from their respective temperature sensing elements thus it may be shown that the heat input rate into a medium may be expressed by the equation q h u03b4t where h is the convection heat transfer co efficient and u03b4t is the temperature difference between the heat source and the medium in this case u03b4t is the temperature difference between the heated and ambient probes the value h is a function of the flow rate of the medium hence h is not constant thus it may be seen that the temperature differential between the probes is inversely proportional to the flow rate of the medium for a given heat input rate q it may be more accurately.