Hello Pete,
The "Reichsanstalt" type resistors require a heated oil bath, and you know, how tedious this is to maintain.
The SR104 as well as the Fluke 720A first decade resistors also have an oil bath, and you as well know, how critical the design of such a sealed assembly is.
The thermometer inside the aluminum block in combination with a quite small and repeatable R(T) curve makes a cumbersome oil bath, as well as a temperature stabilization unnecessary.
In other words: The aluminum mass replaces the stabilization effect of the oil, and the in situ measurement of the temperature replaces the temperature stabilization completely.? ??
For measuring and comparing my 7 resistance standards (5 x VHP202Z, 10k; 1x VHP202, 1k; 1 x Fluke 5450A, 10k), I use a program which directly calculates all required resistance values, see screen shot from my old Turbo Pascal program. 2nd column shows the temperature as measured by a 34401A, 3rd column is the actual resistance as measured by the 3458A (afterwards, a calibration factor is applied), and the 4th column is the calculated nominal R25 value, from its previously determined T.C. Please take notice of the very low StD value!
The 5450A is always used at a constant room temperature, 22 +/- 1¡ãC over the whole year (no active Air Condition used!), and its calculated max. compensated T.C. of about 0.1ppm/K for the 10k resistor chain, similar to the 742A's, eliminates further corrections.
A regulated oil bath is over engineering in my opinion.
Frank?
