High stability resistors are often required for testing multimeters, ohmeters and current measurement circuits. It’s not the absolute accuracy what is important, but the stability of test resistor, so good confidence in comparisons can be maintained.

But most of industry standard resistors are expensive, often bulky and provide only one value. Widely recognized examples are Fluke 742A resistors, Thomas type oil bath barrel resistors such as Guildline 9330, and ESI/IET SR104.

What I had in mind instead is building own portable compact box with array of different hermetic low tempco resistors, to allow use as transfer device between different meters. It should be built sturdy enough also to survive usual lab operation and multiple connections.

Construction

It was decided to make a prototype using 8 resistors from 1 Ω to 10 MΩ.

To get most of accuracy kelvin 4-wire connection technique is a common requirement for resistance measurements. But as name imply, for 8 resistors that will need 32 connectors. High quality copper binding posts can cost a lot, often more than 20$ each. Alternative route was chosen, to use triaxial connectors which can also provide guarding and allow 4-wire connection to each resistor with just two triax cables.

Triaxial connectors aren’t cheap too, but I have a workaround, in form of taking triaxial panel ports from Keithley Model 7172 low current matrix card, which was bought for $200 USD.

Initial calibration (measurement)

K2001 : 1R – 100K – 4wire, OCOMP Enabled, no filter, 1M-10M, 2-wire, OCOMP disabled
K2002 : 1R – 20K – 4wire, OCOMP ENABLED, no filter, 200K+, OCOMP DISABLED
3458 : NPLC 100, OCOMP Enabled, Delay 6sec, 4-wire

Value	Used resistor	K2001	K2002u1	_.3458A(8/8/16)	HHH-Bridge	3458A-3
1 Ω	VPG	1.001537	1.00050	1.00130		1.00475
19 Ω	VPG	19.000069	19.000089	18.99927		18.99939
100 Ω	Wirewound		100.00915	100.00456		100.00168
190 Ω	VPG	190.00335	190.00981	189.9981		190.00000
1.9 KΩ	VPG	1.9000207K	1.899984K	1.899965K		1.899981 K
20 KΩ	Wirewound		19.993345K	19.99269K	19.99336K	19.99353K
95 KΩ	VPG	94.99544K	94.99735K	94.99655K		94.99895K
100 KΩ	VPG	100.00074K	100.00493K	100.00354K		100.00570K
1 MΩ	VPG	0.9998124M	0.9999725M	0.9999547M		0.9999831M
10 MΩ	Caddock USF340	9.985158M	9.99842M	9.999130M		10.000199M

Comparison of calibrated 3458A versus DUT 3458A meter after +3ppm recalibration, using resistance box at +24.0 °C temperature:

Value	2002-1	2002-2	3458A-3 Calibrated	3458A UUT	Dev, ppm
1 Ω	1.001399	1.001540	1.00475	1.00145	—-
19 Ω	18.999425	18.99914	18.99939	18.99951	-1.57
100 Ω	100.00344	100.0039	100.00168	100.00211	+8.2
190 Ω	189.99941	189.9966	190.00000	189.9988	-6.31
1.9 KΩ	1.8999750K	1.8999640k	1.899981 K	1.899973 K	-4.21
10K REF9823			10K
20 KΩ	19.99358K	19.993312k	19.99353K	19.99335 K	-9.00
95 KΩ	94.99890K	94.999054k	94.99895K	94.99847 K	-5.05
100 KΩ	100.00567K	100.00586k	100.00570K	100.00532 K	-3.8
1 MΩ	1.0000043M	0.9999951M	0.9999831M	0.9999817 M	-1.4
10 MΩ	10.001082M	10.000402M	10.000199M	10.000197 M	-0.2

Build video and process

Here is short video for box build and assembly.

On video you can see resistor array in place of 10 MΩ resistor, but currently that is replaced with single Caddock USF340 resistor.

Update R2

Later in July 2016 I had replaced 10MΩ with Caddock USF340 resistor and added 100Ω and 20KΩ wirewound precision resistors.

Final looks:

Side closeups:

Triaxial cables assembly

Since none of multimeters have triaxial input ports, custom pair of cable has to be made to connect resistor box we built to the meter. Here is video of cable assembly and test.

Cable used : CNMC medical low-noise triax
Connectors used: Trompeter 3-lug triaxial

Use examples

One of the usage cases of this box is transferring resistance from different meters, both in my own lab and internationally with help of other enthusiasts. I’ll provide more details as the test data of such experiment arrives.

_This article is in update progress, sorry. _

There are also some other resistors kits available from resistor manufacturers which come in decade steps for test and calibration purposed. One of such examples is MC-7 kit

Here’s test data, taken by freshly calibrated 3458A with OCOMP ON, DELAY 5, 4W NPLC100 measurements. Ambient temperature +24.2 °C. :

Value	3458A-3 (8/26/2016)	Abs.Tol	Unc
1 Ω	1.00047
10 Ω	10.00079
100 Ω	100.00650
1 KΩ	1.0000322K
10 KΩ	10.000073K
100 KΩ	99.99939K
1 MΩ	1.0000599M