Resistors this precise and stable require unusual design and manufacturing skills that absolutely manage the desirable properties of the resistor element wire, while minimizing undesirable characteristics that cause unpredictable errors and instabilities. These same design and manufacturing skills are as critical in RTDs as they are in precision resistors!
A Resistance Temperature Detector (RTD) is a resistor designed to measure temperature using the known resistance vs. temperature relationship of metals. An RTD element is the actual temperature-sensing unit. It is typically unprotected and offers only the element leads for terminations. An RTD probe is an assembly composed of an element, a sheath or tube to protect the element and provide a thermal path to the measurement, lead wires, and a termination or connection.