The present invention relates to a sensor for measuring temperature of a fluid within a vessel, the vessel having a first region and a second region and the fluid having a temperature profile extending between the first region and the second region, the sensor comprising an array of elements, each element having a temperature-dependent parameter, the array being capable of deployment within or adjacent the vessel such that the array extends along the vessel for measuring the temperature profile, the elements of the array being coupled together between an input and an output, the input being coupled or capable of being coupled to a driving source for driving the sensors, and the output being coupled or capable of being coupled to a detector for measuring an aggregate of the temperature-dependent parameter from the array of elements. The invention further relates to a fluid temperature controller comprising a first input for receiving a first signal indicating a measurement of an aggregate of a temperature-dependent parameter from a sensor deployed within or adjacent a vessel containing a fluid having a temperature profile, a second input for receiving a second signal indicating a (preferably absolute) temperature of the fluid in the vessel and a processor configured to calculate a total thermal energy of the fluid in the vessel based on the first and second signals. The invention also relates to a combination comprising a sensing arrangement and a controller; a device; and a system.