In a number of metrology applications a modern long scale multimeter (like Fluke 8508A) can replace the traditional resistance bridge. But when calibrating the highest grade standard resistors, which deviate only a few ppm from each other, the ESI 242D based system is still the king - unless you can afford a current comparator.
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610524;image)
The setup in the photo was originally built for SR104 intercomparison as well as calibrating other standard resistors against those primary standards. Decade transfers were realized with trimmable SR1010 Hamon resistors, allowing the bridge to operate in its 1:1 comfort zone.
The unit shown is a low temperature coefficient version of the 242D, later known as SP3632. It was purchased probably ten years before the special version became a catalogue orderable product and got that new code. The yellow sticker has obviously been added afterwards. The lab also bought a second identical system during the eighties. It has black front panels and SP3632/242D printed on the top name plate instead of the ordinary 242D.
There seems to be no visible difference between the standard and the low TC version, only some additional testing and selection. And knowing the very high build quality, the basic version was probably not much worse anyway.
What draws the attention is the use of Keithley 181 nanovoltmeter instead of the built-in null detector. The goal could have been better sensitivity in low resistance levels. Or maybe it just looked super cool back then.
I remember seeing an old arcticle about SR104 calibrations describing a similar setup and also mentioning the use of a nanovoltmeter. If anyone knows where to find it, please post here.
ESI 240C KELVIN RATIO BRIDGE
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610530;image)
Plenty of Evanohm wire resistors inside. 35 of them wound on mica cards (in SR104 style) and the remaining two 900 kohm high resistance types on ceramic bobbins. The cabling is teflon insulated.
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610536;image)
ESI 801 DC GENERATOR/DETECTOR
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610542;image)
The generator output ranges from 2V/2A up to 600V/6mA max (variable control CW). Enough to kill you. The variable scale is conveniently calibrated in milliwatts dissipated by the resistor. The lever switch turns on the excitation voltage either in positive or negative polarity, which is useful in low resistance measurements where the thermal voltages matter.
The mains wiring of the unused internal null meter has been unsoldered, possibly to minimize leakage. The wire ends can be seen in a transparent sleeve just behind the moving coil instrument. The null meter is actually an HP 419A, but he box was a little bit tricky to open, so no photos were taken this time.
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610548;image)
When measuring in the sub ppm region, you can't just use any DC source. The transformer core and the secondary winding have no physical contact: the DC leakage path is cut by several millimeters of air. A copper guard surrounds the whole secondary to prevent capacitive coupling.
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610554;image)
The connections to the external nanovoltmeter are quite interesting: the original factory made connector has been substituted by crimped copper tubes and a cover made of cellular plastic. Works very well, I have succesfully used the same approach with my Keithley 148.
![](https://www.eevblog.com/forum/metrology/esi-242d-(sp3632)-resistance-bridge-for-sr104-intercomparison/?action=dlattach;attach=1610560;image)