A temperature sensor assembly comprising: a temperature sensing probe having a temperature sensing portion, a terminal portion, and an intermediate portion between said temperature sensing portion and said terminal portion; an open tip thermowell configured to receive said temperature sensing probe; a mounting element configured to attach to the open tip thermowell such that said temperature sensing portion is exposed to a process environment; and a spring element; wherein said thermowell and said temperature sensing probe have different coefficients of thermal expansion, such that said thermowell will expand in an amount different than said temperature sensing probe in response to temperature in use; said temperature sensing probe comprises a stop configured to bear against a seat in said thermowell; said spring element is arranged between said stop and said mounting element and configured to bias said stop and said mounting element linearly away from each other in an axial direction along a first axis ( x-x) and thereby to bias said stop against, and maintain engagement with said seat of said thermowell; said temperature sensor assembly being characterised in that, during operation, said mounting element is in sliding engagement along said first axis ( x - x) with said intermediate portion of the temperature sensing probe such that said temperature sensing probe is movable linearly relative to said mounting element in said axial direction through a displacement range, over which said spring element will compensate for said relative differences in expansion of said temperature sensing probe and said thermowell from said temperature in use. a temperature sensing probe having a temperature sensing portion, a terminal portion, and an intermediate portion between said temperature sensing portion and said terminal portion; a temperature sensing portion, a terminal portion, and an intermediate portion between said temperature sensing portion and said terminal portion; an open tip thermowell configured to receive said temperature sensing probe; a mounting element configured to attach to the open tip thermowell such that said temperature sensing portion is exposed to a process environment; and a spring element; wherein said thermowell and said temperature sensing probe have different coefficients of thermal expansion, such that said thermowell will expand in an amount different than said temperature sensing probe in response to temperature in use; said temperature sensing probe comprises a stop configured to bear against a seat in said thermowell; said spring element is arranged between said stop and said mounting element and configured to bias said stop and said mounting element linearly away from each other in an axial direction along a first axis ( x-x) and thereby to bias said stop against, and maintain engagement with said seat of said thermowell; said temperature sensor assembly being characterised in that, during operation, said mounting element is in sliding engagement along said first axis ( x - x) with said intermediate portion of the temperature sensing probe such that said temperature sensing probe is movable linearly relative to said mounting element in said axial direction through a displacement range, over which said spring element will compensate for said relative differences in expansion of said temperature sensing probe and said thermowell from said temperature in use. thereby to bias said stop against, and maintain engagement with said seat of said thermowell; said temperature sensor assembly being characterised in that, during operation, said mounting element is in sliding engagement along said first axis ( x - x) with said intermediate portion of the temperature sensing probe such that said temperature sensing probe is movable linearly relative to said mounting element in said axial direction through a displacement range, over which said spring element will compensate for said relative differences in expansion of said temperature sensing probe and said thermowell from said temperature in use.