Because of it’s special construction, the wekomm reference resistor does not need an oil bath. Even in non optimal or rugged environments the resistor will perfectly perform up to it’s specifications.
According to our experience, the mechanical and electrical properties of the resistor elements only partly contribute to the excellent specifications. A lot of precision is obtained through special manufacturing processes and construction details. (You can find details in the test results of the PTB). To achieve such results, not only every components needs to be optimized. The well balanced combination of all parts makes the difference here.