Testing of Fluke 720A KVD resistors

Contents

Introduction

While we do not have actual metrology-grade Fluke 720A Kelvin-Varley Divider (yet?), I manage to get some parts from parts unit to check.

Here’s everythink in bulk. This is basically all decades except first “A” decade (which is located in oil tank) and some of trim resistors from calibration board.

Some switching armature, circular labels and gear for switches included as well. I think teal switch missing wiper part and shaft, also top and bottom covers.

Here are decades “B”, “C” (with teal resistors) and “D”, “E” and “F” with wirewound 1K+2.5K resistors on mica substrate. 10 resistors of same value + one higher to form Kelvin-Varley ratio divider arrangement.

One of decades is just as is, with desoldered resistors.

This is Kelvin-Varley trimpot, not a usual trimmer. It manufactured by Precision Control MFG. from Bellevue, Washington. Nominal resistance is 5KΩ, tolerance ±3% and rated power up to 3W. This resistor is used in bridge balance adjustment circuit.

TECH LAB’s switch, with 720A-801 Rev.A marking on it. It manufactured around same date as resistors, 1985. All this stuff is already 31 year old.

Bonus COTO reed relay, model number 3206-12-71. I could not find any datasheet for this series.

Just check that wire thickness on 1KΩ wirewound mica.

Not everyone survive shipping and life… Two more resistors in two decades are dead too (broken wire).

Closeup on Fluke-made nice teal-colored resistors. N1.5, P1.5, N3 labels near date code probably meaning TCR rating and direction? N1.5 could mean negative TCR 1.5ppm/°C when P1.5 positive TCR +1.5ppm/°C? Further testing will reveal this.

Decade switch ring and resistor arrangement.

Overview, decades “B” and “C”

Resistors on them.

Rest of decades with 1K+2.5K setup.

Measurements

I took some measurements using Keithley 2001 in 4W-mode with NPLC10:

Photo Qty Fluke resistor code Resistor case type Manufacturer Resistance value Measured (DMM) TCR Power Device
10 199838 “C” Teal cylinder body, golden leads John Fluke MFG. Co. 4KΩ ±0.01% 4.00057 kΩ ? ? Fluke 720A
1 199836 “C” Teal cylinder body, golden leads John Fluke MFG. Co. 40.31KΩ ±0.02% 40.3160 kΩ ? 1W Fluke 720A
10 199802 “B” Teal cylinder body, golden leads John Fluke MFG. Co. 9.898KΩ ±0.02% 9.89913 kΩ ? ? Fluke 720A
1 199810 “B” Teal cylinder body, golden leads John Fluke MFG. Co. 39.536KΩ ±0.01% 39.5365 kΩ ? 1W Fluke 720A
40 206284 CL6 “D,E,F,G” Wirewound on mica rect,golden leads John Fluke MFG. Co. ? 999.887 Ω ? ? Fluke 720A
4 199869 CL5 “D,E,F,G” Wirewound on mica rect,golden leads John Fluke MFG. Co. ? 2.49976 kΩ ? ? Fluke 720A
4 201715 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.8945 Ω ? ? Fluke 720A
1 201806 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.4370 Ω ? ? Fluke 720A
1 199893 damaged Small WW on mica rect,golden leads John Fluke MFG. Co. ? 249.960 Ω? ? ? Fluke 720A
1 201772 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.6043 Ω ? ? Fluke 720A
1 201749 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.7503 Ω ? ? Fluke 720A
1 201699 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.9161 Ω ? ? Fluke 720A
1 201616 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 6.3992 Ω ? ? Fluke 720A
1 201905 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 4.9310 Ω ? ? Fluke 720A
1 201756 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.7163 Ω ? ? Fluke 720A
1 201798 Small WW on mica rect,golden leads John Fluke MFG. Co. ? 5.4965 Ω ? ? Fluke 720A

Building DIY Hammon divider

Now since we have plenty of high-precision stable resistors, what we can do with them? One of things I wanted to try, is DIY Hammon divider for calibration needs. I had implemented simple online calculator for this clever resistor network, now it’s time to try it on practice.

References:

http://www.pa4tim.nl/?p=2332
http://www.eevblog.com/forum/testgear/how-does-fluke-get-below-5ppm-in-their-kelvin-varley-divider/msg789965/#msg789965
http://www.eevblog.com/forum/testgear/kelvin-varley-reistor-porn/msg773527/#msg773527
http://www.eevblog.com/forum/projects/very-low-bias-current-op-amp-to-buffer-a-kelvin-varley-divider/msg694272/#msg694272
http://www.eevblog.com/forum/testgear/buffer-for-kelvin-varley-divider/
http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/calibration/ratio_calibration—whats_the_big_deal
http://www.amplifier.cd/Test_Equipment/other/Fluke720A.html
http://us.flukecal.com/products/electrical-calibration/electrical-standards/720a-kelvin-varley-divider-0
http://www.eevblog.com/forum/projects/kelvin-varley-divider-(and-precision-voltage-source)/
https://www.febo.com/pipermail/volt-nuts/2010-September/000497.html
http://assets.fluke.com/appnotes/Calibration/NF_ncsl02.pdf
http://assets.fluke.com/download/Calibration/Seminars/TheReferenceMultimeter&RatioMeasurements.pdf

IET KVD-700 is substitute of 720A

AC “KVD

http://www.eevblog.com/forum/reviews/ratio-transformer-gertsch-ratiotran-rt-18/25/

Author: Illya Tsemenko
Published: Feb. 15, 2016, 5:20 p.m.
Modified: Aug. 9, 2016, 4:36 p.m.

References

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