ESI RS925A Decade Resistance Standard - Mini Teardown

[Jay_Diddy_B]

Hi,
On my trip to Dayton this year I picked up an ESI RS925A Resistance Standard. This is a very precise metrology grade decade resistance. It is an alternative to the General Radio 1433 series that Dave did a teardown in video 461.

The ESI decade box dates back to the mid 1960s. The current version RS925D is still available from IET Labs for a mere $16,000. The D version has some trimmers to facilitate calibration.



Schematic



A little different. The lowest decade is a potentiometer.

Specifications



Really outstanding specifications.

Construction

Outside views:







The ESI resistance standard using conventional switches, but four switch decks are wired in parallel.



The resistors are wound on Mica formers.




Several Decades:




This is the variable 10m Ohm decade:




So how good is it?

All the measurements were made with a Datron 1281 in the 4W ohms mode:


10K Ohm (10 ppm error)



100K (42 ppm error)



1M (33 ppm error)



20K (20 ppm error)



Regards,

Jay_Diddy_B

[Vgkid]

Wow impressive score. That unit is really stable considering that its accuracy is 50ppm max for longer than one year. It was also used in the ESI 242 resistance bridge.

[quarks]

very nice
when I looked into precise resistance measurement solutions
https://www.eevblog.com/forum/chat/help-wanted-low-and-high-ohm-measuremet/msg169139/#msg169139
I found the esi rs925 series as part of the esi 242 resistance measuring system is one of the very best ever made piece of gear of this kind

BTW 10k is only 1ppm off in your picture