DIY Calibration resistance box build

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
Author: Illya Tsemenko
Published: March 28, 2016, 3:47 a.m.
Modified: Sept. 4, 2016, 2:50 a.m.

References

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