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What is the practical upper temperature sensing limit for a standard surface mount RTD (1000 ohm)?  Assume it is mounted with 95/5 solder.  Will the sensor remain accurate up to the melting point of the solder (237°C)?

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"A witty saying proves nothing."
-Voltaire

Surface mount RTDs don't seem to be limited by the soldering temperature, rather the manufacturer specifies the maximum upper and lower ranges for the part.    The RTDs that I am familiar with have an upper limit of 155C, other maufacturers may have higher or lower limits.  I assume that the RTD is no longer accurate beyond the specified limits.  Maybe someone else can explain why the parts are not able to go beyond these limits and remain accurate?

The actual surface mount RTD component (SMT)  itself can withstand the 237°C temperarture. The de-rating area is the point of transition between the pad on the SMT and the pad on the carrier (e.g. Polyimide flex circuit, ribbon cable, PCB etc).  If you are talking about just a short dwell time in order to solder on the compoent then the 237°C temperature rating is acceptable.  If you are talking continuous operating temperatures then you are typically limited to the temp rating on the carrier/substrate laminate.   In many current RTD sensor products, 180°C is the maximum continuous rating on a "classic"  SMT RTD system.     If  a mechanical clamping means would work in your application, there are stainless steel encased bolt on RTD sensors rated to 260°C .  There are even some rated up to 500°C.   For example, the stainless steel case which holds the 1K ohm platinum RTD sensor could be 0.265" ID ring lug.

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It is not the answer that enlightens, but the question.

Typically SMD thermistors are specified with a max upper temperature limit of 130-150 Deg C, but by coordinating the thermal expansion properties of the SMD, the board, and termination method, you should be able to use an SMD at your required temp with little problem. For example, the SMD element is made with an Alumina substrate, so mounting the element on an Alumina ceramic substrate will eliminate one source of thermal stress.